为什么我的Stringer接口方法没有被调用?使用fmt.Println时 [英] Why isn't my Stringer interface method getting invoked? When using fmt.Println

问题描述

假设我有以下代码:

package main

import "fmt"

type Car struct{
    year int
    make string
}

func (c *Car)String() string{
    return fmt.Sprintf("{make:%s, year:%d}", c.make, c.year)
}

func main() {
    myCar := Car{year:1996, make:"Toyota"}
    fmt.Println(myCar)
}

当我调用fmt.Println(myCar)且有问题的对象是指针时，我的String()方法将被正确调用.但是，如果该对象是一个值，则使用Go内置的默认格式对我的输出进行格式化，而不会调用格式化该对象的代码.

When I call fmt.Println(myCar) and the object in question is a pointer, my String() method gets called properly. If, however the object is a value, my output is formatted using the default formatting built into Go and my code to format the said object is not called.

有趣的是，无论哪种情况，如果我手动调用myCar.String()，无论我的对象是指针还是值，它都能正常工作.

The interesting thing is in either case if I call myCar.String() manually it works properly whether my object is either a pointer or value.

与Println一起使用时，无论对象是基于值还是基于指针，如何以我想要的方式格式化对象?

How can I get my object formatted the way I want no matter if the object is value-based or pointer-based when used with Println?

我不想为String使用value方法，因为那样就意味着每次调用它时，都会复制对象，从而产生不合理的接缝.而且我也不想总是手动调用.String()，因为我正试图让鸭子键入系统来工作.

I don't want to use a value method for String because then that means every time it's invoked the object is copied which seams unreasonable. And I don't want to have to always manually called .String() either because I'm trying to let the duck-typing system do it's work.

推荐答案

调用函数fmt.Println时，myCar被隐式转换为类型为interface{}的值，如您从函数签名中所见.然后，来自fmt包的代码会执行类型切换，以弄清楚如何打印此值，如下所示:

When calling fmt.Println, myCar is implicitly converted to a value of type interface{} as you can see from the function signature. The code from the fmt package then does a type switch to figure out how to print this value, looking something like this:

switch v := v.(type) {
case string:
    os.Stdout.WriteString(v)
case fmt.Stringer:
    os.Stdout.WriteString(v.String())
// ...
}

但是，由于Car没有实现String(如在*Car上定义的)，因此fmt.Stringer情况失败.手动调用String是有效的，因为编译器发现String需要一个*Car，从而自动将myCar.String()转换为(&myCar).String().对于任何有关接口的内容，您都必须手动进行.因此，您要么必须在Car上实现String，要么总是将指针传递给fmt.Println:

However, the fmt.Stringer case fails because Car doesn't implement String (as it is defined on *Car). Calling String manually works because the compiler sees that String needs a *Car and thus automatically converts myCar.String() to (&myCar).String(). For anything regarding interfaces, you have to do it manually. So you either have to implement String on Car or always pass a pointer to fmt.Println:

fmt.Println(&myCar)

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