Pointer difference when initializing embedded structs

Question

I am playing around with struct embedding and have a problem with keeping the same reference to the embedded struct.

Try out Go Playground and see that there are two different pointer addresses to *strings.Reader.

package main

import (
    "fmt"
    "strings"
)

type Base struct {
    reader *strings.Reader
}

func NewBase() *Base {
    r := strings.NewReader("hello")

    fmt.Printf("document: %#+v\n\n", &r)
    return &Base{r}
}

func (b *Base) Check() {
    fmt.Printf("document: %#+v\n\n", &b.reader)

}

type Concrete struct {
    *Base
}

func NewConcrete() *Concrete {
    return &Concrete{NewBase()}
}

func main() {
    c := NewConcrete()
    c.Check()
}

Why are these addresses not the same? How do I fix this?

OneOfOne · Accepted Answer · 2014-08-10 18:17:26Z

3

You're checking the address of the pointer, not the pointer itself.

func NewBase() *Base {
    r := strings.NewReader("hello")

    fmt.Printf("document: %#p\n\n", r)
    return &Base{r}
}

func (b *Base) Check() {
    fmt.Printf("document: %#p\n\n", b.reader)

}

playground

//edit

r := strings.NewReader("hello")

r is a variable holding a pointer to strings.Reader, &r is the address of the variable holding the pointer to strings.Reader.

fmt.Printf("document: %#+v\n\n", &b.reader)

&b.reader is the address of the variable b.reader, which is holding the pointer of the strings.Reader from earlier.

answered Aug 10, 2014 at 18:17

OneOfOne

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