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std::begin, std::cbegin

Defined in header `<array>`
Defined in header `<deque>`
Defined in header `<forward_list>`
Defined in header `<iterator>`
Defined in header `<list>`
Defined in header `<map>`
Defined in header `<regex>`
Defined in header `<set>`
Defined in header `<span>`		(since C++20)
Defined in header `<string>`
Defined in header `<string_view>`		(since C++17)
Defined in header `<unordered_map>`
Defined in header `<unordered_set>`
Defined in header `<vector>`
	(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 range.

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.

Parameters

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

Return value

An iterator to the beginning of the range.

Exceptions

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

(1,3) 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

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

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)