Switch statement

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

(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)

[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.

(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.

[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

(until C++11)

• attribute-specifier-seq(optional) decl-specifier-seq declarator brace-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 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 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.

[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

[edit] See also

[edit] External links

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