I propose to add SafeLength and DirtyLength method to sync.Map. Currently, we don't get the length of sync.Map. For that reason, we can't prepare length and capacity for map and slice. sync.Map has two map, m contained in readOnly struct and dirty.

So, SafeLength returns the length of m, DirtyLength returns the length of dirty. If a developer wanna get the length of m (i.e. wanna to get the length of immutable struct stored atomically), we'll use SafeLength. Conversely, If a developer wanna to get the length of dirty which contains all values, we'll use DirtyLength.

But, dirty may be initialized. For example, Load and missLocked initialize it. Above the reason, I recommend to use SafeLength, but I also added DirtyLength for every case. If this proposal is accepted, we can execute following code.

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	m := sync.Map{}

	m.Store("Key 1", "Value 1")
	m.Store("Key 2", "Value 2")
	m.Store("Key 3", "Value 3")
	m.Store("Key 4", "Value 4")
	m.Store("Key 5", "Value 5")

	<-time.After(5 * time.Millisecond)

	vals := make([]string, 0, m.SafeLength())

	m.Range(func(key interface{}, value interface{}) bool {
		vals = append(vals, value.(string))
		return true
	})

	fmt.Println(vals)
}