std::begin, std::cbegin
<array>
<deque>
<flat_map>
<flat_set>
<forward_list>
<inplace_vector>
<iterator>
<list>
<map>
<regex>
<set>
<span>
<string>
<string_view>
<unordered_map>
<unordered_set>
<vector>
auto begin( C& c ) -> decltype(c.begin());
(constexpr since C++17)
auto begin( const C& c ) -> decltype(c.begin());
(constexpr since C++17)
T* begin( T (&array)[N] );
(noexcept since C++14)
(constexpr since C++14)
constexpr auto cbegin( const C& c ) noexcept(/* see below */)
Returns an iterator to the beginning of the given range.
[edit] Parameters
begin member function
[edit] Return value
[edit] Exceptions
[edit] 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:
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 .
[edit] Notes
The non-array overloads 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.
[edit] Example
#include <iostream> #include <iterator> #include <vector> 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