9 Statements [stmt.stmt]

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 an if statement then that inner if statement shall contain an else part.90

If the if statement is of the form if constexpr, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement. If the value of the converted condition is false, the first substatement is a discarded statement, otherwise the second substatement, if present, is a discarded statement. During the instantation of an enclosing templated entity, 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 an if statement shall be associated with a switch statement 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) {
  // ... handle p

  if constexpr (sizeof...(rs) > 0)
    g(rs...);       // never instantiated with an empty argument list
}

extern int x;       // no definition of x required

int f() {
  if constexpr (true)
    return 0;
  else if (x)
    return x;
  else
    return -x;
}

 — end example ]

An if statement of the form

if constexpr${}_{opt}$ ( init-statement condition ) statement

is equivalent to

{
	init-statement
	if constexpr${}_{opt}$ ( condition ) statement
}

and an if statement of the form

if constexpr${}_{opt}$ ( init-statement condition ) statement else statement

is equivalent to

{
	init-statement
	if constexpr${}_{opt}$ ( condition ) statement else statement
}

except that names declared in the init-statement are in the same declarative region as those declared in the condition.

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 of class type, the condition is contextually implicitly converted to an integral or enumeration 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:

case constant-expression :

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 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 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. [ 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 ]

A switch statement of the form

switch ( init-statement condition ) statement

is equivalent to

{
	init-statement
	switch ( condition ) statement
}

except that names declared in the init-statement are in the same declarative region as those declared in the condition.

In the while statement the substatement is executed repeatedly until the value of the condition ([stmt.select]) becomes false. The test takes place before each execution of the substatement.

When the condition of a while statement is a declaration, the scope of the variable that is declared extends from its point of declaration to the end of the while statement. A while statement of the form

while (T t = x) statement

is equivalent to

label:
{                   // start of condition scope
  T t = x;
  if (t) {
    statement
    goto label;
  }
}                   // end of condition scope

The variable created in a condition is destroyed and created with each iteration of the loop. [ Example:

struct A {
  int val;
  A(int i) : val(i) { }
  ~A() { }
  operator bool() { return val != 0; }
};
int i = 1;
while (A a = i) {
  // ...
  i = 0;
}

In the while-loop, the constructor and destructor are each called twice, once for the condition that succeeds and once for the condition that fails.  — end example ]

The expression is contextually converted to bool; if that conversion is ill-formed, the program is ill-formed.

In the do statement the substatement is executed repeatedly until the value of the expression becomes false. The test takes place after each execution of the statement.

The for statement

for ( init-statement condition${}_{opt}$ ; expression${}_{opt}$ ) statement

is equivalent to

{
	init-statement
	while ( condition ) {
		statement
		expression ;
	}
}

except that names declared in the init-statement are in the same declarative region as those declared in the condition, and except that a continue in statement (not enclosed in another iteration statement) will execute expression before re-evaluating condition. [ Note: Thus the first statement specifies initialization for the loop; the condition ([stmt.select]) specifies a test, sequenced before each iteration, such that the loop is exited when the condition becomes false; the expression often specifies incrementing that is sequenced after each iteration.  — end note ]

Either or both of the condition and the expression can be omitted. A missing condition makes the implied while clause equivalent to while(true).

If the init-statement is a declaration, the scope of the name(s) declared extends to the end of the for statement. [ Example:

int i = 42;
int a[10];

for (int i = 0; i < 10; i++)
  a[i] = i;

int j = i;          // j = 42

 — end example ]

The range-based for statement

for ( for-range-declaration : for-range-initializer ) statement

is equivalent to

{
	auto &&__range = for-range-initializer ;
	auto __begin = begin-expr ;
	auto __end = end-expr ;
	for ( ; __begin != __end; ++__begin ) {
		for-range-declaration = *__begin;
		statement
	}
}

where

• (1.1)

  if the for-range-initializer is an expression, it is regarded as if it were surrounded by parentheses (so that a comma operator cannot be reinterpreted as delimiting two init-declarators);

• (1.2)

  _­_­range, _­_­begin, and _­_­end are variables defined for exposition only; and

• (1.3)

  begin-expr and end-expr are determined as follows:

  • (1.3.1)

    if the for-range-initializer is an expression of array type R, begin-expr and end-expr are _­_­range and _­_­range + _­_­bound, respectively, where _­_­bound is the array bound. If R is an array of unknown bound or an array of incomplete type, the program is ill-formed;

  • (1.3.2)

    if the for-range-initializer is an expression of class type C, the unqualified-ids begin and end are looked up in the scope of C as if by class member access lookup, and if either (or both) finds at least one declaration, begin-expr and end-expr are _­_­range.begin() and _­_­range.end(), respectively;

  • (1.3.3)

    otherwise, begin-expr and end-expr are begin(_­_­range) and end(_­_­range), respectively, where begin and end are looked up in the associated namespaces. [ Note: Ordinary unqualified lookup is not performed.  — end note ]

[ Example:

int array[5] = { 1, 2, 3, 4, 5 };
for (int& x : array)
  x *= 2;

 — end example ]

In the decl-specifier-seq of a for-range-declaration, each decl-specifier shall be either a type-specifier or constexpr. The decl-specifier-seq shall not define a class or enumeration.

The break statement shall occur only in an iteration-statement or a switch statement and causes termination of the smallest enclosing iteration-statement or switch statement; control passes to the statement following the terminated statement, if any.

The continue statement shall occur only in an iteration-statement and causes control to pass to the loop-continuation portion of the smallest enclosing iteration-statement, that is, to the end of the loop. More precisely, in each of the statements

while (foo) {
  {
    // ...
  }
contin: ;
}

do {
  {
    // ...
  }
contin: ;
} while (foo);

for (;;) {
  {
    // ...
  }
contin: ;
}

a continue not contained in an enclosed iteration statement is equivalent to goto contin.

A function returns to its caller by the return statement.

The expr-or-braced-init-list of a return statement is called its operand. A return statement with no operand shall be used only in a function whose return type is cv void, a constructor, or a destructor. A return statement with an operand of type void shall be used only in a function whose return type is cv void. A return statement with any other operand shall be used only in a function whose return type is not cv void; the return statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization from the operand. [ Note: A return statement can involve an invocation of a constructor to perform a copy or move of the operand if it is not a prvalue or if its type differs from the return type of the function. A copy operation associated with a return statement may be elided or converted to a move operation if an automatic storage duration variable is returned ([class.copy]).  — end note ] [ Example:

std::pair<std::string,int> f(const char* p, int x) {
  return {p,x};
}

 — end example ] Flowing off the end of a constructor, a destructor, or a function with a cv void return type is equivalent to a return with no operand. Otherwise, flowing off the end of a function other than main results in undefined behavior.

The copy-initialization of the result of the call is sequenced before the destruction of temporaries at the end of the full-expression established by the operand of the return statement, which, in turn, is sequenced before the destruction of local variables ([stmt.jump]) of the block enclosing the return statement.

The goto statement unconditionally transfers control to the statement labeled by the identifier. The identifier shall be a label located in the current function.