Range-based for loop (since C++11) - cppreference.com

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

From cppreference.com
< cpp‎ | language

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.

### [edit] Syntax

attr(optional) `for (` range_declaration `:` range_expression `)` loop_statement |  |

attr(C++11) | - | any number of attributes
range_declaration | - | a declaration of a named variable, whose type is the type of the element of the sequence represented by range_expression, or a reference to that type. Often uses the auto specifier for automatic type deduction
range_expression | - | any expression that represents a suitable sequence (either an array or an object for which `begin` and `end` member functions or free functions are defined, see below) or a braced-init-list.
loop_statement | - | any statement, typically a compound statement, which is the body of the loop

### [edit] 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_expression is an expression of array type, 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_expression is an expression of a class type `C` that has a member named begin and/or a member named end, 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 (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] Notes

If the initializer (range_expression) is a braced-init-list, __range is deduced to be std::initializer_list<>&&

It is safe, and in fact, preferable in generic code, to use deduction to forwarding reference, for(auto&& var : sequence).

### [edit] Keywords

`for`

### [edit] Example

Run this code

    #include <iostream>
    #include <vector>

    int main()
    {
        std::vector<int> v = {0, 1, 2, 3, 4, 5};

        for(const int &i : v) // access by const reference
            std::cout << i << ' ';
        std::cout << '\n';

        for(auto i: v) // access by value, the type of i is int
            std::cout << i << ' ';
        std::cout << '\n';

        for(auto&& i: v) // access by reference, the type of i is int&
            std::cout << i << ' ';
        std::cout << '\n';

        for(int n: {0, 1, 2, 3, 4, 5}) // the initializer may be a braced-init-list
            std::cout << n << ' ';
        std::cout << '\n';

        int a[] = {0, 1, 2, 3, 4, 5};
        for(int n: a)  // the initializer may be an array
            std::cout << n << ' ';
        std::cout << '\n';
    }

Output:

    0 1 2 3 4 5
    0 1 2 3 4 5
    0 1 2 3 4 5
    0 1 2 3 4 5
    0 1 2 3 4 5

### [edit] See also

for_each | applies a function to a range of elements (function template) [edit]

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