switch statement

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条件执行语句
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switch statement
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执行代码的整体参数值
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使用一个或几个，满分许多分支的代码根据积分值需要被执行.
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Used where one or several out of many branches of code need to be executed according to an integral value.
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目录

[编辑] 语法

switch ( expression ) {
case constant_expression1 :
statement1 （可选的）
case constant_expression2 :
statement2 （可选的）
... ... ...
case constant_expressionn :
statementn （可选的）
default: default_statement （可选的）

}

[编辑] 解释

expression应是一个表达式，转换为一个整数值.
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expression shall be an expression, convertible to an integer value.
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所有constant_expressions必须是常量表达式，转换为一个整数值，这是唯一在此switch声明
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All constant_expressions shall be constant expressions, convertible to an integer value, which is unique within this switch statement
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如果expression评估值，等于值的定义constant_expressioni，statementi（如果有的话）和所有后续语句（除了default_statement，如果存在的话）被执行。如果该值的expression不匹配任何的constant_expressions，在default_statement执行如果存在的话.
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If the expression evaluates to a value, equal to the value of one of the defined constant_expressioni, the statementi (if present) and all subsequent statements (except default_statement, if present) are executed. If the value of the expression does not match any of the constant_expressions, the default_statement is executed if present.
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要注意，它是有用的，如果后续语句的执行是不可取的，<div class="t-tr-text"> break语句
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break statement
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可以使用。在这种情况下，执行的switch声明终止.
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It is useful to note, that if the execution of subsequent statements is undesirable, the
break语句
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break statement
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</div> can be used. In that case the execution of the switch statement terminates.
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[编辑] 关键字

switch, case, default

[编辑] 为例

下面的代码显示了几个在某些情况下，“开关”叙述
Original:
The following code shows several usage cases of the switch statement
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#include <iostream>
 
int main()
{
    int i = 2;
    switch (i) {
        case 1: std::cout << "1";
        case 2: std::cout << "2";   //execution starts at this case label
        case 3: std::cout << "3";
        case 4:
        case 5: std::cout << "45";
                break;              //execution of subsequent statements is terminated
        case 6: std::cout << "6";
    }
 
    std::cout << '\n';
 
    switch (i) {
        case 4: std::cout << "a";
        default: std::cout << "d"; //there are no applicable constant_expressions 
                                   //therefore default_statement is executed
    }
 
    std::cout << '\n';
 
    switch (i) {
        case 4: std::cout << "a";  //nothing is executed
    }
}

Output: 

2345
d
来自“http://zh.cppreference.com/mwiki/index.php?title=cpp/language/switch&oldid=28616”