在Go中,为什么我的Stringer接口方法不被调用?使用fmt.Println时 [英] In Go, 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
importfmt
类型Car结构{
年int
make字符串
}
func(c * Car)字符串()字符串{
返回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)并且所讨论的对象是一个指针时,我的String()方法被正确调用。如果,但是该对象是一个值,我的输出将使用Go中内置的默认格式进行格式化,而我的代码将不调用该对象的格式。
有趣的事情无论是哪种情况,如果我手动调用myCar.String(),它可以正常工作,无论我的对象是指针还是值。
如何使对象格式化无论在与Println一起使用时,对象是基于值还是基于指针?
我不希望为String使用值方法,因为那意味着每次调用时都会复制该对象,这些接口不合理。我不想总是手动调用.String(),因为我试图让duck-typing系统做到这一点。
谢谢
-Ralph
fmt.Println
, myCar
隐式转换为类型接口{}
你可以从函数签名中看到。然后, fmt
包中的代码执行类型切换如何打印这个值,看起来像这样: switch v:= v。(type){
case string:
os.Stdout.WriteString(v)
case fmt.Stringer:
os.Stdout.WriteString(v.String())
// ...
然而, fmt.Stringer $ c $因为
Car 没有实现
字符串
(因为它在 *车
)。调用 String
手动工作是因为编译器发现 * Car $ c> $ c>,并自动将
myCar.String()
转换为(& myCar).String()
。对于有关接口的任何事情,您必须手动完成。因此,您必须在 Car
上实现 String
,或者总是将指针传递给 fmt.Println
:
fmt.Println(&myCar)
Suppose I have the following code:
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)
}
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.
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.
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?
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.
Thanks in advance!
-Ralph
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())
// ...
}
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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