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)更多推荐
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