英文:
Is this type assertion ever valid?
问题
我正在阅读Go的io包的源代码，我遇到了一个我不完全理解的片段。在这里
func WriteString(w Writer, s string) (n int, err error) {
if sw, ok := w.(stringWriter); ok {
return sw.WriteString(s)
}
return w.Write([]byte(s))
}
其中
type stringWriter interface {
WriteString(s string) (n int, err error)
}
type Writer interface {
Write(p []byte) (n int, err error)
}
类型断言w.(stringWriter)断言w的动态类型（即Writer）实现了stringWriter接口。鉴于stringWriter和Writer的类型定义，我不明白这是如何可能的。假设这段代码是正确的，我错过了什么？
英文:
I am reading the source code for Go's io package and I came across a snippet I don't fully understand. Here it is
func WriteString(w Writer, s string) (n int, err error) {
if sw, ok := w.(stringWriter); ok {
return sw.WriteString(s)
}
return w.Write([]byte(s))
}
where
type stringWriter interface {
WriteString(s string) (n int, err error)
}
type Writer interface {
Write(p []byte) (n int, err error)
}
The type assertion w.(stringWriter) asserts that the dynamic type of w (i.e. Writer) implements the stringWriter interface. I don't see how this is possible given the type definitions of stringWriter and Writer. Assuming that this code is correct, what is it that I am missing?
答案1
得分: 5
你可以轻松地构建一个实现Writer和stringWriter的类型：
type StringWriter struct{}
func (s StringWriter) Write(in []byte) (int, error) {
return 0, nil
}
func (s StringWriter) WriteString(s string) (int, error) {
return s.Write([]byte(s))
}
因此，将StringWriter的实例传递给io.WriteString会调用StringWriter.WriteString。
io.WriteString的逻辑是：如果写入器具有WriteString方法，则使用该方法，否则回退到string到[]byte的转换。为了测试实例是否实现了某个方法，使用接口，因为接口只是方法集，可以轻松地进行测试。
英文:
You can easily build a type which implements Writer as well as stringWriter:
type StringWriter struct{}
func (s StringWriter) Write(in []byte) (int, error) {
return 0, nil
}
func (s StringWriter) WriteString(s string) (int, error) {
return s.Write([]byte(s))
}
So passing an instance of StringWriter to io.WriteString results in StringWriter.WriteString
being called.
The logic behind io.WriteString is: Use WriteString on the writer if it has such a method
or fall back to string to []byte conversion otherwise. To test whether the instance implements
a method or not, interfaces are used, as these are just method sets and can be tested for easily.
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