std::begin,std::cbegin(3)

🌸 std::begin, std::cbegin — C++ Standard Libary (3) 🌸

okay so bestie, this is the C++ Standard Libary manual page for std::begin and std::cbegin, and the title line is std::begin,std::cbegin(3). It’s giving “iterator to the beginning of the given range” energy, and we are going through every single crumb. ✨

💖 NAME 💖

std::begin, std::cbegin — std::begin, std::cbegin

✨ Synopsis ✨

First up, the header glow-up: this stuff is defined in a whole pile of headers. 💅

• Defined in header <array>
• Defined in header <deque>
• Defined in header <flat_map>
• Defined in header <flat_set>
• 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>
• Defined in header <string>
• Defined in header <string_view>
• Defined in header <unordered_map>
• Defined in header <unordered_set>
• Defined in header <vector>

Now the signatures, no cap:

template< class C >
auto begin( C& c ) -> decltype(c.begin());                   (since C++11) (until C++17)   (1)

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)   (2)

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)   (3)

template< class T, std::size_t N >
constexpr T* begin( T (&array)[N] ) noexcept;                (since C++14)                 (3)

template< class C >
constexpr auto cbegin( const C& c ) noexcept(/* see below */)
    -> decltype(std::begin(c));                              (since C++14)                 (4)

Returns an iterator to the beginning of the given range. 🫶

• 1,2) Returns c.begin(), which is typically an iterator to the beginning of the sequence represented by c.
• 1) If C is a standard Container, returns a C::iterator object.
• 2) If C is a standard Container, returns a C::const_iterator object.
• 3) Returns a pointer to the beginning of array.
• 4) Returns std::begin(c), with c always treated as const-qualified. If C is a standard Container, returns a C::const_iterator object.

There’s even a diagram moment: range-begin-end.svg. 🎀

💖 Parameters 💖

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

🎀 Return value 🎀

• 1,2) c.begin()
• 3) array
• 4) c.begin()

🦋 Exceptions 🦋

• 4) 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) — overloads std::begin (C++11) (function template)
• std::begin(std::valarray) — overloads std::begin (C++11) (function template)
• begin(std::filesystem::directory_iterator), end(std::filesystem::directory_iterator) — range-based for loop support (function) (C++17)
• 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 (since C++20) other customization pointer objects depending on std::ranges::begin.

💅 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.

🍓 Example 🍓

Run this code:

#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

⭐ See also ⭐

• end (C++11) — returns an iterator to the end of a container or array (function template)
• cend (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)

👑 Category 👑

• * conditionally noexcept

---

💖 source: http://cppreference.com · version/date: 2024.06.10 · page: std::begin,std::cbegin(3) — that’s the whole page, bestie, mwah! 💖