< cpp‎ | ranges
Defined in header <ranges>
inline namespace /*unspecified*/ {

    inline constexpr /*unspecified*/ begin = /*unspecified*/;

(since C++20)
(customization point object)
Call signature
template< class T >

    requires /* see below */

constexpr std::input_or_output_iterator auto begin(T&& t);

Returns an iterator to the first element of the argument.


Let t be an object of type T. If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to ranges::begin is expression-equivalent to:

  1. t + 0 if T is an array type.
    If std::remove_all_extents_t<T> is incomplete, then ranges::begin(std::forward<T>(t)) is ill-formed, no diagnostic required.
  2. Otherwise, std::forward<T>(t).begin(), if that expression is valid, and its return type models std::input_or_output_iterator.
  3. Otherwise, begin(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, its return type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
    • void begin(auto&) = delete;
    • void begin(const auto&) = delete;
    • any declarations of begin found by argument-dependent lookup.

In all other cases, a call to ranges::begin is ill-formed, which can result in substitution failure when ranges::begin(t) appears in the immediate context of a template instantiation.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Customization point objects

The name ranges::begin denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __begin_fn.

All instances of __begin_fn are equal. The effects of invoking different instances of type __begin_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::begin can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::begin above, __begin_fn models std::invocable<__begin_fn, Args...>, std::invocable<const __begin_fn, Args...>, std::invocable<__begin_fn&, Args...>, and std::invocable<const __begin_fn&, Args...>. Otherwise, no function call operator of __begin_fn participates in overload resolution.

[edit] Notes

If the argument is an rvalue (i.e. T is an object type) and ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to ranges::begin is ill-formed, which also results in substitution failure.

The return type models std::input_or_output_iterator in all cases.

[edit] Example

#include <iostream>
#include <vector>
#include <ranges>
int main() 
    std::vector<int> v = { 3, 1, 4 };
    auto vi = std::ranges::begin(v);
    std::cout << *vi << '\n';
    *vi = 42; // OK
    int a[] = { -5, 10, 15 };
    auto ai = std::ranges::begin(a);
    std::cout << *ai << '\n';
    *ai = 42; // OK



[edit] See also

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