8 Statements [stmt.stmt]

8.5 Iteration statements [stmt.iter]

8.5.1 The while statement [stmt.while]

1
#
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.
2
#
When the condition of a while statement is a declaration, the scope of the variable that is declared extends from its point of declaration ([basic.scope.pdecl]) to the end of the while statement.
A while statement whose condition is an initialized declaration of some variable t is equivalent to 
label:
[0pt][l]{				// start of condition scope
	[0pt][l]condition ;			// declares t
	if (t) {
		statement
		goto label;
	}
[0pt][l]}				// end of condition scope
[ Note
:
The variable created in the 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
 ]
— end note
 ]