std::ranges::begin(3)

NAME
std::ranges::begin - std::ranges::begin

Synopsis
   Defined in header <ranges>
   inline namespace /*unspecified*/ {
   inline constexpr /*unspecified*/ begin = (since C++20)
   /*unspecified*/; (customization point object)
   }
   Call signature
   template< class T >
   requires /* see below */ (since C++20)
   constexpr std::input_or_output_iterator auto begin(
   T&& t );
   Returns an iterator to the first element of the argument.
   range-begin-end.svg
   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 has an array type.
                    If std::remove_all_extents_t<std::remove_reference_t<T>> is
                    incomplete, then the call to ranges::begin is ill-formed, no
                    diagnostic required.
    2. Otherwise, t.begin() converted to its decayed type, if that expression with
       conversion is valid, and its converted type models
       std::input_or_output_iterator.
    3. Otherwise, begin(t) converted to its decayed type, if t has a class or
       enumeration type, the aforementioned unqualified call with conversion is valid,
       and its converted type models std::input_or_output_iterator, where the overload
       resolution is performed with the following candidates:
          * any declarations of begin found by argument-dependent lookup.
          * void begin(auto&) = delete;
          * void begin(const auto&) = delete;
   In all other cases, a call to ranges::begin is ill-formed, which can result in
   substitution failure when the call appears in the immediate context of a template
   instantiation.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  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.

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.
   The C++20 standard requires that if the underlying begin function call returns a
   prvalue, the return value is move-constructed from the materialized temporary
   object. All implementations directly return the prvalue instead. The requirement is
   corrected by the post-C++20 proposal P0849R8 to match the implementations.

Example
// Run this code
 #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
 }

Output:
 3
 -5

See also
   ranges::cbegin returns an iterator to the beginning of a read-only range
   (C++20) (customization point object)
   begin
   cbegin returns an iterator to the beginning of a container or array
   (C++11) (function template)
   (C++14)