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Showing content from https://timsong-cpp.github.io/cppwp/n4659/stmt.iter below:

[stmt.iter]

The substatement in an iteration-statement implicitly defines a block 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 ]

If a name introduced in an init-statement or for-range-declaration is redeclared in the outermost block of the substatement, the program is ill-formed. [ Example:

void f() {
  for (int i = 0; i < 10; ++i)
    int i = 0;            for (int i : { 1, 2, 3 })
    int i = 1;          }

 — 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 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 ]

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 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 ]

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

• 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);

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

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

  • 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;

  • 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;

  • 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 ]

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