Question

I am looking for help understanding how to access struct fields that are inside a container.vector.Vector.

The following code:

package main

import "fmt"
import "container/vector"

func main() {
    type Hdr struct {
        H string
    }
    type Blk struct {
        B string
    }

    a := new(vector.Vector)

    a.Push(Hdr{"Header_1"})
    a.Push(Blk{"Block_1"})

    for i := 0; i < a.Len(); i++ {
        fmt.Printf("a.At(%d) == %+v\n", i, a.At(i))
        x := a.At(i)
        fmt.Printf("%+v\n", x.H)
    }
}

Produces the error prog.go:22: x.H undefined (type interface { } has no field or method H)

removing lines 21 and 22 produces:

a.At(0) == {H:Header_1}
a.At(1) == {B:Block_1}

So, how exactly does one access ‘H’ or ‘B’? It seems like I need to convert those interfaces to structs, but… I dunno. I’m at a loss.

Thanks for any help.

Answer 1

Use a Go type switch or type assertion to distinguish between the Hdr and Blk types. For example,

package main

import (
    "fmt"
    "container/vector"
)

func main() {
    type Hdr struct {
        H string
    }
    type Blk struct {
        B string
    }

    a := new(vector.Vector)

    a.Push(Hdr{"Header_1"})
    a.Push(Blk{"Block_1"})

    for i := 0; i < a.Len(); i++ {
        fmt.Printf("a.At(%d) == %+v\n", i, a.At(i))
        x := a.At(i)
        switch x := x.(type) {
        case Hdr:
            fmt.Printf("%+v\n", x.H)
        case Blk:
            fmt.Printf("%+v\n", x.B)
        }
    }
}

However, effective the weekly.2011-10-18 release:

The container/vector package has been deleted. Slices are better:
SliceTricks.

Therefore, for the latest releases,

package main

import "fmt"

func main() {
    type Hdr struct {
        H string
    }
    type Blk struct {
        B string
    }

    var a []interface{}

    a = append(a, Hdr{"Header_1"})
    a = append(a, Blk{"Block_1"})

    for i := 0; i < len(a); i++ {
        fmt.Printf("a[%d]) == %+v\n", i, a[i])
        x := a[i]
        switch x := x.(type) {
        case Hdr:
            fmt.Printf("%+v\n", x.H)
        case Blk:
            fmt.Printf("%+v\n", x.B)
        }
    }
}