Using struct{} for context keys

[cpu...@gmail.com]

I've recently stumbled across code like this https://go.dev/play/p/u3UER2ywRlr:

type clientContextKey struct{}

ctx := context.Background()

ctx = context.WithValue(ctx, clientContextKey{}, "foo")

_, ok := ctx.Value(clientContextKey{}).(string)

Naively, I had expected this to fail since I assumed that clientContextKey{} would create a new variable instance every time it was used and hence the keys would not match.

Why does this use of instantiating the struct type work here?

Thanks,

Andi

[burak serdar]

Two values A and B are equal (A==B) if A and B have the same type and the same values. In a code where

 a:=clientContextKey{}

  b:=clientContextKey{}

a==b is true, because they have the same types, and they have the same values (i.e. the empty value).

Note however:

a:=&clientContextKey{}

b:=&clientContextKey{}

It is not guaranteed that a==b, neither is it guaranteed that a!=b. The compiler may choose to allocate the same address for the zero-size variables a and b, or it may not,

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[Andreas Götz]

Thank you. Zero values are of course identical. I was only thinking about the pointer case...