std::ranges::begin(3)

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

Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   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,
       decay-copy(t.begin())
       (until C++23)
       auto(t.begin())
       (since C++23), if that expression is valid and its type models
       std::input_or_output_iterator.
    3. Otherwise,
       decay-copy(begin(t))
       (until C++23)
       auto(begin(t))
       (since C++23), if T is a class or enumeration type, that expression is valid and
       its type models std::input_or_output_iterator, where the meaning of begin is
       established as if by performing argument-dependent lookup only.
   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.

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 <cassert>
 #include <ranges>
 #include <vector>
 int main()
 {
     std::vector v{3, 1, 4};
     auto vi = std::ranges::begin(v);
     auto vci = std::ranges::cbegin(v);
     assert(*vi == 3 and *vi == *vci);
     ++vi;
     ++vci; // OK: vci is modifiable object
     *vi = 42; // OK: vi points to mutable element
     // *vci = 13; // Error: vci points to immutable element
     int a[]{-5, 10, 15};
     auto ai = std::ranges::begin(a); // works with C-arrays as well
     assert(*ai == -5);
     *ai = 42; // OK
 }
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P2602R2 C++20 there's machinery to prohibit certain removed such machinery
                      non-member begin found by ADL

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)