[stmt.select] (original) (raw)
8 Statements [stmt.stmt]
8.5 Selection statements [stmt.select]
The substatement in a selection-statement (each substatement, in the else form of the if statement) implicitly defines a block scope ([basic.scope]).
If the substatement in aselection-statement is a single statement and not acompound-statement, it is as if it was rewritten to be acompound-statement containing the original substatement.
[ Example
:
if (x) int i;
can be equivalently rewritten as
if (x) { int i; }
Thus after the if statement, i is no longer in scope.
— end example
]
8.5.1 The if statement [stmt.if]
If the condition ([stmt.select]) yields true the first substatement is executed.
If the else part of the selection statement is present and the condition yields false, the second substatement is executed.
If the first substatement is reached via a label, the condition is not evaluated and the second substatement is not executed.
In the second form of if statement (the one including else), if the first substatement is also anif statement then that inner if statement shall contain an else part.83
If the value of the converted condition is false, the first substatement is adiscarded statement, otherwise the second substatement, if present, is a discarded statement.
During the instantiation of an enclosing templated entity ([temp.pre]), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
[ Note
:
Odr-uses in a discarded statement do not require an entity to be defined.
— end note
]
A case or default label appearing within such anif statement shall be associated with a switchstatement within the same if statement.
A label declared in a substatement of a constexpr if statement shall only be referred to by a statement in the same substatement.
[ Example
:
template<typename T, typename ... Rest> void g(T&& p, Rest&& ...rs) {
if constexpr (sizeof...(rs) > 0)
g(rs...);
}
extern int x;
int f() { if constexpr (true) return 0; else if (x) return x; else return -x; }
— end example
]
8.5.2 The switch statement [stmt.switch]
The switch statement causes control to be transferred to one of several statements depending on the value of a condition.
The condition shall be of integral type, enumeration type, or class type.
If the (possibly converted) type is subject to integral promotions, the condition is converted to the promoted type.
No two of the case constants in the same switch shall have the same value after conversion.
There shall be at most one label of the form
default :
within a switch statement.
Switch statements can be nested; a case or default label is associated with the smallest switch enclosing it.
When the switch statement is executed, its condition is evaluated.
If one of the case constants has the same value as the condition, control is passed to the statement following the matched case label.
If no case constant matches the condition, and if there is adefault label, control passes to the statement labeled by the default label.
If no case matches and if there is no defaultthen none of the statements in the switch is executed.
case and default labels in themselves do not alter the flow of control, which continues unimpeded across such labels.
To exit from a switch, see break.
[ Note
:
Usually, the substatement that is the subject of a switch is compound and case and default labels appear on the top-level statements contained within the (compound) substatement, but this is not required.
Declarations can appear in the substatement of aswitch statement.
— end note
]