在GO中从一个结构复制到另一个结构

Question

I have two structs:

type struct1 struct{
     arr [num1]byte
}

type struct2 struct{
     b1 [n1]uint64
     b2 [n2]uint64
     b3 [n3]uint64
}

Now, I have two pointers to these structs:

p1 := new(struct1);
p2 := new(struct2);

and after some computations, I want to copy p2 to a specified part of p1. Something like:

copy(p1.arr[k:], p2);

Where k is a positive integer. It can be easily done in C using memcpy, but I could not find an easy way to do it in GO without using any external libraries. Does anyone have any suggestions? I'll really appreciate it.

Answer 1

You want to bypass the Go type system, which is unsafe. Therefore, you should be very careful. Implement this as a function and use the Go testing package to write tests. Check for errors. Write simple, readable code. Don't be clever.

For example,

package main

import (
    "fmt"
    "unsafe"
)

const num1 = 42

type struct1 struct {
    arr [num1]byte
}

const (
    n1 = 1
    n2 = 2
    n3 = 1
)

type struct2 struct {
    b1 [n1]uint64
    b2 [n2]uint64
    b3 [n3]uint64
}

func arrInsert(p1 *struct1, i1 int, p2 *struct2) int {
    if p1 == nil || p2 == nil {
        return 0
    }
    if i1 < 0 || i1 >= len(p1.arr) {
        return 0
    }
    s1 := p1.arr[i1:]
    s2 := (*[unsafe.Sizeof(*p2)]byte)(unsafe.Pointer(p2))[:]
    return copy(s1, s2)
}

func main() {
    p1 := new(struct1)
    p2 := new(struct2)
    for i := range p2.b1 {
        p2.b1[i] = uint64(i + 10)
    }
    for i := range p2.b2 {
        p2.b2[i] = uint64(i + 20)
    }
    for i := range p2.b3 {
        p2.b3[i] = uint64(i + 30)
    }
    n := arrInsert(p1, 1, p2)
    fmt.Println(n)
    fmt.Println(*p1)
    fmt.Println((*[unsafe.Sizeof(*p2)]byte)(unsafe.Pointer(p2))[:])
    fmt.Println(*p2)
}

Playground: https://play.golang.org/p/KA0B0xpFR6l

Output:

32
{[0 10 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0 21 0 0 0 0 0 0 0 30 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]}
[10 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0 21 0 0 0 0 0 0 0 30 0 0 0 0 0 0 0]
{[10] [20 21] [30]}