Method values

If the expression x has static type T and M is in the method set of type T, x.M is called a method value. The method value x.M is a function value that is callable with the same arguments as a method call of x.M. The expression x is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later.

The type T may be an interface or non-interface type.

As in the discussion of method expressions above, consider a struct type T with two methods, Mv, whose receiver is of type T, and Mp, whose receiver is of type *T.

type T struct {
    a int
}
func (tv  T) Mv(a int) int         { return 0 }  // value receiver
func (tp *T) Mp(f float32) float32 { return 1 }  // pointer receiver

var t T
var pt *T
func makeT() T

The expression

t.Mv

yields a function value of type

func(int) int

These two invocations are equivalent:

t.Mv(7)
f := t.Mv; f(7)

Similarly, the expression

pt.Mp

yields a function value of type

func(float32) float32

As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv.

As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.

f := t.Mv; f(7)   // like t.Mv(7)
f := pt.Mp; f(7)  // like pt.Mp(7)
f := pt.Mv; f(7)  // like (*pt).Mv(7)
f := t.Mp; f(7)   // like (&t).Mp(7)
f := makeT().Mp   // invalid: result of makeT() is not addressable

Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type.

var i interface { M(int) } = myVal
f := i.M; f(7)  // like i.M(7)