How do I print the pointer value of a Go object?

Question

I am just playing around with Go and do not yet have a good mental model of when structs are passed by value or by reference.

This may be a very dumb question but I just want to experiment a bit and see if I am still working on the same object or I have made a copy of it (passed it by value).

Is there a way to print the pointer (or internal id if pointer value is changed by gc) of an object?
package main

import ( "runtime" )

type Something struct {
    number int
    queue chan int
}

func gotest( s *Something, done chan bool ) {
    println( "from gotest:")
    println( &s )
    for num := range s.queue {
        println( num )
        s.number = num
    }
    done <- true
}

func main() {
    runtime.GOMAXPROCS(4)
    s := new(Something)
    println(&s)
    s.queue = make(chan int)
    done := make(chan bool)
    go gotest(s, done)
    s.queue <- 42
    close(s.queue)
    <- done
    println(&s)
    println(s.number)
}

gives on my windows (8g compiled version):
0x4930d4
from gotest:
0x4974d8
42
0x4930d4
42

Why does the pointer value from within the go routine show a different value? The quantity on the original object did get changed so it was working with the same object. Is there a way to see an object id that is persistent?

Gopher · Accepted Answer

Go function arguments are passed by value. First, let's discard the irrelevant parts of your example, so that we can easily see that you are merely passing an argument by value. For example, package main import "fmt" func byval(q *int) { fmt.Printf("3. byval -- q %T: &q=%p q=&i=%p *q=i=%v
", q, &q, q, *q) *q = 4143 fmt.Printf("4. byval -- q %T: &q=%p q=&i=%p *q=i=%v
", q, &q, q, *q) q = nil } func main() { i := int(42) fmt.Printf("1. main -- i %T: &i=%p i=%v
", i, &i, i) p := &i fmt.Printf("2. main -- p %T: &p=%p p=&i=%p *p=i=%v
", p, &p, p, *p) byval(p) fmt.Printf("5. main -- p %T: &p=%p p=&i=%p *p=i=%v
", p, &p, p, *p) fmt.Printf("6. main -- i %T: &i=%p i=%v
", i, &i, i) } Output: 1. main -- i int: &i=0xf840000040 i=42 2. main -- p *int: &p=0xf8400000f0 p=&i=0xf840000040 *p=i=42 3. byval -- q *int: &q=0xf8400000d8 q=&i=0xf840000040 *q=i=42 4. byval -- q *int: &q=0xf8400000d8 q=&i=0xf840000040 *q=i=4143 5. main -- p *int: &p=0xf8400000f0 p=&i=0xf840000040 *p=i=4143 6. main -- i int: &i=0xf840000040 i=4143 In function main, i is an int variable at memory location (&i) 0xf800000040 with an initial value (i) 42. In function main, p is a pointer to an int variable at memory location (&p) 0xf8000000f0 with a value (p=&i) 0xf800000040 which points to an int value (*p=i) 42. In function main, byval(p) is a function call which assigns the value (p=&i) 0xf800000040 of the argument at memory location (&p) 0xf8000000f0 to the function byval parameter q at memory location (&q) 0xf8000000d8. In other words, memory is allocated for the byval parameter q and the value of the main byval argument p is assigned to it; the values of p and q are initially the same, but the variables p and q are distinct. In function byval, using pointer q (*int), which is a copy of pointer p (*int), integer *q (i) is set to a new int value 4143. At the end before returning. the pointer q is set to nil (zero value), which has no effect on p since q is a copy. In function main, p is a pointer to an int variable at memory location (&p) 0xf8000000f0 with a value (p=&i) 0xf800000040 which points to a new int value (*p=i) 4143. In function main, i is an int variable at memory location (&i) 0xf800000040 with a final value (i) 4143.

Go Language

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