Today I’ll continue where I left off yesterday, talking about…

Method expressions

…

Similarly, the expression

(*T).Mp

yields a function value representing Mp with signature

func(tp *T, f float32) float32

The key difference to notice between this example and yesterday’s is that the first argument (that of the receiver) is a pointer here, but was not a pointer in the previous example:

func(tv T, a int) int          // First argument of type T, not a pointer
func(tp *T, f float32) float32 // First argument of type *T, a pointer

For a method with a value receiver, one can derive a function with an explicit pointer receiver, so

(*T).Mv

yields a function value representing Mv with signature

func(tv *T, a int) int

That is to say, we can get a “pointer” version of the first example if we want to…

func(tv T, a int) int          // First argument of type T, not a pointer
func(tp *T, f float32) float32 // First argument of type *T, a pointer
func(tv *T, a int) int         // Now the first argument of type *T, as well

Such a function indirects through the receiver to create a value to pass as the receiver to the underlying method; the method does not overwrite the value whose address is passed in the function call.

That to say, using the (*T).Mv construct does not render you suddenly able to change the values of the receiver within the method:

type T struct {
	a int
}

func (t T) Mv(a int)  { t.a = a } // value receiver
func (t *T) Mp(a int) { t.a = a } // pointer receiver

var t T

fp := (*T).Mp
fp(&t, 4)
fmt.Println(t) // Prints 4; the pointer receiver allows modifying the receiver

fvp := (*T).Mv
fvp(&t, 5)
fmt.Println(t) // Prints 4; value receivers cannot be modified within the method

The final case, a value-receiver function for a pointer-receiver method, is illegal because pointer-receiver methods are not in the method set of the value type.

See a full example in the playground.

Quotes from The Go Programming Language Specification Version of August 2, 2023