a label can only be part of a statement and a declaration is not a statement

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.

在switch语句的最后一个case之前的所有case中，如果不加{}不能声明变量的，为了防止后面的case没有定义等dentistry

case 1:
     int x;
case 2:
    x = 0;

http://stackoverflow.com/questions/92396/why-cant-variables-be-declared-in-a-switch-statement