std::begin
std::begin
| Defined in header | | |
|---|---|---|
| | (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.
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>, <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:
| std::begin(std::initializer_list) (C++11) | specializes std::begin (function template) |
|---|---|
| std::begin(std::valarray) (C++11) | specializes std::begin (function template) |
| begin(std::filesystem::directory_iterator)end(std::filesystem::directory_iterator) | 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;
}
}
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
| endcend (C++11)(C++14) | returns an iterator to the end of a container or array (function) |
|---|
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