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(+example of the C++17 switch-with-init syntax (taken from the Tony Tables example))
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{{par | {{spar|condition}} | any {{rlp|expressions|expression}} of integral or enumeration type, or of a class type {{rlp|implicit cast|contextually implicitly convertible}} to an integral or enumeration type, or a {{rlp|declarations|declaration}} of a single non-array variable of such type with a brace-or-equals {{rlp|initialization|initializer}}.}}
 
{{par | {{spar|condition}} | any {{rlp|expressions|expression}} of integral or enumeration type, or of a class type {{rlp|implicit cast|contextually implicitly convertible}} to an integral or enumeration type, or a {{rlp|declarations|declaration}} of a single non-array variable of such type with a brace-or-equals {{rlp|initialization|initializer}}.}}
 
{{par | {{spar|init-statement}}{{mark c++17}} | either  
 
{{par | {{spar|init-statement}}{{mark c++17}} | either  
−
* an {{rlp|statements|expression statement}} (which may be a ''null statement'' "{{ttb|;}}")
+
* an {{rlp|statements#Expression_statements|expression statement}} (which may be a ''null statement'' "{{ttb|;}}")
 
* a  {{rlp|declarations|simple declaration}}, typically a declaration of a variable with initializer, but it may declare arbitrary many variables or structured bindings  
 
* a  {{rlp|declarations|simple declaration}}, typically a declaration of a variable with initializer, but it may declare arbitrary many variables or structured bindings  
 
:Note that any {{spar|init-statement}} must end with a semicolon {{ttb|;}}, which is why it is often described informally as an expression or a declaration followed by a semicolon.}}
 
:Note that any {{spar|init-statement}} must end with a semicolon {{ttb|;}}, which is why it is often described informally as an expression or a declaration followed by a semicolon.}}

Revision as of 03:24, 15 September 2018

 
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Transfers control to one of the several statements, depending on the value of a condition.

Contents

Syntax

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

Explanation

The body of a switch statement may have an arbitrary number of case: labels, as long as the values of all constant_expressions are unique (after conversions/promotions). At most one default: label may be present (although nested switch statements may use their own default: labels or have case: labels whose constants are identical to the ones used in the enclosing switch)

If condition evaluates to the value that is equal to the value of one of constant_expressions, then control is transferred to the statement that is labeled with that constant_expression.

If condition evaluates to the value that doesn't match any of the case: labels, and the default: label is present, control is transferred to the statement labeled with the default: label.

The break statement, when encountered in statement exits the switch statement: 

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

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.

(since C++17)

Because transfer of control is not permitted to enter the scope of a variable, if a declaration statement is encountered inside the statement, it has to be scoped in its own compound statement: 

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;
}
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;
}

Keywords

switch, case, default

Example

The following code shows several usage cases of the switch statement

Run this code
#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;
    }
 
    // the C++17 init-statement syntax can be helpful when there is
    // no implicit conversion to integral or enumeration type
    switch (Device dev = get_device(); dev.state())
    {
       case SLEEP: /*...*/ break;
       case READY: /*...*/ break;
       case BAD: /*...*/ 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
}

Output: 

2345
d
red
1

See also

C documentation for switch
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