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Source file src/reflect/value.go

Documentation: reflect

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package reflect
     6  
     7  import (
     8  	"math"
     9  	"runtime"
    10  	"unsafe"
    11  )
    12  
    13  const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const
    14  
    15  // Value is the reflection interface to a Go value.
    16  //
    17  // Not all methods apply to all kinds of values. Restrictions,
    18  // if any, are noted in the documentation for each method.
    19  // Use the Kind method to find out the kind of value before
    20  // calling kind-specific methods. Calling a method
    21  // inappropriate to the kind of type causes a run time panic.
    22  //
    23  // The zero Value represents no value.
    24  // Its IsValid method returns false, its Kind method returns Invalid,
    25  // its String method returns "<invalid Value>", and all other methods panic.
    26  // Most functions and methods never return an invalid value.
    27  // If one does, its documentation states the conditions explicitly.
    28  //
    29  // A Value can be used concurrently by multiple goroutines provided that
    30  // the underlying Go value can be used concurrently for the equivalent
    31  // direct operations.
    32  //
    33  // To compare two Values, compare the results of the Interface method.
    34  // Using == on two Values does not compare the underlying values
    35  // they represent.
    36  type Value struct {
    37  	// typ holds the type of the value represented by a Value.
    38  	typ *rtype
    39  
    40  	// Pointer-valued data or, if flagIndir is set, pointer to data.
    41  	// Valid when either flagIndir is set or typ.pointers() is true.
    42  	ptr unsafe.Pointer
    43  
    44  	// flag holds metadata about the value.
    45  	// The lowest bits are flag bits:
    46  	//	- flagStickyRO: obtained via unexported not embedded field, so read-only
    47  	//	- flagEmbedRO: obtained via unexported embedded field, so read-only
    48  	//	- flagIndir: val holds a pointer to the data
    49  	//	- flagAddr: v.CanAddr is true (implies flagIndir)
    50  	//	- flagMethod: v is a method value.
    51  	// The next five bits give the Kind of the value.
    52  	// This repeats typ.Kind() except for method values.
    53  	// The remaining 23+ bits give a method number for method values.
    54  	// If flag.kind() != Func, code can assume that flagMethod is unset.
    55  	// If ifaceIndir(typ), code can assume that flagIndir is set.
    56  	flag
    57  
    58  	// A method value represents a curried method invocation
    59  	// like r.Read for some receiver r. The typ+val+flag bits describe
    60  	// the receiver r, but the flag's Kind bits say Func (methods are
    61  	// functions), and the top bits of the flag give the method number
    62  	// in r's type's method table.
    63  }
    64  
    65  type flag uintptr
    66  
    67  const (
    68  	flagKindWidth        = 5 // there are 27 kinds
    69  	flagKindMask    flag = 1<<flagKindWidth - 1
    70  	flagStickyRO    flag = 1 << 5
    71  	flagEmbedRO     flag = 1 << 6
    72  	flagIndir       flag = 1 << 7
    73  	flagAddr        flag = 1 << 8
    74  	flagMethod      flag = 1 << 9
    75  	flagMethodShift      = 10
    76  	flagRO          flag = flagStickyRO | flagEmbedRO
    77  )
    78  
    79  func (f flag) kind() Kind {
    80  	return Kind(f & flagKindMask)
    81  }
    82  
    83  func (f flag) ro() flag {
    84  	if f&flagRO != 0 {
    85  		return flagStickyRO
    86  	}
    87  	return 0
    88  }
    89  
    90  // pointer returns the underlying pointer represented by v.
    91  // v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
    92  func (v Value) pointer() unsafe.Pointer {
    93  	if v.typ.size != ptrSize || !v.typ.pointers() {
    94  		panic("can't call pointer on a non-pointer Value")
    95  	}
    96  	if v.flag&flagIndir != 0 {
    97  		return *(*unsafe.Pointer)(v.ptr)
    98  	}
    99  	return v.ptr
   100  }
   101  
   102  // packEface converts v to the empty interface.
   103  func packEface(v Value) interface{} {
   104  	t := v.typ
   105  	var i interface{}
   106  	e := (*emptyInterface)(unsafe.Pointer(&i))
   107  	// First, fill in the data portion of the interface.
   108  	switch {
   109  	case ifaceIndir(t):
   110  		if v.flag&flagIndir == 0 {
   111  			panic("bad indir")
   112  		}
   113  		// Value is indirect, and so is the interface we're making.
   114  		ptr := v.ptr
   115  		if v.flag&flagAddr != 0 {
   116  			// TODO: pass safe boolean from valueInterface so
   117  			// we don't need to copy if safe==true?
   118  			c := unsafe_New(t)
   119  			typedmemmove(t, c, ptr)
   120  			ptr = c
   121  		}
   122  		e.word = ptr
   123  	case v.flag&flagIndir != 0:
   124  		// Value is indirect, but interface is direct. We need
   125  		// to load the data at v.ptr into the interface data word.
   126  		e.word = *(*unsafe.Pointer)(v.ptr)
   127  	default:
   128  		// Value is direct, and so is the interface.
   129  		e.word = v.ptr
   130  	}
   131  	// Now, fill in the type portion. We're very careful here not
   132  	// to have any operation between the e.word and e.typ assignments
   133  	// that would let the garbage collector observe the partially-built
   134  	// interface value.
   135  	e.typ = t
   136  	return i
   137  }
   138  
   139  // unpackEface converts the empty interface i to a Value.
   140  func unpackEface(i interface{}) Value {
   141  	e := (*emptyInterface)(unsafe.Pointer(&i))
   142  	// NOTE: don't read e.word until we know whether it is really a pointer or not.
   143  	t := e.typ
   144  	if t == nil {
   145  		return Value{}
   146  	}
   147  	f := flag(t.Kind())
   148  	if ifaceIndir(t) {
   149  		f |= flagIndir
   150  	}
   151  	return Value{t, e.word, f}
   152  }
   153  
   154  // A ValueError occurs when a Value method is invoked on
   155  // a Value that does not support it. Such cases are documented
   156  // in the description of each method.
   157  type ValueError struct {
   158  	Method string
   159  	Kind   Kind
   160  }
   161  
   162  func (e *ValueError) Error() string {
   163  	if e.Kind == 0 {
   164  		return "reflect: call of " + e.Method + " on zero Value"
   165  	}
   166  	return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
   167  }
   168  
   169  // methodName returns the name of the calling method,
   170  // assumed to be two stack frames above.
   171  func methodName() string {
   172  	pc, _, _, _ := runtime.Caller(2)
   173  	f := runtime.FuncForPC(pc)
   174  	if f == nil {
   175  		return "unknown method"
   176  	}
   177  	return f.Name()
   178  }
   179  
   180  // emptyInterface is the header for an interface{} value.
   181  type emptyInterface struct {
   182  	typ  *rtype
   183  	word unsafe.Pointer
   184  }
   185  
   186  // nonEmptyInterface is the header for an interface value with methods.
   187  type nonEmptyInterface struct {
   188  	// see ../runtime/iface.go:/Itab
   189  	itab *struct {
   190  		ityp *rtype // static interface type
   191  		typ  *rtype // dynamic concrete type
   192  		hash uint32 // copy of typ.hash
   193  		_    [4]byte
   194  		fun  [100000]unsafe.Pointer // method table
   195  	}
   196  	word unsafe.Pointer
   197  }
   198  
   199  // mustBe panics if f's kind is not expected.
   200  // Making this a method on flag instead of on Value
   201  // (and embedding flag in Value) means that we can write
   202  // the very clear v.mustBe(Bool) and have it compile into
   203  // v.flag.mustBe(Bool), which will only bother to copy the
   204  // single important word for the receiver.
   205  func (f flag) mustBe(expected Kind) {
   206  	if f.kind() != expected {
   207  		panic(&ValueError{methodName(), f.kind()})
   208  	}
   209  }
   210  
   211  // mustBeExported panics if f records that the value was obtained using
   212  // an unexported field.
   213  func (f flag) mustBeExported() {
   214  	if f == 0 {
   215  		panic(&ValueError{methodName(), 0})
   216  	}
   217  	if f&flagRO != 0 {
   218  		panic("reflect: " + methodName() + " using value obtained using unexported field")
   219  	}
   220  }
   221  
   222  // mustBeAssignable panics if f records that the value is not assignable,
   223  // which is to say that either it was obtained using an unexported field
   224  // or it is not addressable.
   225  func (f flag) mustBeAssignable() {
   226  	if f == 0 {
   227  		panic(&ValueError{methodName(), Invalid})
   228  	}
   229  	// Assignable if addressable and not read-only.
   230  	if f&flagRO != 0 {
   231  		panic("reflect: " + methodName() + " using value obtained using unexported field")
   232  	}
   233  	if f&flagAddr == 0 {
   234  		panic("reflect: " + methodName() + " using unaddressable value")
   235  	}
   236  }
   237  
   238  // Addr returns a pointer value representing the address of v.
   239  // It panics if CanAddr() returns false.
   240  // Addr is typically used to obtain a pointer to a struct field
   241  // or slice element in order to call a method that requires a
   242  // pointer receiver.
   243  func (v Value) Addr() Value {
   244  	if v.flag&flagAddr == 0 {
   245  		panic("reflect.Value.Addr of unaddressable value")
   246  	}
   247  	return Value{v.typ.ptrTo(), v.ptr, v.flag.ro() | flag(Ptr)}
   248  }
   249  
   250  // Bool returns v's underlying value.
   251  // It panics if v's kind is not Bool.
   252  func (v Value) Bool() bool {
   253  	v.mustBe(Bool)
   254  	return *(*bool)(v.ptr)
   255  }
   256  
   257  // Bytes returns v's underlying value.
   258  // It panics if v's underlying value is not a slice of bytes.
   259  func (v Value) Bytes() []byte {
   260  	v.mustBe(Slice)
   261  	if v.typ.Elem().Kind() != Uint8 {
   262  		panic("reflect.Value.Bytes of non-byte slice")
   263  	}
   264  	// Slice is always bigger than a word; assume flagIndir.
   265  	return *(*[]byte)(v.ptr)
   266  }
   267  
   268  // runes returns v's underlying value.
   269  // It panics if v's underlying value is not a slice of runes (int32s).
   270  func (v Value) runes() []rune {
   271  	v.mustBe(Slice)
   272  	if v.typ.Elem().Kind() != Int32 {
   273  		panic("reflect.Value.Bytes of non-rune slice")
   274  	}
   275  	// Slice is always bigger than a word; assume flagIndir.
   276  	return *(*[]rune)(v.ptr)
   277  }
   278  
   279  // CanAddr reports whether the value's address can be obtained with Addr.
   280  // Such values are called addressable. A value is addressable if it is
   281  // an element of a slice, an element of an addressable array,
   282  // a field of an addressable struct, or the result of dereferencing a pointer.
   283  // If CanAddr returns false, calling Addr will panic.
   284  func (v Value) CanAddr() bool {
   285  	return v.flag&flagAddr != 0
   286  }
   287  
   288  // CanSet reports whether the value of v can be changed.
   289  // A Value can be changed only if it is addressable and was not
   290  // obtained by the use of unexported struct fields.
   291  // If CanSet returns false, calling Set or any type-specific
   292  // setter (e.g., SetBool, SetInt) will panic.
   293  func (v Value) CanSet() bool {
   294  	return v.flag&(flagAddr|flagRO) == flagAddr
   295  }
   296  
   297  // Call calls the function v with the input arguments in.
   298  // For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]).
   299  // Call panics if v's Kind is not Func.
   300  // It returns the output results as Values.
   301  // As in Go, each input argument must be assignable to the
   302  // type of the function's corresponding input parameter.
   303  // If v is a variadic function, Call creates the variadic slice parameter
   304  // itself, copying in the corresponding values.
   305  func (v Value) Call(in []Value) []Value {
   306  	v.mustBe(Func)
   307  	v.mustBeExported()
   308  	return v.call("Call", in)
   309  }
   310  
   311  // CallSlice calls the variadic function v with the input arguments in,
   312  // assigning the slice in[len(in)-1] to v's final variadic argument.
   313  // For example, if len(in) == 3, v.CallSlice(in) represents the Go call v(in[0], in[1], in[2]...).
   314  // CallSlice panics if v's Kind is not Func or if v is not variadic.
   315  // It returns the output results as Values.
   316  // As in Go, each input argument must be assignable to the
   317  // type of the function's corresponding input parameter.
   318  func (v Value) CallSlice(in []Value) []Value {
   319  	v.mustBe(Func)
   320  	v.mustBeExported()
   321  	return v.call("CallSlice", in)
   322  }
   323  
   324  var callGC bool // for testing; see TestCallMethodJump
   325  
   326  func (v Value) call(op string, in []Value) []Value {
   327  	// Get function pointer, type.
   328  	t := (*funcType)(unsafe.Pointer(v.typ))
   329  	var (
   330  		fn       unsafe.Pointer
   331  		rcvr     Value
   332  		rcvrtype *rtype
   333  	)
   334  	if v.flag&flagMethod != 0 {
   335  		rcvr = v
   336  		rcvrtype, t, fn = methodReceiver(op, v, int(v.flag)>>flagMethodShift)
   337  	} else if v.flag&flagIndir != 0 {
   338  		fn = *(*unsafe.Pointer)(v.ptr)
   339  	} else {
   340  		fn = v.ptr
   341  	}
   342  
   343  	if fn == nil {
   344  		panic("reflect.Value.Call: call of nil function")
   345  	}
   346  
   347  	isSlice := op == "CallSlice"
   348  	n := t.NumIn()
   349  	if isSlice {
   350  		if !t.IsVariadic() {
   351  			panic("reflect: CallSlice of non-variadic function")
   352  		}
   353  		if len(in) < n {
   354  			panic("reflect: CallSlice with too few input arguments")
   355  		}
   356  		if len(in) > n {
   357  			panic("reflect: CallSlice with too many input arguments")
   358  		}
   359  	} else {
   360  		if t.IsVariadic() {
   361  			n--
   362  		}
   363  		if len(in) < n {
   364  			panic("reflect: Call with too few input arguments")
   365  		}
   366  		if !t.IsVariadic() && len(in) > n {
   367  			panic("reflect: Call with too many input arguments")
   368  		}
   369  	}
   370  	for _, x := range in {
   371  		if x.Kind() == Invalid {
   372  			panic("reflect: " + op + " using zero Value argument")
   373  		}
   374  	}
   375  	for i := 0; i < n; i++ {
   376  		if xt, targ := in[i].Type(), t.In(i); !xt.AssignableTo(targ) {
   377  			panic("reflect: " + op + " using " + xt.String() + " as type " + targ.String())
   378  		}
   379  	}
   380  	if !isSlice && t.IsVariadic() {
   381  		// prepare slice for remaining values
   382  		m := len(in) - n
   383  		slice := MakeSlice(t.In(n), m, m)
   384  		elem := t.In(n).Elem()
   385  		for i := 0; i < m; i++ {
   386  			x := in[n+i]
   387  			if xt := x.Type(); !xt.AssignableTo(elem) {
   388  				panic("reflect: cannot use " + xt.String() + " as type " + elem.String() + " in " + op)
   389  			}
   390  			slice.Index(i).Set(x)
   391  		}
   392  		origIn := in
   393  		in = make([]Value, n+1)
   394  		copy(in[:n], origIn)
   395  		in[n] = slice
   396  	}
   397  
   398  	nin := len(in)
   399  	if nin != t.NumIn() {
   400  		panic("reflect.Value.Call: wrong argument count")
   401  	}
   402  	nout := t.NumOut()
   403  
   404  	// Compute frame type.
   405  	frametype, _, retOffset, _, framePool := funcLayout(t, rcvrtype)
   406  
   407  	// Allocate a chunk of memory for frame.
   408  	var args unsafe.Pointer
   409  	if nout == 0 {
   410  		args = framePool.Get().(unsafe.Pointer)
   411  	} else {
   412  		// Can't use pool if the function has return values.
   413  		// We will leak pointer to args in ret, so its lifetime is not scoped.
   414  		args = unsafe_New(frametype)
   415  	}
   416  	off := uintptr(0)
   417  
   418  	// Copy inputs into args.
   419  	if rcvrtype != nil {
   420  		storeRcvr(rcvr, args)
   421  		off = ptrSize
   422  	}
   423  	for i, v := range in {
   424  		v.mustBeExported()
   425  		targ := t.In(i).(*rtype)
   426  		a := uintptr(targ.align)
   427  		off = (off + a - 1) &^ (a - 1)
   428  		n := targ.size
   429  		if n == 0 {
   430  			// Not safe to compute args+off pointing at 0 bytes,
   431  			// because that might point beyond the end of the frame,
   432  			// but we still need to call assignTo to check assignability.
   433  			v.assignTo("reflect.Value.Call", targ, nil)
   434  			continue
   435  		}
   436  		addr := add(args, off, "n > 0")
   437  		v = v.assignTo("reflect.Value.Call", targ, addr)
   438  		if v.flag&flagIndir != 0 {
   439  			typedmemmove(targ, addr, v.ptr)
   440  		} else {
   441  			*(*unsafe.Pointer)(addr) = v.ptr
   442  		}
   443  		off += n
   444  	}
   445  
   446  	// Call.
   447  	call(frametype, fn, args, uint32(frametype.size), uint32(retOffset))
   448  
   449  	// For testing; see TestCallMethodJump.
   450  	if callGC {
   451  		runtime.GC()
   452  	}
   453  
   454  	var ret []Value
   455  	if nout == 0 {
   456  		typedmemclr(frametype, args)
   457  		framePool.Put(args)
   458  	} else {
   459  		// Zero the now unused input area of args,
   460  		// because the Values returned by this function contain pointers to the args object,
   461  		// and will thus keep the args object alive indefinitely.
   462  		typedmemclrpartial(frametype, args, 0, retOffset)
   463  
   464  		// Wrap Values around return values in args.
   465  		ret = make([]Value, nout)
   466  		off = retOffset
   467  		for i := 0; i < nout; i++ {
   468  			tv := t.Out(i)
   469  			a := uintptr(tv.Align())
   470  			off = (off + a - 1) &^ (a - 1)
   471  			if tv.Size() != 0 {
   472  				fl := flagIndir | flag(tv.Kind())
   473  				ret[i] = Value{tv.common(), add(args, off, "tv.Size() != 0"), fl}
   474  				// Note: this does introduce false sharing between results -
   475  				// if any result is live, they are all live.
   476  				// (And the space for the args is live as well, but as we've
   477  				// cleared that space it isn't as big a deal.)
   478  			} else {
   479  				// For zero-sized return value, args+off may point to the next object.
   480  				// In this case, return the zero value instead.
   481  				ret[i] = Zero(tv)
   482  			}
   483  			off += tv.Size()
   484  		}
   485  	}
   486  
   487  	return ret
   488  }
   489  
   490  // callReflect is the call implementation used by a function
   491  // returned by MakeFunc. In many ways it is the opposite of the
   492  // method Value.call above. The method above converts a call using Values
   493  // into a call of a function with a concrete argument frame, while
   494  // callReflect converts a call of a function with a concrete argument
   495  // frame into a call using Values.
   496  // It is in this file so that it can be next to the call method above.
   497  // The remainder of the MakeFunc implementation is in makefunc.go.
   498  //
   499  // NOTE: This function must be marked as a "wrapper" in the generated code,
   500  // so that the linker can make it work correctly for panic and recover.
   501  // The gc compilers know to do that for the name "reflect.callReflect".
   502  //
   503  // ctxt is the "closure" generated by MakeFunc.
   504  // frame is a pointer to the arguments to that closure on the stack.
   505  // retValid points to a boolean which should be set when the results
   506  // section of frame is set.
   507  func callReflect(ctxt *makeFuncImpl, frame unsafe.Pointer, retValid *bool) {
   508  	ftyp := ctxt.ftyp
   509  	f := ctxt.fn
   510  
   511  	// Copy argument frame into Values.
   512  	ptr := frame
   513  	off := uintptr(0)
   514  	in := make([]Value, 0, int(ftyp.inCount))
   515  	for _, typ := range ftyp.in() {
   516  		off += -off & uintptr(typ.align-1)
   517  		v := Value{typ, nil, flag(typ.Kind())}
   518  		if ifaceIndir(typ) {
   519  			// value cannot be inlined in interface data.
   520  			// Must make a copy, because f might keep a reference to it,
   521  			// and we cannot let f keep a reference to the stack frame
   522  			// after this function returns, not even a read-only reference.
   523  			v.ptr = unsafe_New(typ)
   524  			if typ.size > 0 {
   525  				typedmemmove(typ, v.ptr, add(ptr, off, "typ.size > 0"))
   526  			}
   527  			v.flag |= flagIndir
   528  		} else {
   529  			v.ptr = *(*unsafe.Pointer)(add(ptr, off, "1-ptr"))
   530  		}
   531  		in = append(in, v)
   532  		off += typ.size
   533  	}
   534  
   535  	// Call underlying function.
   536  	out := f(in)
   537  	numOut := ftyp.NumOut()
   538  	if len(out) != numOut {
   539  		panic("reflect: wrong return count from function created by MakeFunc")
   540  	}
   541  
   542  	// Copy results back into argument frame.
   543  	if numOut > 0 {
   544  		off += -off & (ptrSize - 1)
   545  		if runtime.GOARCH == "amd64p32" {
   546  			off = align(off, 8)
   547  		}
   548  		for i, typ := range ftyp.out() {
   549  			v := out[i]
   550  			if v.typ != typ {
   551  				panic("reflect: function created by MakeFunc using " + funcName(f) +
   552  					" returned wrong type: have " +
   553  					out[i].typ.String() + " for " + typ.String())
   554  			}
   555  			if v.flag&flagRO != 0 {
   556  				panic("reflect: function created by MakeFunc using " + funcName(f) +
   557  					" returned value obtained from unexported field")
   558  			}
   559  			off += -off & uintptr(typ.align-1)
   560  			if typ.size == 0 {
   561  				continue
   562  			}
   563  			addr := add(ptr, off, "typ.size > 0")
   564  			// We are writing to stack. No write barrier.
   565  			if v.flag&flagIndir != 0 {
   566  				memmove(addr, v.ptr, typ.size)
   567  			} else {
   568  				*(*uintptr)(addr) = uintptr(v.ptr)
   569  			}
   570  			off += typ.size
   571  		}
   572  	}
   573  
   574  	// Announce that the return values are valid.
   575  	// After this point the runtime can depend on the return values being valid.
   576  	*retValid = true
   577  
   578  	// We have to make sure that the out slice lives at least until
   579  	// the runtime knows the return values are valid. Otherwise, the
   580  	// return values might not be scanned by anyone during a GC.
   581  	// (out would be dead, and the return slots not yet alive.)
   582  	runtime.KeepAlive(out)
   583  
   584  	// runtime.getArgInfo expects to be able to find ctxt on the
   585  	// stack when it finds our caller, makeFuncStub. Make sure it
   586  	// doesn't get garbage collected.
   587  	runtime.KeepAlive(ctxt)
   588  }
   589  
   590  // methodReceiver returns information about the receiver
   591  // described by v. The Value v may or may not have the
   592  // flagMethod bit set, so the kind cached in v.flag should
   593  // not be used.
   594  // The return value rcvrtype gives the method's actual receiver type.
   595  // The return value t gives the method type signature (without the receiver).
   596  // The return value fn is a pointer to the method code.
   597  func methodReceiver(op string, v Value, methodIndex int) (rcvrtype *rtype, t *funcType, fn unsafe.Pointer) {
   598  	i := methodIndex
   599  	if v.typ.Kind() == Interface {
   600  		tt := (*interfaceType)(unsafe.Pointer(v.typ))
   601  		if uint(i) >= uint(len(tt.methods)) {
   602  			panic("reflect: internal error: invalid method index")
   603  		}
   604  		m := &tt.methods[i]
   605  		if !tt.nameOff(m.name).isExported() {
   606  			panic("reflect: " + op + " of unexported method")
   607  		}
   608  		iface := (*nonEmptyInterface)(v.ptr)
   609  		if iface.itab == nil {
   610  			panic("reflect: " + op + " of method on nil interface value")
   611  		}
   612  		rcvrtype = iface.itab.typ
   613  		fn = unsafe.Pointer(&iface.itab.fun[i])
   614  		t = (*funcType)(unsafe.Pointer(tt.typeOff(m.typ)))
   615  	} else {
   616  		rcvrtype = v.typ
   617  		ms := v.typ.exportedMethods()
   618  		if uint(i) >= uint(len(ms)) {
   619  			panic("reflect: internal error: invalid method index")
   620  		}
   621  		m := ms[i]
   622  		if !v.typ.nameOff(m.name).isExported() {
   623  			panic("reflect: " + op + " of unexported method")
   624  		}
   625  		ifn := v.typ.textOff(m.ifn)
   626  		fn = unsafe.Pointer(&ifn)
   627  		t = (*funcType)(unsafe.Pointer(v.typ.typeOff(m.mtyp)))
   628  	}
   629  	return
   630  }
   631  
   632  // v is a method receiver. Store at p the word which is used to
   633  // encode that receiver at the start of the argument list.
   634  // Reflect uses the "interface" calling convention for
   635  // methods, which always uses one word to record the receiver.
   636  func storeRcvr(v Value, p unsafe.Pointer) {
   637  	t := v.typ
   638  	if t.Kind() == Interface {
   639  		// the interface data word becomes the receiver word
   640  		iface := (*nonEmptyInterface)(v.ptr)
   641  		*(*unsafe.Pointer)(p) = iface.word
   642  	} else if v.flag&flagIndir != 0 && !ifaceIndir(t) {
   643  		*(*unsafe.Pointer)(p) = *(*unsafe.Pointer)(v.ptr)
   644  	} else {
   645  		*(*unsafe.Pointer)(p) = v.ptr
   646  	}
   647  }
   648  
   649  // align returns the result of rounding x up to a multiple of n.
   650  // n must be a power of two.
   651  func align(x, n uintptr) uintptr {
   652  	return (x + n - 1) &^ (n - 1)
   653  }
   654  
   655  // callMethod is the call implementation used by a function returned
   656  // by makeMethodValue (used by v.Method(i).Interface()).
   657  // It is a streamlined version of the usual reflect call: the caller has
   658  // already laid out the argument frame for us, so we don't have
   659  // to deal with individual Values for each argument.
   660  // It is in this file so that it can be next to the two similar functions above.
   661  // The remainder of the makeMethodValue implementation is in makefunc.go.
   662  //
   663  // NOTE: This function must be marked as a "wrapper" in the generated code,
   664  // so that the linker can make it work correctly for panic and recover.
   665  // The gc compilers know to do that for the name "reflect.callMethod".
   666  //
   667  // ctxt is the "closure" generated by makeVethodValue.
   668  // frame is a pointer to the arguments to that closure on the stack.
   669  // retValid points to a boolean which should be set when the results
   670  // section of frame is set.
   671  func callMethod(ctxt *methodValue, frame unsafe.Pointer, retValid *bool) {
   672  	rcvr := ctxt.rcvr
   673  	rcvrtype, t, fn := methodReceiver("call", rcvr, ctxt.method)
   674  	frametype, argSize, retOffset, _, framePool := funcLayout(t, rcvrtype)
   675  
   676  	// Make a new frame that is one word bigger so we can store the receiver.
   677  	// This space is used for both arguments and return values.
   678  	scratch := framePool.Get().(unsafe.Pointer)
   679  
   680  	// Copy in receiver and rest of args.
   681  	// Avoid constructing out-of-bounds pointers if there are no args.
   682  	storeRcvr(rcvr, scratch)
   683  	if argSize-ptrSize > 0 {
   684  		typedmemmovepartial(frametype, add(scratch, ptrSize, "argSize > ptrSize"), frame, ptrSize, argSize-ptrSize)
   685  	}
   686  
   687  	// Call.
   688  	// Call copies the arguments from scratch to the stack, calls fn,
   689  	// and then copies the results back into scratch.
   690  	call(frametype, fn, scratch, uint32(frametype.size), uint32(retOffset))
   691  
   692  	// Copy return values. On amd64p32, the beginning of return values
   693  	// is 64-bit aligned, so the caller's frame layout (which doesn't have
   694  	// a receiver) is different from the layout of the fn call, which has
   695  	// a receiver.
   696  	// Ignore any changes to args and just copy return values.
   697  	// Avoid constructing out-of-bounds pointers if there are no return values.
   698  	if frametype.size-retOffset > 0 {
   699  		callerRetOffset := retOffset - ptrSize
   700  		if runtime.GOARCH == "amd64p32" {
   701  			callerRetOffset = align(argSize-ptrSize, 8)
   702  		}
   703  		// This copies to the stack. Write barriers are not needed.
   704  		memmove(add(frame, callerRetOffset, "frametype.size > retOffset"),
   705  			add(scratch, retOffset, "frametype.size > retOffset"),
   706  			frametype.size-retOffset)
   707  	}
   708  
   709  	// Tell the runtime it can now depend on the return values
   710  	// being properly initialized.
   711  	*retValid = true
   712  
   713  	// Clear the scratch space and put it back in the pool.
   714  	// This must happen after the statement above, so that the return
   715  	// values will always be scanned by someone.
   716  	typedmemclr(frametype, scratch)
   717  	framePool.Put(scratch)
   718  
   719  	// See the comment in callReflect.
   720  	runtime.KeepAlive(ctxt)
   721  }
   722  
   723  // funcName returns the name of f, for use in error messages.
   724  func funcName(f func([]Value) []Value) string {
   725  	pc := *(*uintptr)(unsafe.Pointer(&f))
   726  	rf := runtime.FuncForPC(pc)
   727  	if rf != nil {
   728  		return rf.Name()
   729  	}
   730  	return "closure"
   731  }
   732  
   733  // Cap returns v's capacity.
   734  // It panics if v's Kind is not Array, Chan, or Slice.
   735  func (v Value) Cap() int {
   736  	k := v.kind()
   737  	switch k {
   738  	case Array:
   739  		return v.typ.Len()
   740  	case Chan:
   741  		return chancap(v.pointer())
   742  	case Slice:
   743  		// Slice is always bigger than a word; assume flagIndir.
   744  		return (*sliceHeader)(v.ptr).Cap
   745  	}
   746  	panic(&ValueError{"reflect.Value.Cap", v.kind()})
   747  }
   748  
   749  // Close closes the channel v.
   750  // It panics if v's Kind is not Chan.
   751  func (v Value) Close() {
   752  	v.mustBe(Chan)
   753  	v.mustBeExported()
   754  	chanclose(v.pointer())
   755  }
   756  
   757  // Complex returns v's underlying value, as a complex128.
   758  // It panics if v's Kind is not Complex64 or Complex128
   759  func (v Value) Complex() complex128 {
   760  	k := v.kind()
   761  	switch k {
   762  	case Complex64:
   763  		return complex128(*(*complex64)(v.ptr))
   764  	case Complex128:
   765  		return *(*complex128)(v.ptr)
   766  	}
   767  	panic(&ValueError{"reflect.Value.Complex", v.kind()})
   768  }
   769  
   770  // Elem returns the value that the interface v contains
   771  // or that the pointer v points to.
   772  // It panics if v's Kind is not Interface or Ptr.
   773  // It returns the zero Value if v is nil.
   774  func (v Value) Elem() Value {
   775  	k := v.kind()
   776  	switch k {
   777  	case Interface:
   778  		var eface interface{}
   779  		if v.typ.NumMethod() == 0 {
   780  			eface = *(*interface{})(v.ptr)
   781  		} else {
   782  			eface = (interface{})(*(*interface {
   783  				M()
   784  			})(v.ptr))
   785  		}
   786  		x := unpackEface(eface)
   787  		if x.flag != 0 {
   788  			x.flag |= v.flag.ro()
   789  		}
   790  		return x
   791  	case Ptr:
   792  		ptr := v.ptr
   793  		if v.flag&flagIndir != 0 {
   794  			ptr = *(*unsafe.Pointer)(ptr)
   795  		}
   796  		// The returned value's address is v's value.
   797  		if ptr == nil {
   798  			return Value{}
   799  		}
   800  		tt := (*ptrType)(unsafe.Pointer(v.typ))
   801  		typ := tt.elem
   802  		fl := v.flag&flagRO | flagIndir | flagAddr
   803  		fl |= flag(typ.Kind())
   804  		return Value{typ, ptr, fl}
   805  	}
   806  	panic(&ValueError{"reflect.Value.Elem", v.kind()})
   807  }
   808  
   809  // Field returns the i'th field of the struct v.
   810  // It panics if v's Kind is not Struct or i is out of range.
   811  func (v Value) Field(i int) Value {
   812  	if v.kind() != Struct {
   813  		panic(&ValueError{"reflect.Value.Field", v.kind()})
   814  	}
   815  	tt := (*structType)(unsafe.Pointer(v.typ))
   816  	if uint(i) >= uint(len(tt.fields)) {
   817  		panic("reflect: Field index out of range")
   818  	}
   819  	field := &tt.fields[i]
   820  	typ := field.typ
   821  
   822  	// Inherit permission bits from v, but clear flagEmbedRO.
   823  	fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
   824  	// Using an unexported field forces flagRO.
   825  	if !field.name.isExported() {
   826  		if field.embedded() {
   827  			fl |= flagEmbedRO
   828  		} else {
   829  			fl |= flagStickyRO
   830  		}
   831  	}
   832  	// Either flagIndir is set and v.ptr points at struct,
   833  	// or flagIndir is not set and v.ptr is the actual struct data.
   834  	// In the former case, we want v.ptr + offset.
   835  	// In the latter case, we must have field.offset = 0,
   836  	// so v.ptr + field.offset is still the correct address.
   837  	ptr := add(v.ptr, field.offset(), "same as non-reflect &v.field")
   838  	return Value{typ, ptr, fl}
   839  }
   840  
   841  // FieldByIndex returns the nested field corresponding to index.
   842  // It panics if v's Kind is not struct.
   843  func (v Value) FieldByIndex(index []int) Value {
   844  	if len(index) == 1 {
   845  		return v.Field(index[0])
   846  	}
   847  	v.mustBe(Struct)
   848  	for i, x := range index {
   849  		if i > 0 {
   850  			if v.Kind() == Ptr && v.typ.Elem().Kind() == Struct {
   851  				if v.IsNil() {
   852  					panic("reflect: indirection through nil pointer to embedded struct")
   853  				}
   854  				v = v.Elem()
   855  			}
   856  		}
   857  		v = v.Field(x)
   858  	}
   859  	return v
   860  }
   861  
   862  // FieldByName returns the struct field with the given name.
   863  // It returns the zero Value if no field was found.
   864  // It panics if v's Kind is not struct.
   865  func (v Value) FieldByName(name string) Value {
   866  	v.mustBe(Struct)
   867  	if f, ok := v.typ.FieldByName(name); ok {
   868  		return v.FieldByIndex(f.Index)
   869  	}
   870  	return Value{}
   871  }
   872  
   873  // FieldByNameFunc returns the struct field with a name
   874  // that satisfies the match function.
   875  // It panics if v's Kind is not struct.
   876  // It returns the zero Value if no field was found.
   877  func (v Value) FieldByNameFunc(match func(string) bool) Value {
   878  	if f, ok := v.typ.FieldByNameFunc(match); ok {
   879  		return v.FieldByIndex(f.Index)
   880  	}
   881  	return Value{}
   882  }
   883  
   884  // Float returns v's underlying value, as a float64.
   885  // It panics if v's Kind is not Float32 or Float64
   886  func (v Value) Float() float64 {
   887  	k := v.kind()
   888  	switch k {
   889  	case Float32:
   890  		return float64(*(*float32)(v.ptr))
   891  	case Float64:
   892  		return *(*float64)(v.ptr)
   893  	}
   894  	panic(&ValueError{"reflect.Value.Float", v.kind()})
   895  }
   896  
   897  var uint8Type = TypeOf(uint8(0)).(*rtype)
   898  
   899  // Index returns v's i'th element.
   900  // It panics if v's Kind is not Array, Slice, or String or i is out of range.
   901  func (v Value) Index(i int) Value {
   902  	switch v.kind() {
   903  	case Array:
   904  		tt := (*arrayType)(unsafe.Pointer(v.typ))
   905  		if uint(i) >= uint(tt.len) {
   906  			panic("reflect: array index out of range")
   907  		}
   908  		typ := tt.elem
   909  		offset := uintptr(i) * typ.size
   910  
   911  		// Either flagIndir is set and v.ptr points at array,
   912  		// or flagIndir is not set and v.ptr is the actual array data.
   913  		// In the former case, we want v.ptr + offset.
   914  		// In the latter case, we must be doing Index(0), so offset = 0,
   915  		// so v.ptr + offset is still the correct address.
   916  		val := add(v.ptr, offset, "same as &v[i], i < tt.len")
   917  		fl := v.flag&(flagIndir|flagAddr) | v.flag.ro() | flag(typ.Kind()) // bits same as overall array
   918  		return Value{typ, val, fl}
   919  
   920  	case Slice:
   921  		// Element flag same as Elem of Ptr.
   922  		// Addressable, indirect, possibly read-only.
   923  		s := (*sliceHeader)(v.ptr)
   924  		if uint(i) >= uint(s.Len) {
   925  			panic("reflect: slice index out of range")
   926  		}
   927  		tt := (*sliceType)(unsafe.Pointer(v.typ))
   928  		typ := tt.elem
   929  		val := arrayAt(s.Data, i, typ.size, "i < s.Len")
   930  		fl := flagAddr | flagIndir | v.flag.ro() | flag(typ.Kind())
   931  		return Value{typ, val, fl}
   932  
   933  	case String:
   934  		s := (*stringHeader)(v.ptr)
   935  		if uint(i) >= uint(s.Len) {
   936  			panic("reflect: string index out of range")
   937  		}
   938  		p := arrayAt(s.Data, i, 1, "i < s.Len")
   939  		fl := v.flag.ro() | flag(Uint8) | flagIndir
   940  		return Value{uint8Type, p, fl}
   941  	}
   942  	panic(&ValueError{"reflect.Value.Index", v.kind()})
   943  }
   944  
   945  // Int returns v's underlying value, as an int64.
   946  // It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
   947  func (v Value) Int() int64 {
   948  	k := v.kind()
   949  	p := v.ptr
   950  	switch k {
   951  	case Int:
   952  		return int64(*(*int)(p))
   953  	case Int8:
   954  		return int64(*(*int8)(p))
   955  	case Int16:
   956  		return int64(*(*int16)(p))
   957  	case Int32:
   958  		return int64(*(*int32)(p))
   959  	case Int64:
   960  		return *(*int64)(p)
   961  	}
   962  	panic(&ValueError{"reflect.Value.Int", v.kind()})
   963  }
   964  
   965  // CanInterface reports whether Interface can be used without panicking.
   966  func (v Value) CanInterface() bool {
   967  	if v.flag == 0 {
   968  		panic(&ValueError{"reflect.Value.CanInterface", Invalid})
   969  	}
   970  	return v.flag&flagRO == 0
   971  }
   972  
   973  // Interface returns v's current value as an interface{}.
   974  // It is equivalent to:
   975  //	var i interface{} = (v's underlying value)
   976  // It panics if the Value was obtained by accessing
   977  // unexported struct fields.
   978  func (v Value) Interface() (i interface{}) {
   979  	return valueInterface(v, true)
   980  }
   981  
   982  func valueInterface(v Value, safe bool) interface{} {
   983  	if v.flag == 0 {
   984  		panic(&ValueError{"reflect.Value.Interface", 0})
   985  	}
   986  	if safe && v.flag&flagRO != 0 {
   987  		// Do not allow access to unexported values via Interface,
   988  		// because they might be pointers that should not be
   989  		// writable or methods or function that should not be callable.
   990  		panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
   991  	}
   992  	if v.flag&flagMethod != 0 {
   993  		v = makeMethodValue("Interface", v)
   994  	}
   995  
   996  	if v.kind() == Interface {
   997  		// Special case: return the element inside the interface.
   998  		// Empty interface has one layout, all interfaces with
   999  		// methods have a second layout.
  1000  		if v.NumMethod() == 0 {
  1001  			return *(*interface{})(v.ptr)
  1002  		}
  1003  		return *(*interface {
  1004  			M()
  1005  		})(v.ptr)
  1006  	}
  1007  
  1008  	// TODO: pass safe to packEface so we don't need to copy if safe==true?
  1009  	return packEface(v)
  1010  }
  1011  
  1012  // InterfaceData returns the interface v's value as a uintptr pair.
  1013  // It panics if v's Kind is not Interface.
  1014  func (v Value) InterfaceData() [2]uintptr {
  1015  	// TODO: deprecate this
  1016  	v.mustBe(Interface)
  1017  	// We treat this as a read operation, so we allow
  1018  	// it even for unexported data, because the caller
  1019  	// has to import "unsafe" to turn it into something
  1020  	// that can be abused.
  1021  	// Interface value is always bigger than a word; assume flagIndir.
  1022  	return *(*[2]uintptr)(v.ptr)
  1023  }
  1024  
  1025  // IsNil reports whether its argument v is nil. The argument must be
  1026  // a chan, func, interface, map, pointer, or slice value; if it is
  1027  // not, IsNil panics. Note that IsNil is not always equivalent to a
  1028  // regular comparison with nil in Go. For example, if v was created
  1029  // by calling ValueOf with an uninitialized interface variable i,
  1030  // i==nil will be true but v.IsNil will panic as v will be the zero
  1031  // Value.
  1032  func (v Value) IsNil() bool {
  1033  	k := v.kind()
  1034  	switch k {
  1035  	case Chan, Func, Map, Ptr, UnsafePointer:
  1036  		if v.flag&flagMethod != 0 {
  1037  			return false
  1038  		}
  1039  		ptr := v.ptr
  1040  		if v.flag&flagIndir != 0 {
  1041  			ptr = *(*unsafe.Pointer)(ptr)
  1042  		}
  1043  		return ptr == nil
  1044  	case Interface, Slice:
  1045  		// Both interface and slice are nil if first word is 0.
  1046  		// Both are always bigger than a word; assume flagIndir.
  1047  		return *(*unsafe.Pointer)(v.ptr) == nil
  1048  	}
  1049  	panic(&ValueError{"reflect.Value.IsNil", v.kind()})
  1050  }
  1051  
  1052  // IsValid reports whether v represents a value.
  1053  // It returns false if v is the zero Value.
  1054  // If IsValid returns false, all other methods except String panic.
  1055  // Most functions and methods never return an invalid value.
  1056  // If one does, its documentation states the conditions explicitly.
  1057  func (v Value) IsValid() bool {
  1058  	return v.flag != 0
  1059  }
  1060  
  1061  // Kind returns v's Kind.
  1062  // If v is the zero Value (IsValid returns false), Kind returns Invalid.
  1063  func (v Value) Kind() Kind {
  1064  	return v.kind()
  1065  }
  1066  
  1067  // Len returns v's length.
  1068  // It panics if v's Kind is not Array, Chan, Map, Slice, or String.
  1069  func (v Value) Len() int {
  1070  	k := v.kind()
  1071  	switch k {
  1072  	case Array:
  1073  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  1074  		return int(tt.len)
  1075  	case Chan:
  1076  		return chanlen(v.pointer())
  1077  	case Map:
  1078  		return maplen(v.pointer())
  1079  	case Slice:
  1080  		// Slice is bigger than a word; assume flagIndir.
  1081  		return (*sliceHeader)(v.ptr).Len
  1082  	case String:
  1083  		// String is bigger than a word; assume flagIndir.
  1084  		return (*stringHeader)(v.ptr).Len
  1085  	}
  1086  	panic(&ValueError{"reflect.Value.Len", v.kind()})
  1087  }
  1088  
  1089  // MapIndex returns the value associated with key in the map v.
  1090  // It panics if v's Kind is not Map.
  1091  // It returns the zero Value if key is not found in the map or if v represents a nil map.
  1092  // As in Go, the key's value must be assignable to the map's key type.
  1093  func (v Value) MapIndex(key Value) Value {
  1094  	v.mustBe(Map)
  1095  	tt := (*mapType)(unsafe.Pointer(v.typ))
  1096  
  1097  	// Do not require key to be exported, so that DeepEqual
  1098  	// and other programs can use all the keys returned by
  1099  	// MapKeys as arguments to MapIndex. If either the map
  1100  	// or the key is unexported, though, the result will be
  1101  	// considered unexported. This is consistent with the
  1102  	// behavior for structs, which allow read but not write
  1103  	// of unexported fields.
  1104  	key = key.assignTo("reflect.Value.MapIndex", tt.key, nil)
  1105  
  1106  	var k unsafe.Pointer
  1107  	if key.flag&flagIndir != 0 {
  1108  		k = key.ptr
  1109  	} else {
  1110  		k = unsafe.Pointer(&key.ptr)
  1111  	}
  1112  	e := mapaccess(v.typ, v.pointer(), k)
  1113  	if e == nil {
  1114  		return Value{}
  1115  	}
  1116  	typ := tt.elem
  1117  	fl := (v.flag | key.flag).ro()
  1118  	fl |= flag(typ.Kind())
  1119  	return copyVal(typ, fl, e)
  1120  }
  1121  
  1122  // MapKeys returns a slice containing all the keys present in the map,
  1123  // in unspecified order.
  1124  // It panics if v's Kind is not Map.
  1125  // It returns an empty slice if v represents a nil map.
  1126  func (v Value) MapKeys() []Value {
  1127  	v.mustBe(Map)
  1128  	tt := (*mapType)(unsafe.Pointer(v.typ))
  1129  	keyType := tt.key
  1130  
  1131  	fl := v.flag.ro() | flag(keyType.Kind())
  1132  
  1133  	m := v.pointer()
  1134  	mlen := int(0)
  1135  	if m != nil {
  1136  		mlen = maplen(m)
  1137  	}
  1138  	it := mapiterinit(v.typ, m)
  1139  	a := make([]Value, mlen)
  1140  	var i int
  1141  	for i = 0; i < len(a); i++ {
  1142  		key := mapiterkey(it)
  1143  		if key == nil {
  1144  			// Someone deleted an entry from the map since we
  1145  			// called maplen above. It's a data race, but nothing
  1146  			// we can do about it.
  1147  			break
  1148  		}
  1149  		a[i] = copyVal(keyType, fl, key)
  1150  		mapiternext(it)
  1151  	}
  1152  	return a[:i]
  1153  }
  1154  
  1155  // A MapIter is an iterator for ranging over a map.
  1156  // See Value.MapRange.
  1157  type MapIter struct {
  1158  	m  Value
  1159  	it unsafe.Pointer
  1160  }
  1161  
  1162  // Key returns the key of the iterator's current map entry.
  1163  func (it *MapIter) Key() Value {
  1164  	if it.it == nil {
  1165  		panic("MapIter.Key called before Next")
  1166  	}
  1167  	if mapiterkey(it.it) == nil {
  1168  		panic("MapIter.Key called on exhausted iterator")
  1169  	}
  1170  
  1171  	t := (*mapType)(unsafe.Pointer(it.m.typ))
  1172  	ktype := t.key
  1173  	return copyVal(ktype, it.m.flag.ro()|flag(ktype.Kind()), mapiterkey(it.it))
  1174  }
  1175  
  1176  // Value returns the value of the iterator's current map entry.
  1177  func (it *MapIter) Value() Value {
  1178  	if it.it == nil {
  1179  		panic("MapIter.Value called before Next")
  1180  	}
  1181  	if mapiterkey(it.it) == nil {
  1182  		panic("MapIter.Value called on exhausted iterator")
  1183  	}
  1184  
  1185  	t := (*mapType)(unsafe.Pointer(it.m.typ))
  1186  	vtype := t.elem
  1187  	return copyVal(vtype, it.m.flag.ro()|flag(vtype.Kind()), mapitervalue(it.it))
  1188  }
  1189  
  1190  // Next advances the map iterator and reports whether there is another
  1191  // entry. It returns false when the iterator is exhausted; subsequent
  1192  // calls to Key, Value, or Next will panic.
  1193  func (it *MapIter) Next() bool {
  1194  	if it.it == nil {
  1195  		it.it = mapiterinit(it.m.typ, it.m.pointer())
  1196  	} else {
  1197  		if mapiterkey(it.it) == nil {
  1198  			panic("MapIter.Next called on exhausted iterator")
  1199  		}
  1200  		mapiternext(it.it)
  1201  	}
  1202  	return mapiterkey(it.it) != nil
  1203  }
  1204  
  1205  // MapRange returns a range iterator for a map.
  1206  // It panics if v's Kind is not Map.
  1207  //
  1208  // Call Next to advance the iterator, and Key/Value to access each entry.
  1209  // Next returns false when the iterator is exhausted.
  1210  // MapRange follows the same iteration semantics as a range statement.
  1211  //
  1212  // Example:
  1213  //
  1214  //	iter := reflect.ValueOf(m).MapRange()
  1215  // 	for iter.Next() {
  1216  //		k := iter.Key()
  1217  //		v := iter.Value()
  1218  //		...
  1219  //	}
  1220  //
  1221  func (v Value) MapRange() *MapIter {
  1222  	v.mustBe(Map)
  1223  	return &MapIter{m: v}
  1224  }
  1225  
  1226  // copyVal returns a Value containing the map key or value at ptr,
  1227  // allocating a new variable as needed.
  1228  func copyVal(typ *rtype, fl flag, ptr unsafe.Pointer) Value {
  1229  	if ifaceIndir(typ) {
  1230  		// Copy result so future changes to the map
  1231  		// won't change the underlying value.
  1232  		c := unsafe_New(typ)
  1233  		typedmemmove(typ, c, ptr)
  1234  		return Value{typ, c, fl | flagIndir}
  1235  	}
  1236  	return Value{typ, *(*unsafe.Pointer)(ptr), fl}
  1237  }
  1238  
  1239  // Method returns a function value corresponding to v's i'th method.
  1240  // The arguments to a Call on the returned function should not include
  1241  // a receiver; the returned function will always use v as the receiver.
  1242  // Method panics if i is out of range or if v is a nil interface value.
  1243  func (v Value) Method(i int) Value {
  1244  	if v.typ == nil {
  1245  		panic(&ValueError{"reflect.Value.Method", Invalid})
  1246  	}
  1247  	if v.flag&flagMethod != 0 || uint(i) >= uint(v.typ.NumMethod()) {
  1248  		panic("reflect: Method index out of range")
  1249  	}
  1250  	if v.typ.Kind() == Interface && v.IsNil() {
  1251  		panic("reflect: Method on nil interface value")
  1252  	}
  1253  	fl := v.flag & (flagStickyRO | flagIndir) // Clear flagEmbedRO
  1254  	fl |= flag(Func)
  1255  	fl |= flag(i)<<flagMethodShift | flagMethod
  1256  	return Value{v.typ, v.ptr, fl}
  1257  }
  1258  
  1259  // NumMethod returns the number of exported methods in the value's method set.
  1260  func (v Value) NumMethod() int {
  1261  	if v.typ == nil {
  1262  		panic(&ValueError{"reflect.Value.NumMethod", Invalid})
  1263  	}
  1264  	if v.flag&flagMethod != 0 {
  1265  		return 0
  1266  	}
  1267  	return v.typ.NumMethod()
  1268  }
  1269  
  1270  // MethodByName returns a function value corresponding to the method
  1271  // of v with the given name.
  1272  // The arguments to a Call on the returned function should not include
  1273  // a receiver; the returned function will always use v as the receiver.
  1274  // It returns the zero Value if no method was found.
  1275  func (v Value) MethodByName(name string) Value {
  1276  	if v.typ == nil {
  1277  		panic(&ValueError{"reflect.Value.MethodByName", Invalid})
  1278  	}
  1279  	if v.flag&flagMethod != 0 {
  1280  		return Value{}
  1281  	}
  1282  	m, ok := v.typ.MethodByName(name)
  1283  	if !ok {
  1284  		return Value{}
  1285  	}
  1286  	return v.Method(m.Index)
  1287  }
  1288  
  1289  // NumField returns the number of fields in the struct v.
  1290  // It panics if v's Kind is not Struct.
  1291  func (v Value) NumField() int {
  1292  	v.mustBe(Struct)
  1293  	tt := (*structType)(unsafe.Pointer(v.typ))
  1294  	return len(tt.fields)
  1295  }
  1296  
  1297  // OverflowComplex reports whether the complex128 x cannot be represented by v's type.
  1298  // It panics if v's Kind is not Complex64 or Complex128.
  1299  func (v Value) OverflowComplex(x complex128) bool {
  1300  	k := v.kind()
  1301  	switch k {
  1302  	case Complex64:
  1303  		return overflowFloat32(real(x)) || overflowFloat32(imag(x))
  1304  	case Complex128:
  1305  		return false
  1306  	}
  1307  	panic(&ValueError{"reflect.Value.OverflowComplex", v.kind()})
  1308  }
  1309  
  1310  // OverflowFloat reports whether the float64 x cannot be represented by v's type.
  1311  // It panics if v's Kind is not Float32 or Float64.
  1312  func (v Value) OverflowFloat(x float64) bool {
  1313  	k := v.kind()
  1314  	switch k {
  1315  	case Float32:
  1316  		return overflowFloat32(x)
  1317  	case Float64:
  1318  		return false
  1319  	}
  1320  	panic(&ValueError{"reflect.Value.OverflowFloat", v.kind()})
  1321  }
  1322  
  1323  func overflowFloat32(x float64) bool {
  1324  	if x < 0 {
  1325  		x = -x
  1326  	}
  1327  	return math.MaxFloat32 < x && x <= math.MaxFloat64
  1328  }
  1329  
  1330  // OverflowInt reports whether the int64 x cannot be represented by v's type.
  1331  // It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
  1332  func (v Value) OverflowInt(x int64) bool {
  1333  	k := v.kind()
  1334  	switch k {
  1335  	case Int, Int8, Int16, Int32, Int64:
  1336  		bitSize := v.typ.size * 8
  1337  		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
  1338  		return x != trunc
  1339  	}
  1340  	panic(&ValueError{"reflect.Value.OverflowInt", v.kind()})
  1341  }
  1342  
  1343  // OverflowUint reports whether the uint64 x cannot be represented by v's type.
  1344  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
  1345  func (v Value) OverflowUint(x uint64) bool {
  1346  	k := v.kind()
  1347  	switch k {
  1348  	case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
  1349  		bitSize := v.typ.size * 8
  1350  		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
  1351  		return x != trunc
  1352  	}
  1353  	panic(&ValueError{"reflect.Value.OverflowUint", v.kind()})
  1354  }
  1355  
  1356  // Pointer returns v's value as a uintptr.
  1357  // It returns uintptr instead of unsafe.Pointer so that
  1358  // code using reflect cannot obtain unsafe.Pointers
  1359  // without importing the unsafe package explicitly.
  1360  // It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
  1361  //
  1362  // If v's Kind is Func, the returned pointer is an underlying
  1363  // code pointer, but not necessarily enough to identify a
  1364  // single function uniquely. The only guarantee is that the
  1365  // result is zero if and only if v is a nil func Value.
  1366  //
  1367  // If v's Kind is Slice, the returned pointer is to the first
  1368  // element of the slice. If the slice is nil the returned value
  1369  // is 0.  If the slice is empty but non-nil the return value is non-zero.
  1370  func (v Value) Pointer() uintptr {
  1371  	// TODO: deprecate
  1372  	k := v.kind()
  1373  	switch k {
  1374  	case Chan, Map, Ptr, UnsafePointer:
  1375  		return uintptr(v.pointer())
  1376  	case Func:
  1377  		if v.flag&flagMethod != 0 {
  1378  			// As the doc comment says, the returned pointer is an
  1379  			// underlying code pointer but not necessarily enough to
  1380  			// identify a single function uniquely. All method expressions
  1381  			// created via reflect have the same underlying code pointer,
  1382  			// so their Pointers are equal. The function used here must
  1383  			// match the one used in makeMethodValue.
  1384  			f := methodValueCall
  1385  			return **(**uintptr)(unsafe.Pointer(&f))
  1386  		}
  1387  		p := v.pointer()
  1388  		// Non-nil func value points at data block.
  1389  		// First word of data block is actual code.
  1390  		if p != nil {
  1391  			p = *(*unsafe.Pointer)(p)
  1392  		}
  1393  		return uintptr(p)
  1394  
  1395  	case Slice:
  1396  		return (*SliceHeader)(v.ptr).Data
  1397  	}
  1398  	panic(&ValueError{"reflect.Value.Pointer", v.kind()})
  1399  }
  1400  
  1401  // Recv receives and returns a value from the channel v.
  1402  // It panics if v's Kind is not Chan.
  1403  // The receive blocks until a value is ready.
  1404  // The boolean value ok is true if the value x corresponds to a send
  1405  // on the channel, false if it is a zero value received because the channel is closed.
  1406  func (v Value) Recv() (x Value, ok bool) {
  1407  	v.mustBe(Chan)
  1408  	v.mustBeExported()
  1409  	return v.recv(false)
  1410  }
  1411  
  1412  // internal recv, possibly non-blocking (nb).
  1413  // v is known to be a channel.
  1414  func (v Value) recv(nb bool) (val Value, ok bool) {
  1415  	tt := (*chanType)(unsafe.Pointer(v.typ))
  1416  	if ChanDir(tt.dir)&RecvDir == 0 {
  1417  		panic("reflect: recv on send-only channel")
  1418  	}
  1419  	t := tt.elem
  1420  	val = Value{t, nil, flag(t.Kind())}
  1421  	var p unsafe.Pointer
  1422  	if ifaceIndir(t) {
  1423  		p = unsafe_New(t)
  1424  		val.ptr = p
  1425  		val.flag |= flagIndir
  1426  	} else {
  1427  		p = unsafe.Pointer(&val.ptr)
  1428  	}
  1429  	selected, ok := chanrecv(v.pointer(), nb, p)
  1430  	if !selected {
  1431  		val = Value{}
  1432  	}
  1433  	return
  1434  }
  1435  
  1436  // Send sends x on the channel v.
  1437  // It panics if v's kind is not Chan or if x's type is not the same type as v's element type.
  1438  // As in Go, x's value must be assignable to the channel's element type.
  1439  func (v Value) Send(x Value) {
  1440  	v.mustBe(Chan)
  1441  	v.mustBeExported()
  1442  	v.send(x, false)
  1443  }
  1444  
  1445  // internal send, possibly non-blocking.
  1446  // v is known to be a channel.
  1447  func (v Value) send(x Value, nb bool) (selected bool) {
  1448  	tt := (*chanType)(unsafe.Pointer(v.typ))
  1449  	if ChanDir(tt.dir)&SendDir == 0 {
  1450  		panic("reflect: send on recv-only channel")
  1451  	}
  1452  	x.mustBeExported()
  1453  	x = x.assignTo("reflect.Value.Send", tt.elem, nil)
  1454  	var p unsafe.Pointer
  1455  	if x.flag&flagIndir != 0 {
  1456  		p = x.ptr
  1457  	} else {
  1458  		p = unsafe.Pointer(&x.ptr)
  1459  	}
  1460  	return chansend(v.pointer(), p, nb)
  1461  }
  1462  
  1463  // Set assigns x to the value v.
  1464  // It panics if CanSet returns false.
  1465  // As in Go, x's value must be assignable to v's type.
  1466  func (v Value) Set(x Value) {
  1467  	v.mustBeAssignable()
  1468  	x.mustBeExported() // do not let unexported x leak
  1469  	var target unsafe.Pointer
  1470  	if v.kind() == Interface {
  1471  		target = v.ptr
  1472  	}
  1473  	x = x.assignTo("reflect.Set", v.typ, target)
  1474  	if x.flag&flagIndir != 0 {
  1475  		typedmemmove(v.typ, v.ptr, x.ptr)
  1476  	} else {
  1477  		*(*unsafe.Pointer)(v.ptr) = x.ptr
  1478  	}
  1479  }
  1480  
  1481  // SetBool sets v's underlying value.
  1482  // It panics if v's Kind is not Bool or if CanSet() is false.
  1483  func (v Value) SetBool(x bool) {
  1484  	v.mustBeAssignable()
  1485  	v.mustBe(Bool)
  1486  	*(*bool)(v.ptr) = x
  1487  }
  1488  
  1489  // SetBytes sets v's underlying value.
  1490  // It panics if v's underlying value is not a slice of bytes.
  1491  func (v Value) SetBytes(x []byte) {
  1492  	v.mustBeAssignable()
  1493  	v.mustBe(Slice)
  1494  	if v.typ.Elem().Kind() != Uint8 {
  1495  		panic("reflect.Value.SetBytes of non-byte slice")
  1496  	}
  1497  	*(*[]byte)(v.ptr) = x
  1498  }
  1499  
  1500  // setRunes sets v's underlying value.
  1501  // It panics if v's underlying value is not a slice of runes (int32s).
  1502  func (v Value) setRunes(x []rune) {
  1503  	v.mustBeAssignable()
  1504  	v.mustBe(Slice)
  1505  	if v.typ.Elem().Kind() != Int32 {
  1506  		panic("reflect.Value.setRunes of non-rune slice")
  1507  	}
  1508  	*(*[]rune)(v.ptr) = x
  1509  }
  1510  
  1511  // SetComplex sets v's underlying value to x.
  1512  // It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
  1513  func (v Value) SetComplex(x complex128) {
  1514  	v.mustBeAssignable()
  1515  	switch k := v.kind(); k {
  1516  	default:
  1517  		panic(&ValueError{"reflect.Value.SetComplex", v.kind()})
  1518  	case Complex64:
  1519  		*(*complex64)(v.ptr) = complex64(x)
  1520  	case Complex128:
  1521  		*(*complex128)(v.ptr) = x
  1522  	}
  1523  }
  1524  
  1525  // SetFloat sets v's underlying value to x.
  1526  // It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
  1527  func (v Value) SetFloat(x float64) {
  1528  	v.mustBeAssignable()
  1529  	switch k := v.kind(); k {
  1530  	default:
  1531  		panic(&ValueError{"reflect.Value.SetFloat", v.kind()})
  1532  	case Float32:
  1533  		*(*float32)(v.ptr) = float32(x)
  1534  	case Float64:
  1535  		*(*float64)(v.ptr) = x
  1536  	}
  1537  }
  1538  
  1539  // SetInt sets v's underlying value to x.
  1540  // It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
  1541  func (v Value) SetInt(x int64) {
  1542  	v.mustBeAssignable()
  1543  	switch k := v.kind(); k {
  1544  	default:
  1545  		panic(&ValueError{"reflect.Value.SetInt", v.kind()})
  1546  	case Int:
  1547  		*(*int)(v.ptr) = int(x)
  1548  	case Int8:
  1549  		*(*int8)(v.ptr) = int8(x)
  1550  	case Int16:
  1551  		*(*int16)(v.ptr) = int16(x)
  1552  	case Int32:
  1553  		*(*int32)(v.ptr) = int32(x)
  1554  	case Int64:
  1555  		*(*int64)(v.ptr) = x
  1556  	}
  1557  }
  1558  
  1559  // SetLen sets v's length to n.
  1560  // It panics if v's Kind is not Slice or if n is negative or
  1561  // greater than the capacity of the slice.
  1562  func (v Value) SetLen(n int) {
  1563  	v.mustBeAssignable()
  1564  	v.mustBe(Slice)
  1565  	s := (*sliceHeader)(v.ptr)
  1566  	if uint(n) > uint(s.Cap) {
  1567  		panic("reflect: slice length out of range in SetLen")
  1568  	}
  1569  	s.Len = n
  1570  }
  1571  
  1572  // SetCap sets v's capacity to n.
  1573  // It panics if v's Kind is not Slice or if n is smaller than the length or
  1574  // greater than the capacity of the slice.
  1575  func (v Value) SetCap(n int) {
  1576  	v.mustBeAssignable()
  1577  	v.mustBe(Slice)
  1578  	s := (*sliceHeader)(v.ptr)
  1579  	if n < s.Len || n > s.Cap {
  1580  		panic("reflect: slice capacity out of range in SetCap")
  1581  	}
  1582  	s.Cap = n
  1583  }
  1584  
  1585  // SetMapIndex sets the value associated with key in the map v to val.
  1586  // It panics if v's Kind is not Map.
  1587  // If val is the zero Value, SetMapIndex deletes the key from the map.
  1588  // Otherwise if v holds a nil map, SetMapIndex will panic.
  1589  // As in Go, key's value must be assignable to the map's key type,
  1590  // and val's value must be assignable to the map's value type.
  1591  func (v Value) SetMapIndex(key, val Value) {
  1592  	v.mustBe(Map)
  1593  	v.mustBeExported()
  1594  	key.mustBeExported()
  1595  	tt := (*mapType)(unsafe.Pointer(v.typ))
  1596  	key = key.assignTo("reflect.Value.SetMapIndex", tt.key, nil)
  1597  	var k unsafe.Pointer
  1598  	if key.flag&flagIndir != 0 {
  1599  		k = key.ptr
  1600  	} else {
  1601  		k = unsafe.Pointer(&key.ptr)
  1602  	}
  1603  	if val.typ == nil {
  1604  		mapdelete(v.typ, v.pointer(), k)
  1605  		return
  1606  	}
  1607  	val.mustBeExported()
  1608  	val = val.assignTo("reflect.Value.SetMapIndex", tt.elem, nil)
  1609  	var e unsafe.Pointer
  1610  	if val.flag&flagIndir != 0 {
  1611  		e = val.ptr
  1612  	} else {
  1613  		e = unsafe.Pointer(&val.ptr)
  1614  	}
  1615  	mapassign(v.typ, v.pointer(), k, e)
  1616  }
  1617  
  1618  // SetUint sets v's underlying value to x.
  1619  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
  1620  func (v Value) SetUint(x uint64) {
  1621  	v.mustBeAssignable()
  1622  	switch k := v.kind(); k {
  1623  	default:
  1624  		panic(&ValueError{"reflect.Value.SetUint", v.kind()})
  1625  	case Uint:
  1626  		*(*uint)(v.ptr) = uint(x)
  1627  	case Uint8:
  1628  		*(*uint8)(v.ptr) = uint8(x)
  1629  	case Uint16:
  1630  		*(*uint16)(v.ptr) = uint16(x)
  1631  	case Uint32:
  1632  		*(*uint32)(v.ptr) = uint32(x)
  1633  	case Uint64:
  1634  		*(*uint64)(v.ptr) = x
  1635  	case Uintptr:
  1636  		*(*uintptr)(v.ptr) = uintptr(x)
  1637  	}
  1638  }
  1639  
  1640  // SetPointer sets the unsafe.Pointer value v to x.
  1641  // It panics if v's Kind is not UnsafePointer.
  1642  func (v Value) SetPointer(x unsafe.Pointer) {
  1643  	v.mustBeAssignable()
  1644  	v.mustBe(UnsafePointer)
  1645  	*(*unsafe.Pointer)(v.ptr) = x
  1646  }
  1647  
  1648  // SetString sets v's underlying value to x.
  1649  // It panics if v's Kind is not String or if CanSet() is false.
  1650  func (v Value) SetString(x string) {
  1651  	v.mustBeAssignable()
  1652  	v.mustBe(String)
  1653  	*(*string)(v.ptr) = x
  1654  }
  1655  
  1656  // Slice returns v[i:j].
  1657  // It panics if v's Kind is not Array, Slice or String, or if v is an unaddressable array,
  1658  // or if the indexes are out of bounds.
  1659  func (v Value) Slice(i, j int) Value {
  1660  	var (
  1661  		cap  int
  1662  		typ  *sliceType
  1663  		base unsafe.Pointer
  1664  	)
  1665  	switch kind := v.kind(); kind {
  1666  	default:
  1667  		panic(&ValueError{"reflect.Value.Slice", v.kind()})
  1668  
  1669  	case Array:
  1670  		if v.flag&flagAddr == 0 {
  1671  			panic("reflect.Value.Slice: slice of unaddressable array")
  1672  		}
  1673  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  1674  		cap = int(tt.len)
  1675  		typ = (*sliceType)(unsafe.Pointer(tt.slice))
  1676  		base = v.ptr
  1677  
  1678  	case Slice:
  1679  		typ = (*sliceType)(unsafe.Pointer(v.typ))
  1680  		s := (*sliceHeader)(v.ptr)
  1681  		base = s.Data
  1682  		cap = s.Cap
  1683  
  1684  	case String:
  1685  		s := (*stringHeader)(v.ptr)
  1686  		if i < 0 || j < i || j > s.Len {
  1687  			panic("reflect.Value.Slice: string slice index out of bounds")
  1688  		}
  1689  		var t stringHeader
  1690  		if i < s.Len {
  1691  			t = stringHeader{arrayAt(s.Data, i, 1, "i < s.Len"), j - i}
  1692  		}
  1693  		return Value{v.typ, unsafe.Pointer(&t), v.flag}
  1694  	}
  1695  
  1696  	if i < 0 || j < i || j > cap {
  1697  		panic("reflect.Value.Slice: slice index out of bounds")
  1698  	}
  1699  
  1700  	// Declare slice so that gc can see the base pointer in it.
  1701  	var x []unsafe.Pointer
  1702  
  1703  	// Reinterpret as *sliceHeader to edit.
  1704  	s := (*sliceHeader)(unsafe.Pointer(&x))
  1705  	s.Len = j - i
  1706  	s.Cap = cap - i
  1707  	if cap-i > 0 {
  1708  		s.Data = arrayAt(base, i, typ.elem.Size(), "i < cap")
  1709  	} else {
  1710  		// do not advance pointer, to avoid pointing beyond end of slice
  1711  		s.Data = base
  1712  	}
  1713  
  1714  	fl := v.flag.ro() | flagIndir | flag(Slice)
  1715  	return Value{typ.common(), unsafe.Pointer(&x), fl}
  1716  }
  1717  
  1718  // Slice3 is the 3-index form of the slice operation: it returns v[i:j:k].
  1719  // It panics if v's Kind is not Array or Slice, or if v is an unaddressable array,
  1720  // or if the indexes are out of bounds.
  1721  func (v Value) Slice3(i, j, k int) Value {
  1722  	var (
  1723  		cap  int
  1724  		typ  *sliceType
  1725  		base unsafe.Pointer
  1726  	)
  1727  	switch kind := v.kind(); kind {
  1728  	default:
  1729  		panic(&ValueError{"reflect.Value.Slice3", v.kind()})
  1730  
  1731  	case Array:
  1732  		if v.flag&flagAddr == 0 {
  1733  			panic("reflect.Value.Slice3: slice of unaddressable array")
  1734  		}
  1735  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  1736  		cap = int(tt.len)
  1737  		typ = (*sliceType)(unsafe.Pointer(tt.slice))
  1738  		base = v.ptr
  1739  
  1740  	case Slice:
  1741  		typ = (*sliceType)(unsafe.Pointer(v.typ))
  1742  		s := (*sliceHeader)(v.ptr)
  1743  		base = s.Data
  1744  		cap = s.Cap
  1745  	}
  1746  
  1747  	if i < 0 || j < i || k < j || k > cap {
  1748  		panic("reflect.Value.Slice3: slice index out of bounds")
  1749  	}
  1750  
  1751  	// Declare slice so that the garbage collector
  1752  	// can see the base pointer in it.
  1753  	var x []unsafe.Pointer
  1754  
  1755  	// Reinterpret as *sliceHeader to edit.
  1756  	s := (*sliceHeader)(unsafe.Pointer(&x))
  1757  	s.Len = j - i
  1758  	s.Cap = k - i
  1759  	if k-i > 0 {
  1760  		s.Data = arrayAt(base, i, typ.elem.Size(), "i < k <= cap")
  1761  	} else {
  1762  		// do not advance pointer, to avoid pointing beyond end of slice
  1763  		s.Data = base
  1764  	}
  1765  
  1766  	fl := v.flag.ro() | flagIndir | flag(Slice)
  1767  	return Value{typ.common(), unsafe.Pointer(&x), fl}
  1768  }
  1769  
  1770  // String returns the string v's underlying value, as a string.
  1771  // String is a special case because of Go's String method convention.
  1772  // Unlike the other getters, it does not panic if v's Kind is not String.
  1773  // Instead, it returns a string of the form "<T value>" where T is v's type.
  1774  // The fmt package treats Values specially. It does not call their String
  1775  // method implicitly but instead prints the concrete values they hold.
  1776  func (v Value) String() string {
  1777  	switch k := v.kind(); k {
  1778  	case Invalid:
  1779  		return "<invalid Value>"
  1780  	case String:
  1781  		return *(*string)(v.ptr)
  1782  	}
  1783  	// If you call String on a reflect.Value of other type, it's better to
  1784  	// print something than to panic. Useful in debugging.
  1785  	return "<" + v.Type().String() + " Value>"
  1786  }
  1787  
  1788  // TryRecv attempts to receive a value from the channel v but will not block.
  1789  // It panics if v's Kind is not Chan.
  1790  // If the receive delivers a value, x is the transferred value and ok is true.
  1791  // If the receive cannot finish without blocking, x is the zero Value and ok is false.
  1792  // If the channel is closed, x is the zero value for the channel's element type and ok is false.
  1793  func (v Value) TryRecv() (x Value, ok bool) {
  1794  	v.mustBe(Chan)
  1795  	v.mustBeExported()
  1796  	return v.recv(true)
  1797  }
  1798  
  1799  // TrySend attempts to send x on the channel v but will not block.
  1800  // It panics if v's Kind is not Chan.
  1801  // It reports whether the value was sent.
  1802  // As in Go, x's value must be assignable to the channel's element type.
  1803  func (v Value) TrySend(x Value) bool {
  1804  	v.mustBe(Chan)
  1805  	v.mustBeExported()
  1806  	return v.send(x, true)
  1807  }
  1808  
  1809  // Type returns v's type.
  1810  func (v Value) Type() Type {
  1811  	f := v.flag
  1812  	if f == 0 {
  1813  		panic(&ValueError{"reflect.Value.Type", Invalid})
  1814  	}
  1815  	if f&flagMethod == 0 {
  1816  		// Easy case
  1817  		return v.typ
  1818  	}
  1819  
  1820  	// Method value.
  1821  	// v.typ describes the receiver, not the method type.
  1822  	i := int(v.flag) >> flagMethodShift
  1823  	if v.typ.Kind() == Interface {
  1824  		// Method on interface.
  1825  		tt := (*interfaceType)(unsafe.Pointer(v.typ))
  1826  		if uint(i) >= uint(len(tt.methods)) {
  1827  			panic("reflect: internal error: invalid method index")
  1828  		}
  1829  		m := &tt.methods[i]
  1830  		return v.typ.typeOff(m.typ)
  1831  	}
  1832  	// Method on concrete type.
  1833  	ms := v.typ.exportedMethods()
  1834  	if uint(i) >= uint(len(ms)) {
  1835  		panic("reflect: internal error: invalid method index")
  1836  	}
  1837  	m := ms[i]
  1838  	return v.typ.typeOff(m.mtyp)
  1839  }
  1840  
  1841  // Uint returns v's underlying value, as a uint64.
  1842  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
  1843  func (v Value) Uint() uint64 {
  1844  	k := v.kind()
  1845  	p := v.ptr
  1846  	switch k {
  1847  	case Uint:
  1848  		return uint64(*(*uint)(p))
  1849  	case Uint8:
  1850  		return uint64(*(*uint8)(p))
  1851  	case Uint16:
  1852  		return uint64(*(*uint16)(p))
  1853  	case Uint32:
  1854  		return uint64(*(*uint32)(p))
  1855  	case Uint64:
  1856  		return *(*uint64)(p)
  1857  	case Uintptr:
  1858  		return uint64(*(*uintptr)(p))
  1859  	}
  1860  	panic(&ValueError{"reflect.Value.Uint", v.kind()})
  1861  }
  1862  
  1863  // UnsafeAddr returns a pointer to v's data.
  1864  // It is for advanced clients that also import the "unsafe" package.
  1865  // It panics if v is not addressable.
  1866  func (v Value) UnsafeAddr() uintptr {
  1867  	// TODO: deprecate
  1868  	if v.typ == nil {
  1869  		panic(&ValueError{"reflect.Value.UnsafeAddr", Invalid})
  1870  	}
  1871  	if v.flag&flagAddr == 0 {
  1872  		panic("reflect.Value.UnsafeAddr of unaddressable value")
  1873  	}
  1874  	return uintptr(v.ptr)
  1875  }
  1876  
  1877  // StringHeader is the runtime representation of a string.
  1878  // It cannot be used safely or portably and its representation may
  1879  // change in a later release.
  1880  // Moreover, the Data field is not sufficient to guarantee the data
  1881  // it references will not be garbage collected, so programs must keep
  1882  // a separate, correctly typed pointer to the underlying data.
  1883  type StringHeader struct {
  1884  	Data uintptr
  1885  	Len  int
  1886  }
  1887  
  1888  // stringHeader is a safe version of StringHeader used within this package.
  1889  type stringHeader struct {
  1890  	Data unsafe.Pointer
  1891  	Len  int
  1892  }
  1893  
  1894  // SliceHeader is the runtime representation of a slice.
  1895  // It cannot be used safely or portably and its representation may
  1896  // change in a later release.
  1897  // Moreover, the Data field is not sufficient to guarantee the data
  1898  // it references will not be garbage collected, so programs must keep
  1899  // a separate, correctly typed pointer to the underlying data.
  1900  type SliceHeader struct {
  1901  	Data uintptr
  1902  	Len  int
  1903  	Cap  int
  1904  }
  1905  
  1906  // sliceHeader is a safe version of SliceHeader used within this package.
  1907  type sliceHeader struct {
  1908  	Data unsafe.Pointer
  1909  	Len  int
  1910  	Cap  int
  1911  }
  1912  
  1913  func typesMustMatch(what string, t1, t2 Type) {
  1914  	if t1 != t2 {
  1915  		panic(what + ": " + t1.String() + " != " + t2.String())
  1916  	}
  1917  }
  1918  
  1919  // arrayAt returns the i-th element of p,
  1920  // an array whose elements are eltSize bytes wide.
  1921  // The array pointed at by p must have at least i+1 elements:
  1922  // it is invalid (but impossible to check here) to pass i >= len,
  1923  // because then the result will point outside the array.
  1924  // whySafe must explain why i < len. (Passing "i < len" is fine;
  1925  // the benefit is to surface this assumption at the call site.)
  1926  func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
  1927  	return add(p, uintptr(i)*eltSize, "i < len")
  1928  }
  1929  
  1930  // grow grows the slice s so that it can hold extra more values, allocating
  1931  // more capacity if needed. It also returns the old and new slice lengths.
  1932  func grow(s Value, extra int) (Value, int, int) {
  1933  	i0 := s.Len()
  1934  	i1 := i0 + extra
  1935  	if i1 < i0 {
  1936  		panic("reflect.Append: slice overflow")
  1937  	}
  1938  	m := s.Cap()
  1939  	if i1 <= m {
  1940  		return s.Slice(0, i1), i0, i1
  1941  	}
  1942  	if m == 0 {
  1943  		m = extra
  1944  	} else {
  1945  		for m < i1 {
  1946  			if i0 < 1024 {
  1947  				m += m
  1948  			} else {
  1949  				m += m / 4
  1950  			}
  1951  		}
  1952  	}
  1953  	t := MakeSlice(s.Type(), i1, m)
  1954  	Copy(t, s)
  1955  	return t, i0, i1
  1956  }
  1957  
  1958  // Append appends the values x to a slice s and returns the resulting slice.
  1959  // As in Go, each x's value must be assignable to the slice's element type.
  1960  func Append(s Value, x ...Value) Value {
  1961  	s.mustBe(Slice)
  1962  	s, i0, i1 := grow(s, len(x))
  1963  	for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
  1964  		s.Index(i).Set(x[j])
  1965  	}
  1966  	return s
  1967  }
  1968  
  1969  // AppendSlice appends a slice t to a slice s and returns the resulting slice.
  1970  // The slices s and t must have the same element type.
  1971  func AppendSlice(s, t Value) Value {
  1972  	s.mustBe(Slice)
  1973  	t.mustBe(Slice)
  1974  	typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
  1975  	s, i0, i1 := grow(s, t.Len())
  1976  	Copy(s.Slice(i0, i1), t)
  1977  	return s
  1978  }
  1979  
  1980  // Copy copies the contents of src into dst until either
  1981  // dst has been filled or src has been exhausted.
  1982  // It returns the number of elements copied.
  1983  // Dst and src each must have kind Slice or Array, and
  1984  // dst and src must have the same element type.
  1985  //
  1986  // As a special case, src can have kind String if the element type of dst is kind Uint8.
  1987  func Copy(dst, src Value) int {
  1988  	dk := dst.kind()
  1989  	if dk != Array && dk != Slice {
  1990  		panic(&ValueError{"reflect.Copy", dk})
  1991  	}
  1992  	if dk == Array {
  1993  		dst.mustBeAssignable()
  1994  	}
  1995  	dst.mustBeExported()
  1996  
  1997  	sk := src.kind()
  1998  	var stringCopy bool
  1999  	if sk != Array && sk != Slice {
  2000  		stringCopy = sk == String && dst.typ.Elem().Kind() == Uint8
  2001  		if !stringCopy {
  2002  			panic(&ValueError{"reflect.Copy", sk})
  2003  		}
  2004  	}
  2005  	src.mustBeExported()
  2006  
  2007  	de := dst.typ.Elem()
  2008  	if !stringCopy {
  2009  		se := src.typ.Elem()
  2010  		typesMustMatch("reflect.Copy", de, se)
  2011  	}
  2012  
  2013  	var ds, ss sliceHeader
  2014  	if dk == Array {
  2015  		ds.Data = dst.ptr
  2016  		ds.Len = dst.Len()
  2017  		ds.Cap = ds.Len
  2018  	} else {
  2019  		ds = *(*sliceHeader)(dst.ptr)
  2020  	}
  2021  	if sk == Array {
  2022  		ss.Data = src.ptr
  2023  		ss.Len = src.Len()
  2024  		ss.Cap = ss.Len
  2025  	} else if sk == Slice {
  2026  		ss = *(*sliceHeader)(src.ptr)
  2027  	} else {
  2028  		sh := *(*stringHeader)(src.ptr)
  2029  		ss.Data = sh.Data
  2030  		ss.Len = sh.Len
  2031  		ss.Cap = sh.Len
  2032  	}
  2033  
  2034  	return typedslicecopy(de.common(), ds, ss)
  2035  }
  2036  
  2037  // A runtimeSelect is a single case passed to rselect.
  2038  // This must match ../runtime/select.go:/runtimeSelect
  2039  type runtimeSelect struct {
  2040  	dir SelectDir      // SelectSend, SelectRecv or SelectDefault
  2041  	typ *rtype         // channel type
  2042  	ch  unsafe.Pointer // channel
  2043  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
  2044  }
  2045  
  2046  // rselect runs a select. It returns the index of the chosen case.
  2047  // If the case was a receive, val is filled in with the received value.
  2048  // The conventional OK bool indicates whether the receive corresponds
  2049  // to a sent value.
  2050  //go:noescape
  2051  func rselect([]runtimeSelect) (chosen int, recvOK bool)
  2052  
  2053  // A SelectDir describes the communication direction of a select case.
  2054  type SelectDir int
  2055  
  2056  // NOTE: These values must match ../runtime/select.go:/selectDir.
  2057  
  2058  const (
  2059  	_             SelectDir = iota
  2060  	SelectSend              // case Chan <- Send
  2061  	SelectRecv              // case <-Chan:
  2062  	SelectDefault           // default
  2063  )
  2064  
  2065  // A SelectCase describes a single case in a select operation.
  2066  // The kind of case depends on Dir, the communication direction.
  2067  //
  2068  // If Dir is SelectDefault, the case represents a default case.
  2069  // Chan and Send must be zero Values.
  2070  //
  2071  // If Dir is SelectSend, the case represents a send operation.
  2072  // Normally Chan's underlying value must be a channel, and Send's underlying value must be
  2073  // assignable to the channel's element type. As a special case, if Chan is a zero Value,
  2074  // then the case is ignored, and the field Send will also be ignored and may be either zero
  2075  // or non-zero.
  2076  //
  2077  // If Dir is SelectRecv, the case represents a receive operation.
  2078  // Normally Chan's underlying value must be a channel and Send must be a zero Value.
  2079  // If Chan is a zero Value, then the case is ignored, but Send must still be a zero Value.
  2080  // When a receive operation is selected, the received Value is returned by Select.
  2081  //
  2082  type SelectCase struct {
  2083  	Dir  SelectDir // direction of case
  2084  	Chan Value     // channel to use (for send or receive)
  2085  	Send Value     // value to send (for send)
  2086  }
  2087  
  2088  // Select executes a select operation described by the list of cases.
  2089  // Like the Go select statement, it blocks until at least one of the cases
  2090  // can proceed, makes a uniform pseudo-random choice,
  2091  // and then executes that case. It returns the index of the chosen case
  2092  // and, if that case was a receive operation, the value received and a
  2093  // boolean indicating whether the value corresponds to a send on the channel
  2094  // (as opposed to a zero value received because the channel is closed).
  2095  func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
  2096  	// NOTE: Do not trust that caller is not modifying cases data underfoot.
  2097  	// The range is safe because the caller cannot modify our copy of the len
  2098  	// and each iteration makes its own copy of the value c.
  2099  	runcases := make([]runtimeSelect, len(cases))
  2100  	haveDefault := false
  2101  	for i, c := range cases {
  2102  		rc := &runcases[i]
  2103  		rc.dir = c.Dir
  2104  		switch c.Dir {
  2105  		default:
  2106  			panic("reflect.Select: invalid Dir")
  2107  
  2108  		case SelectDefault: // default
  2109  			if haveDefault {
  2110  				panic("reflect.Select: multiple default cases")
  2111  			}
  2112  			haveDefault = true
  2113  			if c.Chan.IsValid() {
  2114  				panic("reflect.Select: default case has Chan value")
  2115  			}
  2116  			if c.Send.IsValid() {
  2117  				panic("reflect.Select: default case has Send value")
  2118  			}
  2119  
  2120  		case SelectSend:
  2121  			ch := c.Chan
  2122  			if !ch.IsValid() {
  2123  				break
  2124  			}
  2125  			ch.mustBe(Chan)
  2126  			ch.mustBeExported()
  2127  			tt := (*chanType)(unsafe.Pointer(ch.typ))
  2128  			if ChanDir(tt.dir)&SendDir == 0 {
  2129  				panic("reflect.Select: SendDir case using recv-only channel")
  2130  			}
  2131  			rc.ch = ch.pointer()
  2132  			rc.typ = &tt.rtype
  2133  			v := c.Send
  2134  			if !v.IsValid() {
  2135  				panic("reflect.Select: SendDir case missing Send value")
  2136  			}
  2137  			v.mustBeExported()
  2138  			v = v.assignTo("reflect.Select", tt.elem, nil)
  2139  			if v.flag&flagIndir != 0 {
  2140  				rc.val = v.ptr
  2141  			} else {
  2142  				rc.val = unsafe.Pointer(&v.ptr)
  2143  			}
  2144  
  2145  		case SelectRecv:
  2146  			if c.Send.IsValid() {
  2147  				panic("reflect.Select: RecvDir case has Send value")
  2148  			}
  2149  			ch := c.Chan
  2150  			if !ch.IsValid() {
  2151  				break
  2152  			}
  2153  			ch.mustBe(Chan)
  2154  			ch.mustBeExported()
  2155  			tt := (*chanType)(unsafe.Pointer(ch.typ))
  2156  			if ChanDir(tt.dir)&RecvDir == 0 {
  2157  				panic("reflect.Select: RecvDir case using send-only channel")
  2158  			}
  2159  			rc.ch = ch.pointer()
  2160  			rc.typ = &tt.rtype
  2161  			rc.val = unsafe_New(tt.elem)
  2162  		}
  2163  	}
  2164  
  2165  	chosen, recvOK = rselect(runcases)
  2166  	if runcases[chosen].dir == SelectRecv {
  2167  		tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
  2168  		t := tt.elem
  2169  		p := runcases[chosen].val
  2170  		fl := flag(t.Kind())
  2171  		if ifaceIndir(t) {
  2172  			recv = Value{t, p, fl | flagIndir}
  2173  		} else {
  2174  			recv = Value{t, *(*unsafe.Pointer)(p), fl}
  2175  		}
  2176  	}
  2177  	return chosen, recv, recvOK
  2178  }
  2179  
  2180  /*
  2181   * constructors
  2182   */
  2183  
  2184  // implemented in package runtime
  2185  func unsafe_New(*rtype) unsafe.Pointer
  2186  func unsafe_NewArray(*rtype, int) unsafe.Pointer
  2187  
  2188  // MakeSlice creates a new zero-initialized slice value
  2189  // for the specified slice type, length, and capacity.
  2190  func MakeSlice(typ Type, len, cap int) Value {
  2191  	if typ.Kind() != Slice {
  2192  		panic("reflect.MakeSlice of non-slice type")
  2193  	}
  2194  	if len < 0 {
  2195  		panic("reflect.MakeSlice: negative len")
  2196  	}
  2197  	if cap < 0 {
  2198  		panic("reflect.MakeSlice: negative cap")
  2199  	}
  2200  	if len > cap {
  2201  		panic("reflect.MakeSlice: len > cap")
  2202  	}
  2203  
  2204  	s := sliceHeader{unsafe_NewArray(typ.Elem().(*rtype), cap), len, cap}
  2205  	return Value{typ.(*rtype), unsafe.Pointer(&s), flagIndir | flag(Slice)}
  2206  }
  2207  
  2208  // MakeChan creates a new channel with the specified type and buffer size.
  2209  func MakeChan(typ Type, buffer int) Value {
  2210  	if typ.Kind() != Chan {
  2211  		panic("reflect.MakeChan of non-chan type")
  2212  	}
  2213  	if buffer < 0 {
  2214  		panic("reflect.MakeChan: negative buffer size")
  2215  	}
  2216  	if typ.ChanDir() != BothDir {
  2217  		panic("reflect.MakeChan: unidirectional channel type")
  2218  	}
  2219  	t := typ.(*rtype)
  2220  	ch := makechan(t, buffer)
  2221  	return Value{t, ch, flag(Chan)}
  2222  }
  2223  
  2224  // MakeMap creates a new map with the specified type.
  2225  func MakeMap(typ Type) Value {
  2226  	return MakeMapWithSize(typ, 0)
  2227  }
  2228  
  2229  // MakeMapWithSize creates a new map with the specified type
  2230  // and initial space for approximately n elements.
  2231  func MakeMapWithSize(typ Type, n int) Value {
  2232  	if typ.Kind() != Map {
  2233  		panic("reflect.MakeMapWithSize of non-map type")
  2234  	}
  2235  	t := typ.(*rtype)
  2236  	m := makemap(t, n)
  2237  	return Value{t, m, flag(Map)}
  2238  }
  2239  
  2240  // Indirect returns the value that v points to.
  2241  // If v is a nil pointer, Indirect returns a zero Value.
  2242  // If v is not a pointer, Indirect returns v.
  2243  func Indirect(v Value) Value {
  2244  	if v.Kind() != Ptr {
  2245  		return v
  2246  	}
  2247  	return v.Elem()
  2248  }
  2249  
  2250  // ValueOf returns a new Value initialized to the concrete value
  2251  // stored in the interface i. ValueOf(nil) returns the zero Value.
  2252  func ValueOf(i interface{}) Value {
  2253  	if i == nil {
  2254  		return Value{}
  2255  	}
  2256  
  2257  	// TODO: Maybe allow contents of a Value to live on the stack.
  2258  	// For now we make the contents always escape to the heap. It
  2259  	// makes life easier in a few places (see chanrecv/mapassign
  2260  	// comment below).
  2261  	escapes(i)
  2262  
  2263  	return unpackEface(i)
  2264  }
  2265  
  2266  // Zero returns a Value representing the zero value for the specified type.
  2267  // The result is different from the zero value of the Value struct,
  2268  // which represents no value at all.
  2269  // For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
  2270  // The returned value is neither addressable nor settable.
  2271  func Zero(typ Type) Value {
  2272  	if typ == nil {
  2273  		panic("reflect: Zero(nil)")
  2274  	}
  2275  	t := typ.(*rtype)
  2276  	fl := flag(t.Kind())
  2277  	if ifaceIndir(t) {
  2278  		return Value{t, unsafe_New(t), fl | flagIndir}
  2279  	}
  2280  	return Value{t, nil, fl}
  2281  }
  2282  
  2283  // New returns a Value representing a pointer to a new zero value
  2284  // for the specified type. That is, the returned Value's Type is PtrTo(typ).
  2285  func New(typ Type) Value {
  2286  	if typ == nil {
  2287  		panic("reflect: New(nil)")
  2288  	}
  2289  	t := typ.(*rtype)
  2290  	ptr := unsafe_New(t)
  2291  	fl := flag(Ptr)
  2292  	return Value{t.ptrTo(), ptr, fl}
  2293  }
  2294  
  2295  // NewAt returns a Value representing a pointer to a value of the
  2296  // specified type, using p as that pointer.
  2297  func NewAt(typ Type, p unsafe.Pointer) Value {
  2298  	fl := flag(Ptr)
  2299  	t := typ.(*rtype)
  2300  	return Value{t.ptrTo(), p, fl}
  2301  }
  2302  
  2303  // assignTo returns a value v that can be assigned directly to typ.
  2304  // It panics if v is not assignable to typ.
  2305  // For a conversion to an interface type, target is a suggested scratch space to use.
  2306  func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
  2307  	if v.flag&flagMethod != 0 {
  2308  		v = makeMethodValue(context, v)
  2309  	}
  2310  
  2311  	switch {
  2312  	case directlyAssignable(dst, v.typ):
  2313  		// Overwrite type so that they match.
  2314  		// Same memory layout, so no harm done.
  2315  		fl := v.flag&(flagAddr|flagIndir) | v.flag.ro()
  2316  		fl |= flag(dst.Kind())
  2317  		return Value{dst, v.ptr, fl}
  2318  
  2319  	case implements(dst, v.typ):
  2320  		if target == nil {
  2321  			target = unsafe_New(dst)
  2322  		}
  2323  		if v.Kind() == Interface && v.IsNil() {
  2324  			// A nil ReadWriter passed to nil Reader is OK,
  2325  			// but using ifaceE2I below will panic.
  2326  			// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
  2327  			return Value{dst, nil, flag(Interface)}
  2328  		}
  2329  		x := valueInterface(v, false)
  2330  		if dst.NumMethod() == 0 {
  2331  			*(*interface{})(target) = x
  2332  		} else {
  2333  			ifaceE2I(dst, x, target)
  2334  		}
  2335  		return Value{dst, target, flagIndir | flag(Interface)}
  2336  	}
  2337  
  2338  	// Failed.
  2339  	panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
  2340  }
  2341  
  2342  // Convert returns the value v converted to type t.
  2343  // If the usual Go conversion rules do not allow conversion
  2344  // of the value v to type t, Convert panics.
  2345  func (v Value) Convert(t Type) Value {
  2346  	if v.flag&flagMethod != 0 {
  2347  		v = makeMethodValue("Convert", v)
  2348  	}
  2349  	op := convertOp(t.common(), v.typ)
  2350  	if op == nil {
  2351  		panic("reflect.Value.Convert: value of type " + v.typ.String() + " cannot be converted to type " + t.String())
  2352  	}
  2353  	return op(v, t)
  2354  }
  2355  
  2356  // convertOp returns the function to convert a value of type src
  2357  // to a value of type dst. If the conversion is illegal, convertOp returns nil.
  2358  func convertOp(dst, src *rtype) func(Value, Type) Value {
  2359  	switch src.Kind() {
  2360  	case Int, Int8, Int16, Int32, Int64:
  2361  		switch dst.Kind() {
  2362  		case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  2363  			return cvtInt
  2364  		case Float32, Float64:
  2365  			return cvtIntFloat
  2366  		case String:
  2367  			return cvtIntString
  2368  		}
  2369  
  2370  	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  2371  		switch dst.Kind() {
  2372  		case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  2373  			return cvtUint
  2374  		case Float32, Float64:
  2375  			return cvtUintFloat
  2376  		case String:
  2377  			return cvtUintString
  2378  		}
  2379  
  2380  	case Float32, Float64:
  2381  		switch dst.Kind() {
  2382  		case Int, Int8, Int16, Int32, Int64:
  2383  			return cvtFloatInt
  2384  		case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  2385  			return cvtFloatUint
  2386  		case Float32, Float64:
  2387  			return cvtFloat
  2388  		}
  2389  
  2390  	case Complex64, Complex128:
  2391  		switch dst.Kind() {
  2392  		case Complex64, Complex128:
  2393  			return cvtComplex
  2394  		}
  2395  
  2396  	case String:
  2397  		if dst.Kind() == Slice && dst.Elem().PkgPath() == "" {
  2398  			switch dst.Elem().Kind() {
  2399  			case Uint8:
  2400  				return cvtStringBytes
  2401  			case Int32:
  2402  				return cvtStringRunes
  2403  			}
  2404  		}
  2405  
  2406  	case Slice:
  2407  		if dst.Kind() == String && src.Elem().PkgPath() == "" {
  2408  			switch src.Elem().Kind() {
  2409  			case Uint8:
  2410  				return cvtBytesString
  2411  			case Int32:
  2412  				return cvtRunesString
  2413  			}
  2414  		}
  2415  	}
  2416  
  2417  	// dst and src have same underlying type.
  2418  	if haveIdenticalUnderlyingType(dst, src, false) {
  2419  		return cvtDirect
  2420  	}
  2421  
  2422  	// dst and src are non-defined pointer types with same underlying base type.
  2423  	if dst.Kind() == Ptr && dst.Name() == "" &&
  2424  		src.Kind() == Ptr && src.Name() == "" &&
  2425  		haveIdenticalUnderlyingType(dst.Elem().common(), src.Elem().common(), false) {
  2426  		return cvtDirect
  2427  	}
  2428  
  2429  	if implements(dst, src) {
  2430  		if src.Kind() == Interface {
  2431  			return cvtI2I
  2432  		}
  2433  		return cvtT2I
  2434  	}
  2435  
  2436  	return nil
  2437  }
  2438  
  2439  // makeInt returns a Value of type t equal to bits (possibly truncated),
  2440  // where t is a signed or unsigned int type.
  2441  func makeInt(f flag, bits uint64, t Type) Value {
  2442  	typ := t.common()
  2443  	ptr := unsafe_New(typ)
  2444  	switch typ.size {
  2445  	case 1:
  2446  		*(*uint8)(ptr) = uint8(bits)
  2447  	case 2:
  2448  		*(*uint16)(ptr) = uint16(bits)
  2449  	case 4:
  2450  		*(*uint32)(ptr) = uint32(bits)
  2451  	case 8:
  2452  		*(*uint64)(ptr) = bits
  2453  	}
  2454  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  2455  }
  2456  
  2457  // makeFloat returns a Value of type t equal to v (possibly truncated to float32),
  2458  // where t is a float32 or float64 type.
  2459  func makeFloat(f flag, v float64, t Type) Value {
  2460  	typ := t.common()
  2461  	ptr := unsafe_New(typ)
  2462  	switch typ.size {
  2463  	case 4:
  2464  		*(*float32)(ptr) = float32(v)
  2465  	case 8:
  2466  		*(*float64)(ptr) = v
  2467  	}
  2468  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  2469  }
  2470  
  2471  // makeComplex returns a Value of type t equal to v (possibly truncated to complex64),
  2472  // where t is a complex64 or complex128 type.
  2473  func makeComplex(f flag, v complex128, t Type) Value {
  2474  	typ := t.common()
  2475  	ptr := unsafe_New(typ)
  2476  	switch typ.size {
  2477  	case 8:
  2478  		*(*complex64)(ptr) = complex64(v)
  2479  	case 16:
  2480  		*(*complex128)(ptr) = v
  2481  	}
  2482  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  2483  }
  2484  
  2485  func makeString(f flag, v string, t Type) Value {
  2486  	ret := New(t).Elem()
  2487  	ret.SetString(v)
  2488  	ret.flag = ret.flag&^flagAddr | f
  2489  	return ret
  2490  }
  2491  
  2492  func makeBytes(f flag, v []byte, t Type) Value {
  2493  	ret := New(t).Elem()
  2494  	ret.SetBytes(v)
  2495  	ret.flag = ret.flag&^flagAddr | f
  2496  	return ret
  2497  }
  2498  
  2499  func makeRunes(f flag, v []rune, t Type) Value {
  2500  	ret := New(t).Elem()
  2501  	ret.setRunes(v)
  2502  	ret.flag = ret.flag&^flagAddr | f
  2503  	return ret
  2504  }
  2505  
  2506  // These conversion functions are returned by convertOp
  2507  // for classes of conversions. For example, the first function, cvtInt,
  2508  // takes any value v of signed int type and returns the value converted
  2509  // to type t, where t is any signed or unsigned int type.
  2510  
  2511  // convertOp: intXX -> [u]intXX
  2512  func cvtInt(v Value, t Type) Value {
  2513  	return makeInt(v.flag.ro(), uint64(v.Int()), t)
  2514  }
  2515  
  2516  // convertOp: uintXX -> [u]intXX
  2517  func cvtUint(v Value, t Type) Value {
  2518  	return makeInt(v.flag.ro(), v.Uint(), t)
  2519  }
  2520  
  2521  // convertOp: floatXX -> intXX
  2522  func cvtFloatInt(v Value, t Type) Value {
  2523  	return makeInt(v.flag.ro(), uint64(int64(v.Float())), t)
  2524  }
  2525  
  2526  // convertOp: floatXX -> uintXX
  2527  func cvtFloatUint(v Value, t Type) Value {
  2528  	return makeInt(v.flag.ro(), uint64(v.Float()), t)
  2529  }
  2530  
  2531  // convertOp: intXX -> floatXX
  2532  func cvtIntFloat(v Value, t Type) Value {
  2533  	return makeFloat(v.flag.ro(), float64(v.Int()), t)
  2534  }
  2535  
  2536  // convertOp: uintXX -> floatXX
  2537  func cvtUintFloat(v Value, t Type) Value {
  2538  	return makeFloat(v.flag.ro(), float64(v.Uint()), t)
  2539  }
  2540  
  2541  // convertOp: floatXX -> floatXX
  2542  func cvtFloat(v Value, t Type) Value {
  2543  	return makeFloat(v.flag.ro(), v.Float(), t)
  2544  }
  2545  
  2546  // convertOp: complexXX -> complexXX
  2547  func cvtComplex(v Value, t Type) Value {
  2548  	return makeComplex(v.flag.ro(), v.Complex(), t)
  2549  }
  2550  
  2551  // convertOp: intXX -> string
  2552  func cvtIntString(v Value, t Type) Value {
  2553  	return makeString(v.flag.ro(), string(v.Int()), t)
  2554  }
  2555  
  2556  // convertOp: uintXX -> string
  2557  func cvtUintString(v Value, t Type) Value {
  2558  	return makeString(v.flag.ro(), string(v.Uint()), t)
  2559  }
  2560  
  2561  // convertOp: []byte -> string
  2562  func cvtBytesString(v Value, t Type) Value {
  2563  	return makeString(v.flag.ro(), string(v.Bytes()), t)
  2564  }
  2565  
  2566  // convertOp: string -> []byte
  2567  func cvtStringBytes(v Value, t Type) Value {
  2568  	return makeBytes(v.flag.ro(), []byte(v.String()), t)
  2569  }
  2570  
  2571  // convertOp: []rune -> string
  2572  func cvtRunesString(v Value, t Type) Value {
  2573  	return makeString(v.flag.ro(), string(v.runes()), t)
  2574  }
  2575  
  2576  // convertOp: string -> []rune
  2577  func cvtStringRunes(v Value, t Type) Value {
  2578  	return makeRunes(v.flag.ro(), []rune(v.String()), t)
  2579  }
  2580  
  2581  // convertOp: direct copy
  2582  func cvtDirect(v Value, typ Type) Value {
  2583  	f := v.flag
  2584  	t := typ.common()
  2585  	ptr := v.ptr
  2586  	if f&flagAddr != 0 {
  2587  		// indirect, mutable word - make a copy
  2588  		c := unsafe_New(t)
  2589  		typedmemmove(t, c, ptr)
  2590  		ptr = c
  2591  		f &^= flagAddr
  2592  	}
  2593  	return Value{t, ptr, v.flag.ro() | f} // v.flag.ro()|f == f?
  2594  }
  2595  
  2596  // convertOp: concrete -> interface
  2597  func cvtT2I(v Value, typ Type) Value {
  2598  	target := unsafe_New(typ.common())
  2599  	x := valueInterface(v, false)
  2600  	if typ.NumMethod() == 0 {
  2601  		*(*interface{})(target) = x
  2602  	} else {
  2603  		ifaceE2I(typ.(*rtype), x, target)
  2604  	}
  2605  	return Value{typ.common(), target, v.flag.ro() | flagIndir | flag(Interface)}
  2606  }
  2607  
  2608  // convertOp: interface -> interface
  2609  func cvtI2I(v Value, typ Type) Value {
  2610  	if v.IsNil() {
  2611  		ret := Zero(typ)
  2612  		ret.flag |= v.flag.ro()
  2613  		return ret
  2614  	}
  2615  	return cvtT2I(v.Elem(), typ)
  2616  }
  2617  
  2618  // implemented in ../runtime
  2619  func chancap(ch unsafe.Pointer) int
  2620  func chanclose(ch unsafe.Pointer)
  2621  func chanlen(ch unsafe.Pointer) int
  2622  
  2623  // Note: some of the noescape annotations below are technically a lie,
  2624  // but safe in the context of this package. Functions like chansend
  2625  // and mapassign don't escape the referent, but may escape anything
  2626  // the referent points to (they do shallow copies of the referent).
  2627  // It is safe in this package because the referent may only point
  2628  // to something a Value may point to, and that is always in the heap
  2629  // (due to the escapes() call in ValueOf).
  2630  
  2631  //go:noescape
  2632  func chanrecv(ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, received bool)
  2633  
  2634  //go:noescape
  2635  func chansend(ch unsafe.Pointer, val unsafe.Pointer, nb bool) bool
  2636  
  2637  func makechan(typ *rtype, size int) (ch unsafe.Pointer)
  2638  func makemap(t *rtype, cap int) (m unsafe.Pointer)
  2639  
  2640  //go:noescape
  2641  func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)
  2642  
  2643  //go:noescape
  2644  func mapassign(t *rtype, m unsafe.Pointer, key, val unsafe.Pointer)
  2645  
  2646  //go:noescape
  2647  func mapdelete(t *rtype, m unsafe.Pointer, key unsafe.Pointer)
  2648  
  2649  // m escapes into the return value, but the caller of mapiterinit
  2650  // doesn't let the return value escape.
  2651  //go:noescape
  2652  func mapiterinit(t *rtype, m unsafe.Pointer) unsafe.Pointer
  2653  
  2654  //go:noescape
  2655  func mapiterkey(it unsafe.Pointer) (key unsafe.Pointer)
  2656  
  2657  //go:noescape
  2658  func mapitervalue(it unsafe.Pointer) (value unsafe.Pointer)
  2659  
  2660  //go:noescape
  2661  func mapiternext(it unsafe.Pointer)
  2662  
  2663  //go:noescape
  2664  func maplen(m unsafe.Pointer) int
  2665  
  2666  // call calls fn with a copy of the n argument bytes pointed at by arg.
  2667  // After fn returns, reflectcall copies n-retoffset result bytes
  2668  // back into arg+retoffset before returning. If copying result bytes back,
  2669  // the caller must pass the argument frame type as argtype, so that
  2670  // call can execute appropriate write barriers during the copy.
  2671  //
  2672  //go:linkname call runtime.reflectcall
  2673  func call(argtype *rtype, fn, arg unsafe.Pointer, n uint32, retoffset uint32)
  2674  
  2675  func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)
  2676  
  2677  // memmove copies size bytes to dst from src. No write barriers are used.
  2678  //go:noescape
  2679  func memmove(dst, src unsafe.Pointer, size uintptr)
  2680  
  2681  // typedmemmove copies a value of type t to dst from src.
  2682  //go:noescape
  2683  func typedmemmove(t *rtype, dst, src unsafe.Pointer)
  2684  
  2685  // typedmemmovepartial is like typedmemmove but assumes that
  2686  // dst and src point off bytes into the value and only copies size bytes.
  2687  //go:noescape
  2688  func typedmemmovepartial(t *rtype, dst, src unsafe.Pointer, off, size uintptr)
  2689  
  2690  // typedmemclr zeros the value at ptr of type t.
  2691  //go:noescape
  2692  func typedmemclr(t *rtype, ptr unsafe.Pointer)
  2693  
  2694  // typedmemclrpartial is like typedmemclr but assumes that
  2695  // dst points off bytes into the value and only clears size bytes.
  2696  //go:noescape
  2697  func typedmemclrpartial(t *rtype, ptr unsafe.Pointer, off, size uintptr)
  2698  
  2699  // typedslicecopy copies a slice of elemType values from src to dst,
  2700  // returning the number of elements copied.
  2701  //go:noescape
  2702  func typedslicecopy(elemType *rtype, dst, src sliceHeader) int
  2703  
  2704  // Dummy annotation marking that the value x escapes,
  2705  // for use in cases where the reflect code is so clever that
  2706  // the compiler cannot follow.
  2707  func escapes(x interface{}) {
  2708  	if dummy.b {
  2709  		dummy.x = x
  2710  	}
  2711  }
  2712  
  2713  var dummy struct {
  2714  	b bool
  2715  	x interface{}
  2716  }
  2717  

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