问题描述
我想知道为什么 golang 的类型 switch 语句中不允许 fallthrough.
I'm wondering why fallthrough isn't allowed in a type switch statement in golang.
根据规范:类型切换中不允许使用fallthrough"语句.",这不详细解释为什么不允许这样做.
According to the specification: "The "fallthrough" statement is not permitted in a type switch.", which doesn't explain much about WHY it isn't allowed.
附加的代码是为了模拟一个可能的场景,如果类型 switch 语句中的失败可能有用.
The code attached is to simulate a possible scenario were a fallthrough in a type switch statement might have been useful.
注意!这段代码不起作用,它会产生错误:cannot fallthrough in type switch".我只是想知道不允许在类型切换中使用 fallthrough 语句的可能原因是什么.
Notice! This code doesn't work, it will produce the error: "cannot fallthrough in type switch". I'm just wondering what possible reasons might have been for not allowing the fallthrough statement in a type switch.
//A type switch question
package main
import "fmt"
//Why isn't fallthrough in type switch allowed?
func main() {
//Empty interface
var x interface{}
x = //A int, float64, bool or string value
switch i := x.(type) {
case int:
fmt.Println(i + 1)
case float64:
fmt.Println(i + 2.0)
case bool:
fallthrough
case string:
fmt.Printf("%v", i)
default:
fmt.Println("Unknown type. Sorry!")
}
}
推荐答案
您希望 fallthrough
怎样工作?在此类型开关中,i
变量的类型取决于调用的特定情况.因此,在 case bool
中,i
变量的类型为 bool
.但在 case string
中,它被输入为 string
.所以要么你要求 i
神奇地改变它的类型,这是不可能的,或者你要求它被一个新的变量 i string
遮蔽,它没有值,因为它的值来自 x
,实际上它不是 string
.
How would you expect fallthrough
to work? In this type switch, the i
variable has a type that depends on the particular case that's invoked. So in the case bool
the i
variable is typed as bool
. But in case string
it's typed as string
. So either you're asking for i
to magically morph its type, which isn't possible, or you're asking for it to be shadowed by a new variable i string
, which will have no value because its value comes from x
which is not, in fact, a string
.
这是一个尝试说明问题的示例:
Here's an example to try and illustrate the problem:
switch i := x.(type) {
case int:
// i is an int
fmt.Printf("%T\n", i); // prints "int"
case bool:
// i is a bool
fmt.Printf("%T\n", i); // prints "bool"
fallthrough
case string:
fmt.Printf("%T\n", i);
// What does that type? It should type "string", but if
// the type was bool and we hit the fallthrough, what would it do then?
}
唯一可能的解决方案是让 fallthrough
导致随后的 case 表达式将 i
作为 interface{}
,但是将是一个令人困惑和糟糕的定义.
The only possible solution would be to make the fallthrough
cause the subsequent case expression to leave i
as an interface{}
, but that would be a confusing and bad definition.
如果你真的需要这种行为,你已经可以用现有的功能来实现:
If you really need this behavior you can already accomplish this with the existing functionality:
switch i := x.(type) {
case bool, string:
if b, ok := i.(bool); ok {
// b is a bool
}
// i is an interface{} that contains either a bool or a string
}
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