switch 조건문

 

C++

컴파일러 지원

Freestanding and hosted

언어

표준 라이브러리 헤더

명명된 요구사항

Feature test macros (C++20)

언어 지원 라이브러리

개념(C++20)

Diagnostics library

유틸리티 라이브러리

문자열 라이브러리

컨테이너 라이브러리

반복자 라이브러리

범위 라이브러리(C++20)

알고리즘 라이브러리

수치 라이브러리

지역화 라이브러리

입/출력 라이브러리

파일시스템 라이브러리(C++17)

정규 표현식 라이브러리(C++11)

아토믹 연산 라이브러리(C++11)

쓰레드 지원 라이브러리(C++11)

Technical specifications

Symbols index

외부 라이브러리들

 

C++ 언어

 

Statements

Labels

label : statement

Expression statements

expression ;

Compound statements

{ statement... }

Selection statements

if

switch

Iteration statements

while

do-while

for

range for(C++11)

Jump statements

break

continue

return

goto

Declaration statements

declaration ;

Try blocks

try compound-statementhandler-sequence

 

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

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

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

Keywords

switch, case, default

Example

아래의 코드는 switch조건문의 다양한 사용 방식입니다.

코드 실행

#include <iostream>   int main(){int i =2;switch(i){case1:std::cout<<"1";case2:std::cout<<"2";//execution starts at this case labelcase3:std::cout<<"3";case4:case5: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;}   // pathological examples   // the statement doesn't have to be a compound statementswitch(0)std::cout<<"this does nothing\n";   // labels don't require a compound statement eitherswitch(int n =1)case0:case1: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