interface-receiver.md

Receivers and Interfaces

Methods with value receivers can be called on pointers as well as values. Methods with pointer receivers can only be called on pointers or addressable values.

For example,

type S struct {
  data string
}

func (s S) Read() string {
  return s.data
}

func (s *S) Write(str string) {
  s.data = str
}

// We cannot get pointers to values stored in maps, because they are not
// addressable values.
sVals := map[int]S{1: {"A"}}

// We can call Read on values stored in the map because Read
// has a value receiver, which does not require the value to
// be addressable.
sVals[1].Read()

// We cannot call Write on values stored in the map because Write
// has a pointer receiver, and it's not possible to get a pointer
// to a value stored in a map.
//
//  sVals[1].Write("test")

sPtrs := map[int]*S{1: {"A"}}

// You can call both Read and Write if the map stores pointers,
// because pointers are intrinsically addressable.
sPtrs[1].Read()
sPtrs[1].Write("test")

Similarly, an interface can be satisfied by a pointer, even if the method has a value receiver.

type F interface {
  f()
}

type S1 struct{}

func (s S1) f() {}

type S2 struct{}

func (s *S2) f() {}

s1Val := S1{}
s1Ptr := &S1{}
s2Val := S2{}
s2Ptr := &S2{}

var i F
i = s1Val
i = s1Ptr
i = s2Ptr

// The following doesn't compile, since s2Val is a value, and there is no value receiver for f.
//   i = s2Val

Effective Go has a good write up on Pointers vs. Values.