golang return object from a function

Question

I am struggling to understand exactly what is happening when you return a new object from a function in go.

I have this

func createPointerToInt() *int {
    i := new(int)
    fmt.Println(&i);
    return i;
}

func main() {
    i := createPointerToInt();
    fmt.Println(&i);
}

The values printed returned are

0x1040a128
0x1040a120

I would expect these two values to be the same. I do not understand why there is an 8 byte difference.

In what I see as the equivalent C code:

int* createPointerToInt() {
    int* i = new int;
    printf("%#08x\n", i);
    return i;
}

int main() {
    int* r = createPointerToInt();
    printf("%#08x\n", r);
    return 0;
}

The address returned is the same:

0x8218008
0x8218008

Am I missing something blindingly obvious here? Any clarification would be greatly appreciated!

Sadique · Accepted Answer · 2015-03-04 09:57:13Z

7

You are printing the address of the pointer here fmt.Println(&i);. Try this:

func main() {
    i := createPointerToInt();
    fmt.Println(i); //--> Remove the ampersand
}

i is the pointer returned from createPointerToInt - while &i is the address of the pointer you are trying to print. Note in your C sample you are printing it correctly:

printf("%#08x\n", r);
                 ^No ampersand here

edited Mar 4, 2015 at 9:57

answered Mar 4, 2015 at 9:50

Sadique

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2 Comments

Tom Over a year ago

A of course! What a fool, thank you! I should have known that :P I think I was getting confused as go doesn't require you to deference the pointer when you use it like (*object).function or object->function in C, so I thought taking the address there would be the address of the pointer, not the pointer itself. Thanks!

Tom Over a year ago

A small addendum to this, and I think why I was initially getting confused. if you create a custom type (type MyType struct { }) and use that instead of int, when you use Println, you get '&{}' output, not the address. For a custom object do you know how to get the address to print?

Grzegorz Żur · Accepted Answer · 2015-03-04 09:53:58Z

2

Change &i to i. You are printing address of i while you should print the value of i.

func createPointerToInt() *int {
     i := new(int)
     fmt.Println(i);
     return i;
}

func main() {
    i := createPointerToInt();
    fmt.Println(i);
}

answered Mar 4, 2015 at 9:53

Grzegorz Żur

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1 Comment

Tom Over a year ago

Hi there, small update to this and one of the reasons why I think I'd tried to use the ampersands in the wrong place is when using a custom object (type Example struct {}) using fmt.Println produces &{} not a memory address. Is there a way to force fmt.Println to convert that to a memory address?

Akavall · Accepted Answer · 2015-03-05 14:46:37Z

0

But how come in your original code the addresses of the two pointers (not memory addresses the pointers are pointing too) are different?

func main() {
    i := createPointerToInt();
    fmt.Println(&i);
}

Is equivalent to:

func main() {
    var i *int  // declare variable i
    i = createPointerToInt(); // assign value of
                              // a different i that was 
                              // declared and initialized
                              // inside the function
    fmt.Println(&i);
}

Edit:

To print the address of a struct you need to use:

fmt.Printf("%p\n", &your_struct)

golang.org/pkg/fmt/

For example:

goplayground

edited Mar 5, 2015 at 14:46

answered Mar 5, 2015 at 3:04

Akavall

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4 Comments

Tom Over a year ago

In the original code the first block was the function and the second was main calling that function. The address being returned was that of the local variable pointer in each case, not the address to the region in memory the pointer was referring to. The reason I made this mistake in the first place is in the online golang tool, when you use a custom type (one you create) and use that in place of int in the example, &{} is printed to the console, not an address, which seemed kind of odd, and I'm not sure how to force an address to print

Akavall Over a year ago

Yes, yes, I was actually talking about the addresses of the two pointers in the original example. Maybe it is too obvious for most people, but it had me confused at first.

Akavall Over a year ago

@Tom, You need to use printf to print address of a struct, I updated my answer.

Tom Over a year ago

Awesome that's just what I was looking for :) Thank you! Much appreciated! @Akavall

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