对结构字段进行持续更改*并*满足 Writer 接口？

Question

为了实际更改struct方法中的字段，您需要一个指针类型的接收器。我明白那个。

为什么我不能满足io.Writer具有指针接收器的接口，以便我可以更改结构字段？有没有惯用的方法来做到这一点？

// CountWriter is a type representing a writer that also counts

type CountWriter struct {

    Count      int

    Output     io.Writer

}

func (cw *CountWriter) Write(p []byte) (int, error) {

    cw.Count++

    return cw.Output.Write(p)

}

func takeAWriter(w io.Writer) {

    w.Write([]byte("Testing"))

}

func main() {

    boo := CountWriter{0, os.Stdout}

    boo.Write([]byte("Hello\n"))

    fmt.Printf("Count is incremented: %d", boo.Count)

    takeAWriter(boo)

}

该代码产生此错误：

prog.go:27:13: cannot use boo (type CountWriter) as type io.Writer in argument to takeAWriter:

    CountWriter does not implement io.Writer (Write method has pointer receiver)

看来您既可以满足Writer界面要求，也可以对实际的struct. 如果我将 Write 方法更改为值接收器 ( func (cw CountWriter) Write...)，我可以避免错误，但值不会增加。:(

https://play.golang.org/p/pEUwwTj0zrb

Answer 1

boo不实现接口，因为 Write takes*CountWriter而不是 aCountWriter但是，&booWrite 会接受，所以你必须传递它。