Range-based for loop (since C++11)
Executes a for loop over a range.
Used as a more readable equivalent to the traditional for loop operating over a range of values, such as all elements in a container.
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[edit] Syntax
attr(optional) for ( range_declaration : range_expression ) loop_statement
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[edit] Explanation
The above syntax produces code similar to the following (__range
, __begin
and __end
are for exposition only):
{
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range_expression is evaluated to determine the sequence or range to iterate. Each element of the sequence, in turn, is dereferenced and assigned to the variable with the type and name given in range_declaration.
begin_expr
and end_expr
are defined as follows:
- If __range is an array, then
begin_expr
is __range andend_expr
is (__range + __bound), where__bound
is the number of elements in the array (if the array has unknown size or is of an incomplete type, the program is ill-formed) - If __range's type is a class type with either or both a begin or an end member function, then
begin_expr
is__range.begin()
andend_expr
is__range.end()
; - Otherwise,
begin_expr
is begin(__range) andend_expr
is end(__range), which are found via argument-dependent lookup (non-ADL lookup is not performed).
If range_expression returns a temporary, its lifetime is extended until the end of the loop, as indicated by binding to the rvalue reference __range
, but beware that the lifetime of any temporary within range_expression is not extended.
Just as with a traditional loop, a break statement can be used to exit the loop early and a continue statement can be used to restart the loop with the next element.
attr represents an optional number of attributes.
[edit] Keywords
[edit] Example
#include <iostream> #include <vector> int main() { std::vector<int> v = {0, 1, 2, 3, 4, 5}; for (int &i : v) // access by reference (const allowed) std::cout << i << ' '; std::cout << '\n'; for (auto i : v) // compiler uses type inference to determine the right type std::cout << i << ' '; std::cout << '\n'; for (int i : v) // access by value as well std::cout << i << ' '; std::cout << '\n'; }
Output:
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5