# std::begin, std::cbegin

From cppreference.com
< cpp‎ | iterator

Defined in header `<array>` |  |
Defined in header `<deque>` |  |
Defined in header `<flat_map>` |  |
Defined in header `<flat_set>` |  |
Defined in header `<forward_list>` |  |
Defined in header `<inplace_vector>` |  |
Defined in header `<iterator>` |  |
Defined in header `<list>` |  |
Defined in header `<map>` |  |
Defined in header `<regex>` |  |
Defined in header `<set>` |  |
Defined in header `<span>` |  |
Defined in header `<string>` |  |
Defined in header `<string_view>` |  |
Defined in header `<unordered_map>` |  |
Defined in header `<unordered_set>` |  |
Defined in header `<vector>` |  |
template< class C > auto begin( C& c ) -> decltype(c.begin()); | (1) | (since C++11) (constexpr since C++17)
template< class C > auto begin( const C& c ) -> decltype(c.begin()); | (2) | (since C++11) (constexpr since C++17)
template< class T, std::size_t N > T* begin( T (&array)[N] ); | (3) | (since C++11) (noexcept since C++14) (constexpr since C++14)
template< class C > constexpr auto cbegin( const C& c ) noexcept(/* see below */) -> decltype(std::begin(c)); | (4) | (since C++14)

Returns an iterator to the beginning of the given range.

1,2) Returns c.begin(), which is typically an iterator to the beginning of the sequence represented by c.
1) If `C` is a standard Container, returns a `C::iterator` object.
2) If `C` is a standard Container, returns a `C::const_iterator` object.
3) Returns a pointer to the beginning of array.
4) Returns std::begin(c), with c always treated as const-qualified.
If `C` is a standard Container, returns a `C::const_iterator` object.

### Parameters

c | - | a container or view with a `begin` member function
array | - | an array of arbitrary type

### Return value

1,2) c.begin()
3) array
4) c.begin()

### Exceptions

4)
`noexcept` specification:
noexcept(noexcept(std::begin(c)))

### Overloads

Custom overloads of `begin` may be provided for classes and enumerations that do not expose a suitable `begin()` member function, yet can be iterated. The following overloads are already provided by the standard library:

std::begin(std::initializer_list) (C++11) | overloads std::begin (function template)
std::begin(std::valarray) (C++11) | overloads std::begin (function template)
begin(std::filesystem::directory_iterator)end(std::filesystem::directory_iterator) (C++17) | range-based for loop support (function)
begin(std::filesystem::recursive_directory_iterator)end(std::filesystem::recursive_directory_iterator) | range-based for loop support (function)

Similar to the use of `swap` (described in Swappable), typical use of the `begin` function in generic context is an equivalent of using std::begin; begin(arg);, which allows both the ADL-selected overloads for user-defined types and the standard library function templates to appear in the same overload set.

    template<typename Container, typename Function>
    void for_each(Container&& cont, Function f)
    {
        using std::begin;
        auto it = begin(cont);
        using std::end;
        auto end_it = end(cont);
        while (it != end_it)
        {
            f(*it);
            ++it;
        }
    }

Overloads of `begin` found by argument-dependent lookup can be used to customize the behavior of std::ranges::begin, std::ranges::cbegin, and other customization pointer objects depending on std::ranges::begin. | (since C++20)

### Notes

The non-array overloads exactly reflect the behavior of `C::begin`. Their effects may be surprising if the member function does not have a reasonable implementation.

`std::cbegin` is introduced for unification of member and non-member range accesses. See also LWG issue 2128.

If `C` is a shallow-const view, `std::cbegin` may return a mutable iterator. Such behavior is unexpected for some users. See also P2276 and P2278.

### Example

Run this code

    #include <iostream>
    #include <iterator>
    #include <vector>

    int main()
    {
        std::vector<int> v = {3, 1, 4};
        auto vi = std::begin(v);
        std::cout << std::showpos << *vi << '\n';

        int a[] = {-5, 10, 15};
        auto ai = std::begin(a);
        std::cout << *ai << '\n';
    }

Output:

    +3
    -5

### See also

endcend (C++11)(C++14) | returns an iterator to the end of a container or array (function template)
ranges::begin (C++20) | returns an iterator to the beginning of a range (customization point object)
ranges::cbegin (C++20) | returns an iterator to the beginning of a read-only range (customization point object)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/begin&oldid=177482"