调用提升函数时发生未定义的错误

Question

I am getting an undefined error when using a promoted function in GO.

import "log"
type logger struct {
    log.Logger
}

logError := logger.New(os.Stderr, "ERROR", log.LstdFlags)
logOut := logger.New(os.Stdout, "INFO", log.LstdFlags)

This results in compiler error:

logger.go: logger.New undefined (type logger has no method New)

However, New is a promoted function of the log.Logger. What is wrong here?

Answer 1

There is no Logger.New() method, only a log.New() function that returns a value of *log.Logger:

func New(out io.Writer, prefix string, flag int) *Logger

Generated go doc deliberately lists such "constructor" functions under the type they return so you can easily find them, but this grouping doesn't make them methods. log.New() has no receiver so it cannot be a method, only a function.

You may use log.New() to obtain a value of type *log.Logger, which if you dereference, you get a value of type log.Logger, the value you embed.

However, if something is handed to you as a pointer in the first place, that is a strong indication that you should work with it as a pointer (and shouldn't dereference it). So I'd embed *log.Logger, methods will still get promoted (because they have pointer receiver). This is how you could initialize your loggers:

type logger struct {
    *log.Logger
}

var logError = logger{
    Logger: log.New(os.Stderr, "ERROR", log.LstdFlags),
}

var logOut = logger{
    Logger: log.New(os.Stdout, "INFO", log.LstdFlags),
}

Although if you add nothing else to your logger type, I don't see its existence justified, you could use *log.Logger as well.