通过引用实例化对象

Question

Here is a sample of Go code which I do not really understand:

type MyClass struct {
    field1      string
    field2      string
}
...
objectinstance1 := MyClass{field1:"Hello", field2:"World"}
objectinstance2 := &MyClass{field1:"Hello", field2:"World"}

I can do exactly the same thing with objectinstance1 and objectinstance2 (method call for example) with the same syntax.

So I do not understand the role of the & operator. What I understand is that objectinstance1 contains an object whereas objectinstance2 contains a pointer.

It is for me the same thing in C than the difference between char and char*.

But in this case I should use -> instead of . (dot)?

Answer 1

Difference is not visible because it’s hidden in 2 things:

1. Operator := which assigns value and type for a variable simultaneously. So it looks like objectinstance1 and objectinstance2 are the same. But in fact first is a MyClass instance and second is a pointer to it. It will be more palpable if use long-form operator:

   var objectinstace1 MyClass = MyClass{} var objectinstance2 *MyClass = &MyClass{}

If you omit * or & variable and type become incompatible and assignment fails.

1. Implicit pointer indirection. Go does it automatically in statements like ptr1.Field1 to access a specific field in struct by pointer to it.

   Only on rare cases when there’s ambiguity you have to use full form:

   *ptr1.Value1

   or sometimes even:

   (*ptr1).Value1

UPDATE:

Explicit pointer usage for disambiguation:

type M map[int]int

func (m *M) Add1() {
    // this doesn't work -  invalid operation: m[1] (type *M does not support indexing)
    // m[1] = 1

    // the same - invalid operation: m[1] (type *M does not support indexing)
    // *m[1] = 1

    // this works
    (*m)[1] = 1
}

https://play.golang.org/p/JcXd_oNIAw