switch statement

开关语句

根据条件的值，将控制转移到多个语句之一。

句法

attr(optional) switch ( condition ) statement	attr(optional) switch ( condition ) statement	(until C++17)
attr(optional) switch ( condition ) statement
attr(optional) switch ( init-statement(optional) condition ) statement	attr(optional) switch ( init-statement(optional) condition ) statement	(since C++17)
attr(optional) switch ( init-statement(optional) condition ) statement

attr(C++11)	-	any number of attributes
condition	-	any expression of integral or enumeration type, or of a class type contextually implicitly convertible to an integral or enumeration type, or a declaration of a single non-array variable of such type with a brace-or-equals initializer.
init-statement(C++17)	-	either an expression statement (which may be a null statement ";") a simple declaration, typically a declaration of a variable with initializer, but it may declare arbitrary many variables or structured bindings Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.
statement	-	any statement (typically a compound statement). case: and default: labels are permitted in statement and break; statement has special meaning.

安表达式语句%28，这可能是空语句“;“%29

阿简单声明，通常是带有初始化项的变量声明，但它可以声明任意多个变量或结构化绑定。

注意，任何init语句都必须以分号结尾。;，这就是为什么它经常被非正式地描述为一个表达式或一个声明，后面跟着分号。声明-任何陈述%28通常是复合语句%29。case:和default:在声明中允许使用标签break;语句具有特殊的含义。

attr(optional) case constant_expression : statement	(1)
attr(optional) default : statement	(2)

constant_expression	-	a constant expression of the same type as the type of condition after conversions and integral promotions

解释

开关语句的主体可能有任意数目的case:标签，只要所有常量的值[医]转换/升级后的表达式是唯一的%28%29。最多一次default:标签可能存在%28，尽管嵌套的开关语句可能使用它们自己的default:标签或有case:其常量与封闭开关%29中使用的相同的标签。

如果条件计算为等于常量值的值。[医]表达式，然后将控件转移到标记为该常量的语句。[医]表情。

如果条件计算为不匹配%27T的值，则为case:标签和default:标签存在，则控件被转移到标记为default:标签。

大破断语句中遇到的开关语句退出：

二次

switch(1) {
    case 1 : cout << '1'; // prints "1",
    case 2 : cout << '2'; // then prints "2"
}

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switch(1) {
    case 1 : cout << '1'; // prints "1"
             break;       // and exits the switch
    case 2 : cout << '2';
             break;
}

二次

Compilers may issue warnings on fallthrough (reaching the next case label without a break) unless the attribute [fallthrough] appears immediately before the case label to indicate that the fallthrough is intentional. If init-statement is used, the switch statement is equivalent to. { init_statement switch ( condition ) statement } Except that names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a declaration) are in the same scope, which is also the scope of statement.	{ init_statement switch ( condition ) statement }			(since C++17)
{ init_statement switch ( condition ) statement }

因为控制权的转移不允许进入范围对于变量，如果在语句中遇到声明语句，则必须在其自己的复合语句中限定其作用域：

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switch(1) {
    case 1: int x = 0; // initialization
            std::cout << x << '\n';
            break;
    default: // compilation error: jump to default: would enter the scope of 'x'
             // without initializing it
             std::cout << "default\n";
             break;
}

二次

switch(1) {
    case 1: {  int x = 0;
               std::cout << x << '\n';
               break;
            } // scope of 'x' ends here
    default: std::cout << "default\n"; // no error
             break;
}

二次

关键词

switch，，，case，，，default...

例

下面的代码显示了开关声明。

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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 is executed
    }
 
    std::cout << '\n';
 
    switch (i) {
        case 4: std::cout << "a";  //nothing is executed
    }
 
    // when enumerations are used in a switch statement, many compilers
    // issue warnings if one of the enumerators is not handled
    enum color {RED, GREEN, BLUE};
    switch(RED) {
        case RED:   std::cout << "red\n"; break;
        case GREEN: std::cout << "green\n"; break;
        case BLUE:  std::cout << "blue\n"; break;
    }
 
    // pathological examples
 
    // the statement doesn't have to be a compound statement
    switch(0)
        std::cout << "this does nothing\n";
 
    // labels don't require a compound statement either
    switch(int n = 1)
        case 0:
        case 1: std::cout << n << '\n';
 
    // Duff's Device: http://en.wikipedia.org/wiki/Duff's_device
}

二次

产出：

二次

2345
d
red
1

二次

另见

C开关文件

*。

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