投射兼容但不同的地图片段

Question

I'm working with two libraries, and one defines a type:

type Attrs map[string]string

while the other defines:

type StringMap map[string]string

A function from the first library returns an []Attrs, and struct required by the other has a field as []StringMap which needs to be set. Attempting to either use a simple assignment, or a cast in the form of ([]StringMap)(attrs), just results in an error:

./wscmd.go:8:22: cannot convert attrs (type []mpd.Attrs) to type []StringMap

So, how can those be bridged?

Edit: Ok, apparently this is a language limitation (booo-hooo). Can it be stepped aside with unsafe pointers?

Answer 1

You can do this, but it circumvents Go's type safety, which can lead to trouble depending on implementation type.

package main
import (
    "fmt"
    "reflect"
    "unsafe"
)

func main() {
    type Attrs map[string]string
    type StringMap map[string]string
    a := Attrs{"key1": "val1", "key2": "val2"}
    b := Attrs{"key3": "val3", "key4": "val4"}

    attrs := []Attrs{a, b}

    // This is what you're asking for, keep in mind this circumvents the type safety provided by go

    sh := *(*reflect.SliceHeader)(unsafe.Pointer(&attrs))
    unsafeStrMaps := *(*[]StringMap)(unsafe.Pointer(&sh))
    fmt.Println(unsafeStrMaps)

    // This would be the preferred way of casting the array

    strMaps := []StringMap{}
    for _, v := range attrs {
        strMaps = append(strMaps, StringMap(v))
    }

    fmt.Println(strMaps)
}

It is much better for type safety to just iterate the []Attrs slice and append to a []StringMap.