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Range-based for loop (since C++11)

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Revision as of 18:58, 8 January 2014 by Cubbi (Talk | contribs)

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.

Contents

Syntax

attr(optional) for ( range_declaration : range_expression ) loop_statement

Explanation

The above syntax produces code similar to the following (__range, __begin and __end are for exposition only):

{
auto && __range = range_expression ;
for (auto __begin = begin_expr,
__end = end_expr;
__begin != __end; ++__begin) {
range_declaration = *__begin;
loop_statement
}

}

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 and end_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() and end_expr is __range.end();
  • Otherwise, begin_expr is begin(__range) and end_expr is end(__range), which are found via argument-dependent lookup with std as an associated namespace.

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.

Keywords

for

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