casting overlapping structs in golang

Question

I'm new to golang and trying to figure out the correct way of casting a block of bytes to the correct struct. All structs start with two bytes that dictate the layout of the remaining bytes. In CI would point to the beginning of the block of memory and cast it as a simple struct that only contained those two bytes (X below) but here I get an invalid type assertion. I'm probably way off base here any help you be appreciated.

package main

import (
    "fmt"
)

type A struct {
    tag   byte
    ver   byte
    data1 int
    data2 int
    data3 int
}

type B struct {
    tag   byte
    ver   byte
    data1 float32
}

type X struct {
    tag byte
    ver byte
}

func main() {
    var a A
    a.tag = 1
    a.ver = 1
    x := a.(X)

    fmt.Printf("%d,%d", x.tag, x.ver)
}

playground link

Answer 1

Go generally tries to discourage C-like memory fiddling as it leads to memory leaks, incorrect behavior, and security vulnerabilities unless extraordinary caution and testing are applied. That doesn't mean it's impossible though; in fact, the aptly-named unsafe.Pointer is exposed for exactly this purpose. Use it with caution.

Answer 2

Here's my solution. It involves a few different tips:

1. Embed the shared struct in the individual structs.
2. Use encoding/binary package to load bytes into structs.
3. Fill header struct with first two bytes, then make a decision on which subtype to make and fill.
4. Always use fixed length int types for this kind of thing.
5. Your field names must be UpperCase to be fillable from encoding/binary
6. This is a pretty brittle way to manage marshalling.unmarshalling of data, but I'm sure you know that.

Here's my solution:

package main

import (
    "bytes"
    "encoding/binary"
    "fmt"
    "log"
)

type A struct {
    X
    Data1 int32
    Data2 int32
    Data3 int32
}

type B struct {
    X
    Data1 int32
}

type X struct {
    Tag byte
    Ver byte
}

func main() {
    var err error
    data := []byte{1, 1, 0, 0, 0, 42}
    hdr := X{}

    err = binary.Read(bytes.NewReader(data[:2]), binary.BigEndian, &hdr)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(hdr.Tag, hdr.Ver)

    if hdr.Tag == 1 {
        b := B{}
        err = binary.Read(bytes.NewReader(data), binary.BigEndian, &b)
        if err != nil {
            log.Fatal(err)
        }
        fmt.Println(b.Data1)
    }

}

playground link