os.PathError没有实现错误

Question

The PathError type found in Golang's os library:

type PathError struct {
    Op   string
    Path string
    Err  error
}

func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }

nearly fulfills Go's error interface:

type error interface {
    Error() string
}

However, when trying to pass it as an error, you get the following compile time error:

cannot use (type os.PathError) as type error in argument... 
os.PathError does not implement error (Error method has pointer receiver)

Why would os.PathError use a pointer receiver for the Error method, and just avoid meeting the requirements of the error interface?

Full example:

package main

import (
    "fmt"
    "os"
)

func main() {
    e := os.PathError{Path: "/"}
    printError(e)
}

func printError(e error) {
    fmt.Println(e)
}

Answer 1

Read about method sets here: https://golang.org/ref/spec#Method_sets

The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T)

You are trying to call a function taking the error interface using type os.PathError. According to the above, it does not implement Error() string, because that method is defined on type *os.PathError.

Having os.PathError you can get *os.PathError using the & operator:

printError(&os.PathError{...})