为什么不能变量switch语句声明？ [英] Why can't variables be declared in a switch statement?

问题描述

我一直想知道这 - 你为什么不能在一个switch语句中的case标签后声明变量？在C ++中，可以声明变量pretty随时随地得多（并宣布他们接近第一次使用显然是一件好事），但下面依然是行不通的：

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;
}

以上给了我下面的错误（MSC）：

The above gives me the following error (MSC):

的newval'的初始化是由案例标签

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

这似乎是在其他语言的限制太多。这是为什么这样的问题？

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

推荐答案

案例声明仅是标签。这意味着，编译器会间preT这与直接跳转到的标签。在C ++中，这里的问题是范围之一。你花括号定义范围为开关语句中的一切。这意味着你只剩下在那里跳将进一步进行到code跳过初始化范围。以正确的操作方法是定义特定于该情况下，语句的范围，并在其中定义变量。

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. 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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