Difference between revisions of "cpp/language/switch"
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===Explanation=== | ===Explanation=== | ||
| − | The body of a switch statement may have an arbitrary number of {{tt|case:}} labels, as long as the values of all {{spar|constant_expressions}} are unique (after conversions/promotions). | + | The body of a switch statement may have an arbitrary number of {{tt|case:}} labels, as long as the values of all {{spar|constant_expressions}} are unique (after conversions/promotions). At most one {{tt|default:}} label may be present (although nested switch statements may use their own {{tt|default:}} labels or have {{tt|case:}} labels whose constants are identical to the ones used in the enclosing switch) |
If {{spar|condition}} evaluates to the value that is equal to the value of one of {{spar|constant_expression}}s, then control is transferred to the statement that is labeled with that {{spar|constant_expression}}. | If {{spar|condition}} evaluates to the value that is equal to the value of one of {{spar|constant_expression}}s, then control is transferred to the statement that is labeled with that {{spar|constant_expression}}. | ||
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| + | ===See also=== | ||
| + | {{dsc begin}} | ||
| + | {{dsc see c | c/language/switch}} | ||
| + | {{dsc end}} | ||
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[[de:cpp/language/switch]] | [[de:cpp/language/switch]] | ||
Revision as of 07:10, 26 December 2014
Transfers control to one of the several statements, depending on the value of a condition.
Contents |
Syntax
attr(optional) switch ( condition ) statement
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| 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. |
| statement | - | any statement (typically a compound statement). case: and default: labels are permitted in statement and break; statement has special meaning.
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attr(optional) case constant_expression : statement
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(1) | ||||||||
attr(optional) default : statement
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(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; }
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:
Keywords
Example
The following code shows several usage cases of the switch statement
#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 }
Output:
2345 d red 1
See also
| C documentation for switch
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