问题
package main
type Writeable interface {
OnWrite() interface{}
}
type Result struct {
Message string
}
func (r *Result) OnWrite() interface{} {
return r.Message
}
// what does this line mean? what is the purpose?
var _ Writeable = (*Result)(nil)
func main() {
}
The comments in the code snippet expressed my confusion. As I understood, the line with comment notifies the compiler to check whether a struct has implemented the interface, but I am not sure very much. Could someone help explaining the purpose?
回答1:
As you say it's a way to verify that Result implements Writeable. From the GO FAQ:
You can ask the compiler to check that the type T implements the interface I by attempting an assignment:
type T struct{} var _ I = T{} // Verify that T implements I.
The blank identifier _ stands for the variable name which is not needed here (and thus prevents a "declared but not used" error).
(*Result)(nil) creates an uninitialized pointer to a value of type Result by converting nil to *Result. This avoids allocation of memory for an empty struct as you'd get with new(Result) or &Result{}.
来源：https://stackoverflow.com/questions/30701755/have-trouble-understanding-a-piece-of-golang-code