3.5 KiB
[stmt.ranged]
8 Statements [stmt]
8.6 Iteration statements [stmt.iter]
8.6.5 The range-based for statement [stmt.ranged]
The range-based for statement
for ( init-statementopt for-range-declaration : for-range-initializer ) statement
is equivalent to
{
init-statementopt
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 type of range is a reference to an array type R, begin-expr and end-expr arerange and range + N, respectively, where N 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 type of range is a reference to a class type C, and searches in the scope of C ([class.member.lookup]) for the names begin and end each find at least one declaration,begin-expr and end-expr arerange.begin() and range.end(), respectively;
-
otherwise, begin-expr and end-expr arebegin(range) and end(range), respectively, where begin and end undergo argument-dependent lookup ([basic.lookup.argdep]). [Note 1: Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed. â end note]
[Example 1: int array[5] = { 1, 2, 3, 4, 5 };for (int& x : array) x *= 2; â end example]
[Note 2:
The lifetime of some temporaries in the for-range-initializer is extended to cover the entire loop ([class.temporary]).
â end note]
[Example 2: using T = std::list;const T& f1(const T& t) { return t; }const T& f2(T t) { return t; } T g();
void foo() {for (auto e : f1(g())) {} // OK, lifetime of return value of g() extendedfor (auto e : f2(g())) {} // undefined behavior} â end example]