在 golang 中，为什么我不能将结构用作嵌套结构类型？

Question

以下代码（play）：

func main() {
    buf := bytes.NewBuffer(make([]byte, 0))
    rw := bufio.NewReadWriter(bufio.NewReader(buf), bufio.NewWriter(buf))
    var r *bufio.Writer
    r = rw
}

给出以下编译时错误：

cannot use rw (type *bufio.ReadWriter) as type *bufio.Writer in assignment

我期望的是将结构用作嵌套结构类型。但是如果我把r声明为io.Reader，这样就可以了，那我是不是应该转到interface？

Answer 1

不同类型无法赋值，GO不支持扩展

Answer 2

bufio.NewReadWriter() returns a concrete type, a pointer to a struct and bufio.Writer is also a concrete type, a struct. Neither *ReadWriter and *bufio.Writer is an interface!

Go 中没有自动类型转换，您不能将不同具体类型的值赋给变量。

您有 2 个选择：

由于 bufio.ReadWriter 嵌入了 *bufio.Writer，您可以简单地引用它并在作业中使用它：
```
var r *bufio.Writer
r = rw.Writer
```
或者你可以声明r为一个io.Writer（它是一个接口类型）这样你就可以给它赋值rw因为rw 实现 io.Writer:
```
var r io.Writer
r = rw
```
尽管我认为在这种情况下创建 r 不是特别有用，因为无论何时您使用 r 也可以使用 rw.

查看 Go spec: Assignability:

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:

x's type is identical to T.

x's type V and T have identical underlying types and at least one of V or T is not a named type.

T is an interface type and x implements T.

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a named type.

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.

x is an untyped constant representable by a value of type T.

None 个案例适用于您的代码，因此它是无效的分配。

当r被声明为io.Writer时，是下面的情况，因此它是有效的：

T is an interface type and x implements T.

在 golang 中，为什么我不能将结构用作嵌套结构类型？

In golang why I can't use the struct as nested struct type?

go

interface

type-conversion

embedding

您有 2 个选择：