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encoding/json: Unmarshal to nil pointer documentation discrepancy #18730

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JovanCe opened this issue Jan 20, 2017 · 2 comments
Closed

encoding/json: Unmarshal to nil pointer documentation discrepancy #18730

JovanCe opened this issue Jan 20, 2017 · 2 comments

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@JovanCe
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JovanCe commented Jan 20, 2017

When trying to unmarshal a json string into a nil pointer, the operation fails as can be seen here:
https://play.golang.org/p/wL6UDpaJ-h

The error returned is InvalidUnmarshalError whose docs clearly state:
"The argument to Unmarshal must be a non-nil pointer." (https://golang.org/pkg/encoding/json/#InvalidUnmarshalError)

But the docs for the Unmarshal function state:
"Unmarshal unmarshals the JSON into the value pointed at by the pointer. If the pointer is nil, Unmarshal allocates a new value for it to point to."
(https://golang.org/pkg/encoding/json/#Unmarshal)

Obviously, the behaviour implemented is the one from the error docs.

Curiously, this works:
https://play.golang.org/p/w-Sg24qcCo
So, a reference to a nil pointer is accepted but the nil pointer itself is not. Anyone care to clarify?

go1.7.4 darwin/amd64

@josharian josharian changed the title Json Unmarshal to nil pointer documentation discrepancy encoding/json: Unmarshal to nil pointer documentation discrepancy Jan 20, 2017
@rakyll rakyll added this to the Go1.9 milestone Jan 20, 2017
@shantuo
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shantuo commented Feb 7, 2017

My understanding is that this is a documentation bug. The behavior of returning an InvalidUnmarshalError looks work-as-intended. I guess the docs for the Unmarshal function means "If the pointer points to nil"? Because Unmarshal cannot modify the passed nil pointer.

@shantuo shantuo self-assigned this Feb 7, 2017
@gopherbot
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CL https://golang.org/cl/36478 mentions this issue.

@golang golang locked and limited conversation to collaborators Feb 10, 2018
@rsc rsc unassigned shantuo Jun 23, 2022
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