我可以把code例之外的交换机？ [英] Can I put code outside of cases in a switch?

问题描述

假设：

switch ( test ) {

  // Set some variables, call some functions ?
  int x = 1 ;
  int y = function(x) ;
  //

  case 1 : 
    // Process for test = 1
    ...
    break;

  case 5 : 
    // Process for test = 5
    ...
    break;

  default : 
    // Process for all other cases.
    ...

}

是不是法律来执行code的额外位我的第一个情况前加入？我从来没有见过这样的例子。

Is it 'legal' to execute that extra bit of code I added before the first case? I've never seen this in examples.

推荐答案

一是如何交换机上的一些背景（真）的作品：

First some background on how switch (really) works:

A 开关通常被看作是选择一张code的依赖于一些前pression的值来执行，如在结构

A switch is usually thought of as a construct that selects a piece of code to execute depending on the value of some expression, as in

switch (x) {
case 1:
    foo();
    break;

case 2:
    bar();
    break;
}

然而，这是更准确的认为开关作为计算转到语句的形式。例如，以下是完全合法的：

However, it's more accurate to think of a switch as a form of computed goto statement. For example, the following is perfectly legal:

switch (x) {
    puts("I can't be reached");
case 1:
    if (cond) {
case 2:
        puts("Either x == 1 && cond, or x == 2");
    }
}

根据值 X ，程序将跳转为案例1 或案例2 （或过去开关如果 X 既不是1也不是2）。

Depending on the value of x, the program will jump to either case 1 or case 2 (or past the switch if x is neither 1 nor 2).

您的程序将编译为C（与 X 和是的开关，因为初始化被跳过），而不是像C ++。原因是，C ++不允许跳转到一个情况标签跨越一个变量的初始化。对于简单的类型，如 INT ，跳过 INT X; 是允许的（因为没有初始化参与），而不是跳过在 INT X = 1;

Your program will compile as C (with junk values for x and y inside the switch, since the initializations are skipped), but not as C++. The reason is that C++ does not allow a jump to a case label to cross the initialization of a variable. For simple types like int, skipping over int x; is allowed (since no initialization is involved), but not skipping over int x = 1;.

造成这种差别的主要动机是可能是让跳转到一个情况标签交叉使用C初始化时++构造参与将是不安全的。例如，如果C ++允许一个情况标签的定义之后发生 My_class my_object 某些范围内，则跳转到情况标签将跳过 my_object 的构造函数，但在退出作用域时仍然运行其析构函数。

The main motivation for this difference is probably that letting a jump to a case label cross an initialization in C++ would be unsafe when constructors are involved. For example, if C++ allowed a case label to occur after a definition My_class my_object within some scope, then jumping to that case label would skip my_object's constructor but still run its destructor when exiting the scope.

同样的限制适用于转到在C ++中。你不能用它跳进块，过去变量的初始化。

The same restrictions apply to goto in C++. You can't use it to jump into a block and past a variable initialization.

作为一个方面说明，开关遵循相同的一般语法为如果和而。如果作为C11标准（ISO / IEC 9899：2011，第6.8.4节）给出的的语法为

As a side note, switch follows the same general syntax as if and while. The syntax of if as given in the C11 standard (ISO/IEC 9899:2011, section 6.8.4) is

如果（例如pression）语句

if ( expression ) statement

，而开关的语法是

开关（例如pression）语句

switch ( expression ) statement

据的语句的来讲唯一的区别（在C - C ++增加了上述一些限制）是它允许包含情况标签（和破）的开关而不是一个如果（除非如果 A 开关内发生）。

The only difference as far as statement is concerned (in C -- C++ adds some more limitations as mentioned above) is that it is allowed to contain case labels (and break) for a switch but not for an if (unless the if occurs within a switch).

就像用如果，你甚至可以离开花括号，写code如下所示。 （这是否是不必要的混乱是另一个讨论。）

Just as with an if, you can even leave off the braces and write code like the following. (Whether this is unnecessarily confusing is another discussion.)

switch (x) case 1: case 2: puts("x is 1 or 2");

在语法上，情况和默认标签属于同一类别为转到标签。在C11标准的第6.8.1节定义如下：

Syntactically, case and default labels belong in the same category as goto labels. Section 6.8.1 of the C11 standard has the following definition:

的标记的语句：的结果
  ＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;的标识的： 语句的结果
  ＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP; 情况 恒前pression 的： 语句的结果
  ＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP;＆NBSP; 默认 语句的

labeled-statement:
        identifier : statement
        case constant-expression : statement
        default : statement

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