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

[edit]Syntax

[edit]Condition

Acondition can either be anexpression or asimple declaration.

• If it can be syntactically resolved as an expression, it is treated as an expression. Otherwise, it is treated as a declaration that is not a structured binding declaration(since C++26).

When control reaches condition, the condition will yield a value, which is used to determine which label the control will go to.

[edit]Expression

Ifcondition is an expression, the value it yields is the the value of the expression.

[edit]Declaration

Ifcondition is a simple declaration, the value it yields is the value of the decision variable (see below).

[edit]Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• The declarator cannot specify afunction or anarray.
• Thetype specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers andconstexpr, and it(since C++11) cannot define aclass orenumeration.

The decision variable of the declaration is the declared variable.

[edit]Type

condition can only yield the following types:

• integral types
• enumeration types
• class types

If the yielded value is of a class type, it is contextually implicitly converted to an integral or enumeration type.

If the (possibly converted) type is subject tointegral promotions , the yielded value is converted to the promoted type.

[edit]Labels

Any statement within theswitch statement can be labeled with one or more following labels:

Acase ordefault label is associated with the innermostswitch statement enclosing it.

If any of the following conditions is satisfied, the program is ill-formed:

• Aswitch statement is associated with multiplecase labels whoseconstant-expression s have the same value after conversions.
• Aswitch statement is associated with multipledefault labels.

[edit]Control flow transfer

When the condition of aswitch statement yields a (possibly converted) value:

• If one of the associatedcase label constants has the same value, control is passed to the statement labeled by the matchedcase label.
• Otherwise, if there is an associateddefault label, control is passed to the statement labeled by thedefault label.
• Otherwise, none of the statements in theswitch statement will be executed.

case anddefault labels in themselves do not alter the flow of control. To exit from aswitch statement from the middle, seebreak statements.

Compilers may issue warnings on fallthrough (reaching the nextcase ordefault label without abreak) unless the attribute[[fallthrough]] appears immediately before thecase label to indicate that the fallthrough is intentional(since C++17).

switch(1){case1:std::cout<<'1';// prints "1",case2:std::cout<<'2';// then prints "2"}

switch(1){case1:std::cout<<'1';// prints "1"break;// and exits the switchcase2:std::cout<<'2';break;}

[edit]Notes

Because transfer of control isnot permitted to enter the scope of a variable, if a declaration statement is encountered inside thestatement, it has to be scoped in its own compound statement:

switch(1){case1:int x=0;// initializationstd::cout<< x<<'\n';break;default:// compilation error: jump to default:// would enter the scope of 'x' without initializing itstd::cout<<"default\n";break;}

switch(1){case1:{int x=0;std::cout<< x<<'\n';break;}// scope of 'x' ends heredefault:std::cout<<"default\n";// no errorbreak;}

[edit]Keywords

switch,case,default

[edit]Example

#include <iostream> int main(){constint i=2;switch(i){case1:std::cout<<'1';case2:// execution starts at this case labelstd::cout<<'2';case3:std::cout<<'3';[[fallthrough]];// C++17 attribute to silent the warning on fallthroughcase5:std::cout<<"45";break;// execution of subsequent statements is terminatedcase6:std::cout<<'6';} std::cout<<'\n'; switch(i){case4:std::cout<<'a';default:std::cout<<'d';// there are no applicable constant expressions// therefore default is executed} std::cout<<'\n'; switch(i){case4: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 handledenum 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 typestruct Device{enum State{ SLEEP, READY, BAD};auto state()const{return m_state;} /* ... */ private:        State m_state{};}; switch(auto dev= Device{}; dev.state()){case Device::SLEEP:/* ... */break;case Device::READY:/* ... */break;case Device::BAD:/* ... */break;} // pathological examples // the statement does not have to be a compound statementswitch(0)std::cout<<"this does nothing\n"; // labels do not require a compound statement eitherswitch(int n=1)case0:case1:std::cout<< n<<'\n';}

2345dred1

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

[edit]External links

• In other languages

• العربية
• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文