Transfers control to one of several statements, depending on the value of a condition.
- 1 Syntax
- 2 Condition
- 2.1 Expression
- 2.2 Declaration
- 2.2.1 Non-structured binding declaration
- 2.2.2 Structured binding declaration
- 2.3 Type
- 3 Labels
- 4 Control flow transfer
- 5 switch statements with initializer
- 6 Notes
- 7 Keywords
- 8 Example
- 9 Defect reports
- 10 See also
- 11 External links
[edit] 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
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)[edit] 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.
- 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
If condition is an expression, the value it yields is the the value of the expression.
[edit] Declaration
If condition 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:
- type-specifier-seq declarator = assignment-expression
- attribute-specifier-seq(optional) decl-specifier-seq declarator brace-or-equal-initializer
- 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 e introduced by the declaration.
(since C++26)[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 to integral promotions , the yielded value is converted to the promoted type.
[edit] Labels
Any statement within the switch statement can be labeled with one or more following labels:
attr (optional) case constant-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 conditionA 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.
[edit] 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 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)[edit] 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:
[edit] Keywords
switch, case, default
[edit] Example
[edit] 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 tointegral promotion could not be promoted do not promotecondition s of these types CWG 2629 C++98 condition could be a declaration of a floating-point variable prohibited