switch statement

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for(C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications(until C++17*)
noexcept specifier(C++11)
Exceptions
throw-expression try block	catch handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype(C++11) auto(C++11)	constexpr(C++11) consteval(C++20) constinit(C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization(C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr(C++11)
User-defined(C++11)
Utilities
Attributes(C++11)
Types
typedef declaration
Type alias declaration(C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier(C++11)   final specifier(C++11)	explicit(C++11) static
Special member functions
Default constructor Copy constructor Move constructor(C++11)	Copy assignment Move assignment(C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs(C++11)
Miscellaneous
Inline assembly	History of C++

 

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

Assertions

contract_assert(C++26)

Declaration statements

declaration ;

Try blocks

try block

Transactional memory

synchronized, atomic_commit, etc (TM TS)

 

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

Syntax

attr (optional)switch(init-statement (optional)condition)statement

attr	-	(since C++11) any number of attributes
init-statement	-	(since C++17) any of the following: an expression statement (which may be a null statement ;) a simple declaration, typically a declaration of a variable with initializer, but it may declare arbitrarily many variables or structured bindings an alias declaration (since C++23) Note that any init-statement must end with a semicolon. This is why it is often described informally as an expression or a declaration followed by a semicolon.
condition	-	a condition
statement	-	a statement (typically a compound statement)

Condition

A condition can either be an expression or a simple declaration.

If it can be syntactically resolved as a structured binding declaration, it is interpreted as a structured binding declaration.

(since C++26)

• 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.

Expression

If condition is an expression, the value it yields is the the value of the expression.

Declaration

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

Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

type-specifier-seqdeclarator=assignment-expression	(until C++11)
attribute-specifier-seq(optional)decl-specifier-seqdeclaratorbrace-or-equal-initializer	(since C++11)

• The declarator cannot specify a function or an array.
• The type specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers and constexpr, and it(since C++11) cannot define a class or enumeration.

The decision variable of the declaration is the declared variable.

Structured binding declaration The declaration has the following restrictions: The expression in its initializer cannot be of an array type. The declaration specifier sequence can only contain type specifiers and constexpr. The decision variable of the declaration is the invented variable eintroduced by the declaration.

(since C++26)

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 to integral promotions , the yielded value is converted to the promoted type.

Labels

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

attr (optional)caseconstant-expression:	(1)
attr (optional)default:	(2)

attr	-	(since C++11) any number of attributes
constant-expression	-	a converted constant expression of the adjusted type of the switch condition

A case or default label is associated with the innermost switch statement enclosing it.

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

• A switch statement is associated with multiple case labels whose constant-expression s have the same value after conversions.
• A switch statement is associated with multiple default labels.

Control flow transfer

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

• If one of the associated case label constants has the same value, control is passed to the statement labeled by the matched case label.
• Otherwise, if there is an associated default label, control is passed to the statement labeled by the default label.
• Otherwise, none of the statements in the switch statement will be executed.

case and default labels in themselves do not alter the flow of control. To exit from a switch statement from the middle, see break statements.

Compilers may issue warnings on fallthrough (reaching the next case or default label without a break) unless the attribute [[fallthrough]] appears immediately before the case 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;}

switch statements with initializer 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)

Notes

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

The following code shows several usage cases of the switch statement:

Run this code

#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';}

Output:

2345 d red 1

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 1767	C++98	condition s of types that are not subject to integral promotion could not be promoted	do not promote condition s of these types
CWG 2629	C++98	condition could be a declaration of a floating-point variable	prohibited

See also

C documentation for switch

External links

1.	Loop unrolling using Duff's Device
2.	Duff's device can be used to implement coroutines in C/C++