Why Can sync.WaitGroup Methods Be Called on Non-Pointer Values?

Method Set of sync.WaitGroup

The sync.WaitGroup is a synchronization primitive that enables you to wait for a collection of goroutines to complete. While its documentation suggests that the Add, Done, and Wait methods require a pointer receiver, you can also use them on non-pointer values. How is this possible?

The secret lies in the empty method set of sync.WaitGroup. This means that all the methods of sync.WaitGroup are pointer receivers. When you create a value of type sync.WaitGroup, such as:

var wg sync.WaitGroup

You are actually creating a pointer to a sync.WaitGroup struct. This pointer is automatically dereferenced when you call methods on the wg variable. For instance, the following code compiles successfully:

wg.Add(1)
wg.Done()
wg.Wait()

The Go compiler implicitly converts wg to a pointer to sync.WaitGroup before calling the methods. This behavior is defined in the Go language specification:

If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m().

Therefore, you can call methods that have pointer receivers on non-pointer values if those values are addressable. Non-pointer values are always addressable in Go.

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