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Showing content from https://timsong-cpp.github.io/cppwp/n4140/stmt.iter below:

[stmt.iter]

The substatement in an iteration-statement implicitly defines a block scope ([basic.scope]) which is entered and exited each time through the loop.

If the substatement in an iteration-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 statement. [ Example:

while (--x >= 0)
  int i;

can be equivalently rewritten as

while (--x >= 0) {
  int i;
}

Thus after the while statement, i is no longer in scope.  — end example ]

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 ([basic.scope.pdecl]) to the end of the while statement. A while statement of the form

while (T t = x) statement

is equivalent to

label:{                     T t = x;
  if (t) {
    statement
    goto label;
  }
}                   

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 (Clause [conv]); 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.

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 for-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;          

 — end example ]

For a range-based for statement of the form

for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses89

( expression )

and for a range-based for statement of the form

for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

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

where __range, __begin, and __end are variables defined for exposition only, and _RangeT is the type of the expression, and begin-expr and end-expr are determined as follows:

• if _RangeT is an array type, begin-expr and end-expr are __range and __range + __bound, respectively, where __bound is the array bound. If _RangeT is an array of unknown size or an array of incomplete type, the program is ill-formed;

• if _RangeT is a class type, the unqualified-ids begin and end are looked up in the scope of class _RangeT as if by class member access lookup ([basic.lookup.classref]), and if either (or both) finds at least one declaration, begin-expr and end-expr are __range.begin() and __range.end(), respectively;

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

[ Example:

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

 — end example ]

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