Selection statements choose one of several flows of control.
selection-statement: if ( condition ) statement if ( condition ) statement else statement switch ( condition ) statement
condition: expression attribute-specifier-seqopt decl-specifier-seq declarator = initializer-clause attribute-specifier-seqopt decl-specifier-seq declarator braced-init-list
See [dcl.meaning] for the optional attribute-specifier-seq in a condition. In Clause [stmt.stmt], the term substatement refers to the contained statement or statements that appear in the syntax notation. 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 a selection-statement is a single statement and not a compound-statement, it is as if it was rewritten to be a compound-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 ]
The rules for conditions apply both to selection-statements and to the for and while statements ([stmt.iter]). The declarator shall not specify a function or an array. If the auto type-specifier appears in the type-specifier-seq, the type of the identifier being declared is deduced from the initializer as described in [dcl.spec.auto].
A name introduced by a declaration in a condition (either introduced by the type-specifier-seq or the declarator of the condition) is in scope from its point of declaration until the end of the substatements controlled by the condition. If the name is re-declared in the outermost block of a substatement controlled by the condition, the declaration that re-declares the name is ill-formed. [ Example:
if (int x = f()) {
int x; // ill-formed, redeclaration of x
}
else {
int x; // ill-formed, redeclaration of x
}
— end example ]
The value of a condition that is an initialized declaration in a statement other than a switch statement is the value of the declared variable contextually converted to bool (Clause [conv]). If that conversion is ill-formed, the program is ill-formed. The value of a condition that is an initialized declaration in a switch statement is the value of the declared variable if it has integral or enumeration type, or of that variable implicitly converted to integral or enumeration type otherwise. The value of a condition that is an expression is the value of the expression, contextually converted to bool for statements other than switch; if that conversion is ill-formed, the program is ill-formed. The value of the condition will be referred to as simply “the condition” where the usage is unambiguous.
If a condition can be syntactically resolved as either an expression or the declaration of a block-scope name, it is interpreted as a declaration.
In the decl-specifier-seq of a condition, each decl-specifier shall be either a type-specifier or constexpr.
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. In the second form of if statement (the one including else), if the first substatement is also an if statement then that inner if statement shall contain an else part.85
In other words, the else is associated with the nearest un-elsed if.
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 of a class type for which a single non-explicit conversion function to integral or enumeration type exists ([class.conv]). If the condition is of class type, the condition is converted by calling that conversion function, and the result of the conversion is used in place of the original condition for the remainder of this section. Integral promotions are performed. Any statement within the switch statement can be labeled with one or more case labels as follows:
case constant-expression :
where the constant-expression shall be a converted constant expression ([expr.const]) of the promoted type of the switch condition. No two of the case constants in the same switch shall have the same value after conversion to the promoted type of the switch condition.
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 and compared with each case constant. If one of the case constants is equal to the value of the condition, control is passed to the statement following the matched case label. If no case constant matches the condition, and if there is a default label, control passes to the statement labeled by the default label. If no case matches and if there is no default then 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, [stmt.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 a switch-statement. — end note ]