std::ranges::size(3)

NAME
std::ranges::size - std::ranges::size

Synopsis
   Defined in header <ranges>
   inline namespace /*unspecified*/ {
                        (since C++20)
   inline constexpr auto size = /*unspecified*/; (customization point object)
   }
   Call signature
   template< class T >
   requires /* see below */ (since C++20)
   constexpr auto size( T&& t );
   Calculates the number of elements in t in constant time.
   Let t be an object of type T. A call to ranges::size is expression-equivalent to:
    1. std::extent_v<T>, if T is an array type with a known bound.
    2. Otherwise, t.size() converted to its decayed type, if
       ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted
       expression is valid and has an integer-like type.
    3. Otherwise, size(t) converted to its decayed type, if
       ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted
       expression is valid and has an integer-like type, where the overload resolution
       is performed with the following candidates:
          * void size(auto&) = delete;
          * void size(const auto&) = delete;
    4. Otherwise, /*to-unsigned-like*/(ranges::end(t) - ranges::begin(t)), if T models
       ranges::forward_range and ranges::sentinel_t<T> models
       std::sized_sentinel_for<ranges::iterator_t<T>>,
                    where /*to-unsigned-like*/ denotes an explicit conversion to an
                    unsigned-integer-like type.
   In all other cases, a call to ranges::size is ill-formed, which can result in
   substitution failure when ranges::size(t) 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::size 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 __size_fn.
   All instances of __size_fn are equal. The effects of invoking different instances of
   type __size_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::size 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::size above, __size_fn models
     * std::invocable<__size_fn, Args...>,
     * std::invocable<const __size_fn, Args...>,
     * std::invocable<__size_fn&, Args...>, and
     * std::invocable<const __size_fn&, Args...>.
   Otherwise, no function call operator of __size_fn participates in overload
   resolution.

Notes
   Whenever ranges::size(e) is valid for an expression e, the return type is
   integer-like.
   The C++20 standard requires that if the underlying size 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 <ranges>
 #include <type_traits>
 #include <vector>
 int main()
 {
     auto v = std::vector<int>{};
     std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
     auto il = {7};
     std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n';
     int array[] = {4, 5}; // array has a known bound
     std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n';
     std::cout << std::boolalpha << "is_signed: "
               << std::is_signed_v<decltype(std::ranges::size(v))> << '\n';
 }

Output:
 ranges::size(v) == 0
 ranges::size(il) == 1
 ranges::size(array) == 2
 is_signed: false

See also
   ranges::ssize returns a signed integer equal to the size of a range
   (C++20) (customization point object)
   ranges::sized_range specifies that a range knows its size in constant time
   (C++20) (concept)
   size
   ssize returns the size of a container or array
   (C++17) (function template)
   (C++20)