Difference between revisions of "cpp/language/switch"
m (→Example: + [[fallthrough}] with note.) |
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| Line 113: | Line 113: | ||
case 1: std::cout << "1"; | case 1: std::cout << "1"; | ||
case 2: std::cout << "2"; // execution starts at this case label | case 2: std::cout << "2"; // execution starts at this case label | ||
| − | case 3: std::cout << "3"; | + | case 3: std::cout << "3"; [[fallthrough]]; // C++17 attribute to silent |
| − | case 4: | + | case 4: // the warning on fall through |
case 5: std::cout << "45"; | case 5: std::cout << "45"; | ||
break; // execution of subsequent statements is terminated | break; // execution of subsequent statements is terminated | ||
Revision as of 05:29, 15 September 2021
Transfers control to one of the several statements, depending on the value of a condition.
Contents |
Syntax
attr(optional) switch ( init-statement(optional) condition ) statement
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| attr | - | (since C++11) any number of attributes |
| init-statement | - | (since C++17) either
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| 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; }
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Compilers may issue warnings on fallthrough (reaching the next case label without a break) unless the attribute If init-statement is used, the switch statement is equivalent to
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:
Keywords
Example
The following code shows several usage cases of the switch statement
#include <iostream> int main() { const 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"; [[fallthrough]]; // C++17 attribute to silent case 4: // the warning on fall through 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 struct Device { enum { SLEEP, READY, BAD } state_{}; /*...*/ auto state() const { return state_; } }; switch (auto dev = Device{}; dev.state()) { case Device::SLEEP: /*...*/ break; case Device::READY: /*...*/ break; case Device::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'; }
Output:
2345 d red 1
See also
| Loop unrolling using Duff's Device | |
| Duff's device can be used to implement coroutines in C/C++ | |
| C documentation for switch
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