为什么不能在 switch 语句中声明变量? [英] Why can't variables be declared in a switch statement?

问题描述

我一直想知道 - 为什么不能在 switch 语句中的 case 标签后声明变量?在 C++ 中，您几乎可以在任何地方声明变量(并且在第一次使用时声明它们显然是一件好事)但以下仍然不起作用:

I've always wondered this - why can't you declare variables after a case label in a switch statement? In C++ you can declare variables pretty much anywhere (and declaring them close to first use is obviously a good thing) but the following still won't work:

switch (val)  
{  
case VAL:  
  // This won't work
  int newVal = 42;  
  break;
case ANOTHER_VAL:  
  ...
  break;
}  

以上给了我以下错误(MSC):

The above gives me the following error (MSC):

'newVal' 的初始化被 'case' 标签跳过

initialization of 'newVal' is skipped by 'case' label

这似乎也是其他语言的限制.为什么会出现这样的问题?

This seems to be a limitation in other languages too. Why is this such a problem?

推荐答案

Case 语句只是标签.这意味着编译器会将其解释为直接跳转到标签.在 C++ 中，这里的问题是范围之一.您的大括号将范围定义为 switch 语句中的所有内容.这意味着您将留下一个范围，在该范围内将进一步跳转到跳过初始化的代码中.

Case statements are only labels. This means the compiler will interpret this as a jump directly to the label. In C++, the problem here is one of scope. Your curly brackets define the scope as everything inside the switch statement. This means that you are left with a scope where a jump will be performed further into the code skipping the initialization.

处理此问题的正确方法是定义特定于该 case 语句的范围并在其中定义您的变量:

The correct way to handle this is to define a scope specific to that case statement and define your variable within it:

switch (val)
{   
case VAL:  
{
  // This will work
  int newVal = 42;  
  break;
}
case ANOTHER_VAL:  
...
break;
}

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