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# std::begin

From cppreference.com
< cpp‎ | iterator

Defined in header `<iterator>` |  |
| (1) |
template< class C > auto begin( C& c ) -> decltype(c.begin()); |  | (since C++11) (until C++17)
template< class C > constexpr auto begin( C& c ) -> decltype(c.begin()); |  | (since C++17)
| (1) |
template< class C > auto begin( const C& c ) -> decltype(c.begin()); |  | (since C++11) (until C++17)
template< class C > constexpr auto begin( const C& c ) -> decltype(c.begin()); |  | (since C++17)
| (2) |
template< class T, std::size_t N > T* begin( T (&array)[N] ); |  | (since C++11) (until C++14)
template< class T, std::size_t N > constexpr T* begin( T (&array)[N] ) noexcept; |  | (since C++14)
template< class C > constexpr auto cbegin( const C& c ) noexcept(/* see below */) -> decltype(std::begin(c)); | (3) | (since C++14)

Returns an iterator to the beginning of the given container `c` or array `array`. These templates rely on C::begin() having a reasonable implementation.

1) Returns exactly c.begin(), which is typically an iterator to the beginning of the sequence represented by `c`. If `C` is a standard `Container`, this returns C::iterator when `c` is not const-qualified, and C::const_iterator otherwise.
2) Returns a pointer to the beginning of the `array`.
3) Returns exactly std::begin(c), with `c` always treated as const-qualified. If `C` is a standard `Container`, this always returns C::const_iterator.

| This section is incomplete Reason: an explanation why cbegin was introduced

### [edit] Parameters

c | - | a container with a `begin` method
array | - | an array of arbitrary type

### [edit] Return value

An iterator to the beginning of `c` or `array`

### [edit] Exceptions

3)
`noexcept` specification:
noexcept(noexcept(std::begin(c)))

### [edit] Notes

In addition to being included in `<iterator>`, `std::begin` and `std::cbegin` are guaranteed to become available if any of the following headers are included: `<array>`, `<deque>`, `<forward_list>`, `<list>`, `<map>`, `<regex>`, `<set>`, `<string>`, <string_view> (since C++17), `<unordered_map>`, `<unordered_set>`, and `<vector>`.

### [edit] User-defined overloads

Custom overloads of `begin` may be provided for classes 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) | specializes std::begin (function template) [edit]
std::begin(std::valarray) (C++11) | specializes std::begin (function template) [edit]
begin(std::filesystem::directory_iterator)end(std::filesystem::directory_iterator) | range-based for loop support (function) [edit]
begin(std::filesystem::recursive_directory_iterator)end(std::filesystem::recursive_directory_iterator) | range-based for loop support (function) [edit]

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;
        }
    }

### [edit] Example

Run this code

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

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

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

Output:

    3
    -5

### [edit] See also

end cend (C++11)(C++14) | returns an iterator to the end of a container or array (function) [edit]

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Categories:

- Todo with reason
- conditionally noexcept