std::begin, std::cbegin

From cppreference.com
< cpplrm; | 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.

range-begin-end.svg

Parameters

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

Return value

An iterator to the beginning of c or array

Exceptions

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

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>, <span> (since C++20), <string>, <string_view> (since C++17), <unordered_map>, <unordered_set>, and <vector>.

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:

specializes std::begin
(function template)
specializes std::begin
(function template)
range-based for loop support
(function)
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;
    }
}

Example

#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

See also

(C++11)(C++14)
returns an iterator to the end of a container or array
(function)