英文:
downcast higher-type to lower
问题
除了手动复制内部Box值之外，是否有一种语言特性可以将RatedBox向下转换为Box？
type Box struct {
Name string
}
type RatedBox struct {
Box
Points int
}
func main() {
rated := RatedBox{Box: Box{Name: "foo"}, Points: 10}
box := Box(rated) // 不起作用
}
[go-playground](https://play.golang.org/p/iNpPgiIWqv)
// 对于具有更多成员的结构来说，这种方法可行，但比较冗长
box := Box{Name: rated.Name}
英文:
Is there beside copying the inner Box value by hand a language feature to down-cast RatedBox into a Box?
type Box struct {
Name string
}
type RatedBox struct {
Box
Points int
}
func main() {
rated := RatedBox{Box: Box{Name: "foo"}, Points: 10}
box := Box(rated) // does not work
}
// works, but is quite verbose for structs with more members
box := Box{Name: rated.Name}
答案1
得分: 9
在结构体中嵌入一个类型会向结构体添加一个字段，你可以使用未限定的类型名称来引用它（未限定意味着省略包名和可选的指针符号）。
例如：
box := rated.Box
fmt.Printf("%T %+v", box, box)
输出结果（在Go Playground上尝试）：
main.Box {Name:foo}
请注意，赋值会复制值，所以box局部变量将保存RatedBox.Box字段的值的副本。如果你希望它们是“相同的”（指向相同的Box值），可以使用指针，例如：
box := &rated.Box
fmt.Printf("%T %+v", box, box)
但是这里box的类型当然是*Box。
或者你可以选择嵌入指针类型：
type RatedBox struct {
*Box
Points int
}
然后（在Go Playground上尝试）：
rated := RatedBox{Box: &Box{Name: "foo"}, Points: 10}
box := rated.Box
fmt.Printf("%T %+v", box, box)
最后两个输出结果：
*main.Box &{Name:foo}
英文:
Embedding a type in a struct adds a field to the struct, and you can use the unqualified type name to refer to it (unqualified means omit the package name and the optional pointer sign).
For example:
box := rated.Box
fmt.Printf("%T %+v", box, box)
Output (try it on the Go Playground):
main.Box {Name:foo}
Note that assignment copies the value, so the box local variable will hold a copy of the value of the RatedBox.Box field. If you want them to be the "same" (to point to the same Box value), use a pointer, e.g.:
box := &rated.Box
fmt.Printf("%T %+v", box, box)
But here of course type of box will be *Box.
Or you may choose to embed the pointer type:
type RatedBox struct {
*Box
Points int
}
And then (try it on the Go Playground):
rated := RatedBox{Box: &Box{Name: "foo"}, Points: 10}
box := rated.Box
fmt.Printf("%T %+v", box, box)
Output of the last 2:
*main.Box &{Name:foo}
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