Switch-Case：声明初始化和声明然后赋值

在 switch-case 语句中声明与初始化无效但是声明然后赋值被允许。如下面的代码片段所示。

从编译器方面来看，这两种类型的初始化有什么区别？为什么第一种类型的初始化无效而第二种类型的初始化有效。

switch(val)  
{  
case 0:  
  int newVal = 42;  //Invalid
  break;
case 1:  
  int newVal2;      //Valid
  newVal2 = 42;  
  break;
case 2:
  break;
}

实际上，规则是您不能跳入经过具有初始化的声明（或经过非 POD 类型变量的声明）的块。 C++ 标准规定 (C++03 §6.7)：

It is possible to transfer into a block, but not in a way that bypasses declarations with initialization. A program that jumps⁽⁷⁷⁾ from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer (8.5).

^{(*) The transfer from the condition of a switch statement to a case label is considered a jump in this respect.}

int newVal = 42;是一个具有初始值设定项的声明（= 42部分）。该程序格式错误，因为如果val is 1 or 2，您将跳入初始化后的 switch 块。

int newVal2;也是一个声明；因为int是 POD 类型并且声明没有初始化程序，您可以跳过此声明。