9 Statements [stmt.stmt]

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.