std::begin
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
.
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) |
---|
? cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
本文档系腾讯云开发者社区成员共同维护,如有问题请联系 cloudcommunity@tencent.com