Go 编译器检测类型是否实现指定接口

开源库里会有一些类似下面这种奇怪的语法：

var _ io.Writer = (*myWriter)(nil)
var _ io.Writer = myWriter{
    
    }

这时候会有点懵，这其实是 编译器检查 *myWriter 类型是否实现了 io.Writer 接口。

来看一个例子：

package main

import "io"

type myWriter struct {
    
    

}

/*func (w myWriter) Write(p []byte) (n int, err error) {
	return
}*/

func main() {
    
    
    // 检查 *myWriter 类型是否实现了 io.Writer 接口
    var _ io.Writer = (*myWriter)(nil)

    // 检查 myWriter 类型是否实现了 io.Writer 接口
    var _ io.Writer = myWriter{
    
    }
}

注释掉为 myWriter 定义的 Write 函数后，运行程序：

src/main.go:14:6: cannot use (*myWriter)(nil) (type *myWriter) as type io.Writer in assignment:
	*myWriter does not implement io.Writer (missing Write method)
src/main.go:15:6: cannot use myWriter literal (type myWriter) as type io.Writer in assignment:
	myWriter does not implement io.Writer (missing Write method)

报错信息：*myWriter/myWriter 未实现 io.Writer 接口，也就是未实现 Write 方法。

解除注释后，运行程序不报错。

实际上，上述赋值语句会发生隐式地类型转换，在转换的过程中，编译器会检测等号右边的类型是否实现了等号左边接口所规定的函数。

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