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std::begin, std::cbegin

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
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(C++20)
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(C++11)
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Range access
begincbegin
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
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.

range-begin-end.svg

Contents

[edit] Parameters

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

[edit] Return value

An iterator to the beginning the range.

[edit] Exceptions

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

[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:

overloads std::begin
(function template) [edit]
overloads std::begin
(function template) [edit]
range-based for loop support
(function) [edit]
range-based for loop support
(function) [edit]

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)

[edit] 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.

[edit] 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

[edit] See also

(C++11)(C++14)
returns an iterator to the end of a container or array
(function template) [edit]
returns an iterator to the beginning of a range
(customization point object) [edit]
returns an iterator to the beginning of a read-only range
(customization point object) [edit]