golang 中的 sizeof 以及 golang中的 union

golang 中的 sizeof:

1:

int(unsafe.Sizeof(uint32(0)))

2:

int(reflect.TypeOf(uint32(0)).Size())

golang中的 union:

package main

import (
    "fmt"
    "reflect"
    "unsafe"
)

type I struct {
    a int32
}

type B struct {
    c []int16
}

func main() {
    a := I{0x060302}
    b := (*B)(unsafe.Pointer(&a))
    fmt.Printf("%x, %d\n", a.a, a.a)
    fmt.Printf("%v\n", b.c)
    fmt.Printf("%v\n", b.c[])
    b.c[] = 0x0008
    fmt.Printf("%v\n", b.c)
    fmt.Printf("%x, %d\n", a.a, a.a)
    fmt.Printf("%d\n", reflect.TypeOf(b.c).Size())
    fmt.Printf("%d\n", reflect.TypeOf(a).Size())
}

注意，如果你要以 (*B)(unsafe.Pointer(&a)) 这种方式来作为 union，必须保证这两个union的类型字节大小一样大，golang不会为你检查越界。上面的例子就越界了，编译和运行时，golang没有任何警告。

下面是更完善的实现:

package main

import (
    "fmt"
    "reflect"
    "unsafe"
)

// ----- union begin ---------------------------------------------------
type IorBUnion interface {
    toB() *B
    toI() *I
}

type I struct {
    a int32
}

func (i *I) toB() *B {
    return (*B)(unsafe.Pointer(i))
}

func (i *I) toI() *I {
    return i
}

type B struct {
    c []int16
}

func (b *B) toB() *B {
    return b
}

func (b *B) toI() *I {
    return (*I)(unsafe.Pointer(b))
}

// ------- union end -------------------------------------------------

type myStruct struct {
    iOrB IorBUnion
    aaa  int
}

func main() {
    a := &I{0x060302}
    mystruct := myStruct{a, }
    b := (*B)(unsafe.Pointer(a))
    fmt.Printf("%x, %d\n", a.a, a.a)
    fmt.Printf("%v\n", b.c)
    fmt.Printf("%v\n", b.c[])
    b.c[] = 0x0008
    fmt.Printf("%v\n", b.c)
    fmt.Printf("%x, %d\n", a.a, a.a)
    fmt.Printf("%d\n", reflect.TypeOf(b.c).Size())
    fmt.Printf("%d\n", reflect.TypeOf(a).Size())
    fmt.Println(b.toB())
    fmt.Println(b.toI())
    fmt.Println(b.toI().toB())
    fmt.Println(a.toI().toB())
    fmt.Println(mystruct.iOrB.toI().toB())
}