对于这些接口实现和调用的代码感到困惑吗？

Question

Version of Go

• go version go1.11 darwin/amd64

Code 1:

package main

import "fmt"

type myintf interface {
    GotU()
}

type esc struct {
     i int
}

//func GotU(t esc);
func (e esc)GotU() {
    e.i = 10
}

func TestFunc(it myintf) string {
    it.GotU()
    return  "kk"
}

func main() {

    var test esc
    test.i = 9

    TestFunc(test)

    fmt.Println(test.i)

}

Code 2:

package main

import "fmt"

type myintf interface {
    GotU()
}

type esc struct {
     i int
}

func (e esc)GotU() {
    e.i = 10
}

func TestFunc(it myintf) string {
    it.GotU()
    return  "kk"
}

func main() {

    var test esc
    test.i = 9

    TestFunc(&test)

    fmt.Println(test.i)

}

Code 3:

package main

import "fmt"

type myintf interface {
    GotU()
}

type esc struct {
     i int
}

func (e *esc)GotU() {
    e.i = 10
}

func TestFunc(it myintf) string {
    it.GotU()
    return  "kk"
}

func main() {

    var test esc
    test.i = 9

    TestFunc(test)

    fmt.Println(test.i)

}

The outputs:

• code 1 output: 9
• code 2 output: 9
• code 3 cannot be compiled due to a type mismatch

---

Since only func (e esc)GotU() implemented, why should both pieces of code work and deliver the same result? It's kind of confusing for me to pass a pointer of struct to that function (TestFunc) to get the same answer.

Answer 1

The last code snippet has implemented a method receiver of pointer type. This will consider the situation if you want to modify the value of receiver.

func (e *esc) GotU() {
    e.i = 10
}

In above case Since you are passing pointer type receiver on a method which is implementing the interface.

type myintf interface {
    GotU()
}

So you need to pass address of struct in TestFunc. This is the reason you are getting type mismatch error, because you are passing variable of esc type while your method requires variable of *esc.

func main() {

    var test esc
    test.i = 9
    TestFunc(&test)
    fmt.Println(test.i)

}

Working example on Go playground

In Golang there are two ways to pass a method receiver.

func (s *MyStruct) pointerMethod() { } // method on pointer
func (s MyStruct)  valueMethod()   { } // method on value

For programmers unaccustomed to pointers, the distinction between these two examples can be confusing, but the situation is actually very simple. When defining a method on a type, the receiver (s in the above examples) behaves exactly as if it were an argument to the method. Whether to define the receiver as a value or as a pointer is the same question, then, as whether a function argument should be a value or a pointer. There are several considerations

First, and most important, does the method need to modify the receiver? If it does, the receiver must be a pointer. (Slices and maps act as references, so their story is a little more subtle, but for instance to change the length of a slice in a method the receiver must still be a pointer.) In the examples above, if pointerMethod modifies the fields of s, the caller will see those changes, but valueMethod is called with a copy of the caller's argument (that's the definition of passing a value), so changes it makes will be invisible to the caller.