为什么struct引用的内存地址会发生变化？

Question

I have a struct and a method that's working on the structs reference. The pointer address is changing every time I call the method. Why is it like that?

Code

package main

import "k8s.io/contrib/compare/Godeps/_workspace/src/github.com/emicklei/go-restful/log"

type Whatever struct{
    Name string
}

func (whatever *Whatever) GetNameByReference() (string) {
log.Printf("Whatever.GetNameByReference() memory address: %v", &whatever)

    return whatever.Name
}

func evaluateMemoryAddressWhenNotWritingAnything()  {
    whatever := Whatever{}

    whatever.GetNameByReference()
    whatever.GetNameByReference()
    whatever.GetNameByReference()
}

func main() {
    evaluateMemoryAddressWhenNotWritingAnything()
}

Output:

log.go:30: Whatever.GetNameByReference() memory address: 0xc420034020
log.go:30: Whatever.GetNameByReference() memory address: 0xc420034030
log.go:30: Whatever.GetNameByReference() memory address: 0xc420034038

Answer 1

Never think and never talk about references. Go has no notion of "reference", everything is a value. Some things are pointer values. Your problem stems from thinking about *X as "a reference to an X" which it isn't: It is a value holding the memory address of an X (or nil).

So in func (whatever *Whatever) the variable whatever is a pointer to a Whatever . The value of whatever is the memory address of the Whatever the pointer points to. You would like to print this memory address, ie the value of whatever .

You do Printf("%v", &whatever) . Remember: whatever is a variable (holding a memory address). So &whatever is the address of the variable itself: &whatever is of type **Whatever . What you find at the address &whatever is not the value you are interested in; it is just the temporary variable used to store the address of the original Whatever . Of course this temporary variable is not pinned in memory an may change freely.

You should do Printf("%p", whatever) . The verb %p is for pointer values and whatever is a pointer and you are interested in its value, so print this value.

Answer 2

You are not displaying the address of the struct, you are displaying the address of the address (address of the pointer). The pointer is passed as a parameter and thus new each time. Remove the & in log.Printf("Whatever.GetNameByReference() memory address: %v", &whatever) to get what you want (and use %p instead of %v).

Answer 3

Calling method

func (whatever *Whatever) GetNameByReference() (string) {

is much the same as calling function providing receiver as first argument

func GetNameByReference(whatever *Whatever) (string) {

Go calling convention is to always pass arguments copy_by_value. So each time you call whatever.GetNameByReference() it receives fresh copy of whatever which itself holds same *Whatever address. As already written doing

log.Printf("Whatever.GetNameByReference() memory address: %v", whatever) //instead of &whatever

will log the same whatever value which is address.