Creating a array of extended types

Question

I have a struct A, extending ("subclassing") it with struct B, like this:

package main

type A struct {
    x int
}

type B struct {
    A
    y int
}

I want to create a array where I can append A or B to it, so that this code works:

func main() {
    var m [2]B

    m[0] = B { A { 1 }, 2 }
    m[0].x = 0
    m[0].y = 0

    m[1] = A { 3 }
    m[1].x = 0
}

It doesn't. If I create the array of the type B, I get "cannot use struct literal (type A) as type B in assignment". If I try to create the array of the type A, I get the the same error (just with the types reversed).

So my question is: which type should the array be?

peterSO · Accepted Answer · 2011-07-11 22:01:20Z

3

You could use struct values. For example,

package main

import "fmt"

type A struct {
    x int
}

type B struct {
    A
    y int
}

func main() {
    var m []interface{}
    m = append(m, B{A{1}, 2})
    m = append(m, A{3})
    fmt.Println(m[0], m[1])
    if b, ok := m[0].(B); ok {
        b.x = 0
        b.y = 0
        m[0] = b
    }
    if a, ok := m[1].(A); ok {
        a.x = 0
        m[1] = a
    }
    fmt.Println(m[0], m[1])
}

Output:
{{1} 2} {3}
{{0} 0} {0}

Or, you could use struct pointers. For example,

package main

import "fmt"

type A struct {
    x int
}

type B struct {
    A
    y int
}

func main() {
    var m []interface{}
    m = append(m, &B{A{1}, 2})
    m = append(m, &A{3})
    fmt.Println(m[0], m[1])
    if b, ok := m[0].(*B); ok {
        b.x = 0
        b.y = 0
    }
    if a, ok := m[1].(*A); ok {
        a.x = 0
    }
    fmt.Println(m[0], m[1])
}

Output:
&{{1} 2} &{3}
&{{0} 0} &{0}

edited Jul 11, 2011 at 22:01

answered Jul 11, 2011 at 21:29

peterSO

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1 Comment

André Wagner Over a year ago

Ok, thank you! I suppose my question now is: is this idiomatic? Am I doing in the "Go" way? I ask this because it seems a lot of code just to access the field of a "superstruct".

Tyler Egeto · Accepted Answer · 2011-07-11 20:12:00Z

2

You'll want to define the array type to interface{} rather than B. Then you can store both types in there. That's the only way to accomplish this. If both types implement a specific interface, then you can type to that instead of the generic interface{}

answered Jul 11, 2011 at 20:12

Tyler Egeto

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3 Comments

André Wagner Over a year ago

It doesn't work. Using var m [2]interface{} gets me test.go:16: m[0].x undefined (type interface { } has no field or method x) test.go:17: m[0].y undefined (type interface { } has no field or method y) test.go:20: m[1].x undefined (type interface { } has no field or method x)

Tyler Egeto Over a year ago

You'll have to do a type cast when you pull it out, see here: golang.org/doc/go_spec.html#Type_assertions

André Wagner Over a year ago

Then what'd be the correct syntax to change a field? I tried to replace m[0].x = 0 for m[0].(B).x = 0, but now I get "cannot assign to m[0].(B).A.x".

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