[stmt.select] (original) (raw)

8.5.2 The if statement [stmt.if]

If the condition ([stmt.pre]) 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.72

If the if statement is of the form if constexpr, the value of the condition is contextually converted to bool and the converted expression shall be a constant expression ([expr.const]); this form is called a constexpr if statement.

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.

Each substatement of a constexpr if statement is a control-flow-limited statement ([stmt.label]).

[Example 1: if constexpr (sizeof(int[2])) {} — _end example_]

[Note 1:

Odr-uses ([basic.def.odr]) in a discarded statement do not require an entity to be defined.

— _end note_]

[Example 2: 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_]

An if statement of the form

is equivalent to

and an if statement of the form

is equivalent to

except that the init-statement is in the same scope as the condition.

An if statement of the form if constevalis called a consteval if statement.

[Example 3: constexpr void f(bool b) { if (true) if consteval { } else ; } — _end example_]

If a consteval if statement is evaluated in a context that is manifestly constant-evaluated ([expr.const]), the first substatement is executed.

[Note 2:

The first substatement is an immediate function context.

— _end note_]

Otherwise, if the else part of the selection statement is present, then the second substatement is executed.

Each substatement of a consteval if statement is a control-flow-limited statement ([stmt.label]).

An if statement of the form

is not itself a consteval if statement, but is equivalent to the consteval if statement

An if statement of the form

is not itself a consteval if statement, but is equivalent to the consteval if statement

8.5.3 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.

If the condition is an expression, the value of the condition is the value of the expression; otherwise, it is the value of the decision variable.

The value of 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.

Any statement within the switch statement can be labeled with one or more case labels as follows:

where the constant-expression shall be a converted constant expression of the adjusted type of the switch condition.

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 formdefault : 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.

[Note 1:

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_]

A switch statement of the form

is equivalent to

except that the init-statement is in the same scope as the condition.