std::ranges::views::as_const,std::ranges::as_const_view(3)

NAME
std::ranges::views::as_const,std::ranges::as_const_view - std::ranges::views::as_const,std::ranges::as_const_view

Synopsis
   Defined in header <ranges>
   template< ranges::view V >
       requires ranges::input_range<V> (1) (since C++23)
   class as_const_view
       : public ranges::view_interface<as_const_view<V>>
   namespace views {
       inline constexpr /* unspecified */ as_const = /* unspecified (2) (since C++23)
   */;
   }
   Call signature
   template< ranges::viewable_range R >
       requires /* see below */ (since C++23)
   constexpr ranges::view auto as_const( R&& r );
   1) A range adaptor that represents a view of underlying view that is also a
   constant_range. An as_const_view always has read-only elements (if not empty).
   2) RangeAdaptorObject. Let e be a subexpression, let T be decltype((e)), and let U
   be std::remove_cvref_t<T>. Then the expression views::as_const(e) is
   expression-equivalent to:
     * views::all(e), if it is a well-formed expression and views::all_t<T> models
       constant_range;
     * otherwise, std::span<const X, Extent>(e) for some type X and some extent Extent
       if U denotes std::span<X, Extent>;
     * otherwise, ranges::ref_view(static_cast<const X&>(e.base())) if U denotes
       ranges::ref_view<X> for some type X and const X models constant_range;
     * otherwise, ranges::ref_view(static_cast<const U&>(e)) if e is an lvalue, const U
       models constant_range, and U does not model view.
     * otherwise, as_const_view{e}.
   as_const_view always models constant_range, and it models the contiguous_range,
   random_access_range, bidirectional_range, forward_range, borrowed_range,
   common_range, and sized_range when the underlying view V models respective concepts.

Member functions
   constructor constructs an as_const_view
                 (public member function)
   base returns the underlying view V
                 (public member function)
   begin returns the beginning iterator of the as_const_view
                 (public member function)
   end returns the end iterator of the as_const_view
                 (public member function)
   size returns the size of the view if it is bounded
                 (public member function)
         Inherited from std::ranges::view_interface
   empty returns whether the derived view is empty. Provided if it satisfies
   (C++20) sized_range or forward_range.
                 (public member function of std::ranges::view_interface<D>)
   cbegin returns a constant iterator to the beginning of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   cend returns a sentinel for the constant iterator of the range.
   (C++23) (public member function of std::ranges::view_interface<D>)
   operator bool returns whether the derived view is not empty. Provided if
   (C++20) ranges::empty is applicable to it.
                 (public member function of std::ranges::view_interface<D>)
   data gets the address of derived view's data. Provided if its iterator type
   (C++20) satisfies contiguous_iterator.
                 (public member function of std::ranges::view_interface<D>)
   front returns the first element in the derived view. Provided if it
   (C++20) satisfies forward_range.
                 (public member function of std::ranges::view_interface<D>)
   back returns the last element in the derived view. Provided if it satisfies
   (C++20) bidirectional_range and common_range.
                 (public member function of std::ranges::view_interface<D>)
   operator[] returns the n^th element in the derived view. Provided if it satisfies
   (C++20) random_access_range.
                 (public member function of std::ranges::view_interface<D>) std::ranges::as_const_view::as_const_view
   as_const_view() requires std::default_initializable<V> = default; (1) (since C++23)
   constexpr explicit as_const_view( V base ); (2) (since C++23)
   1) Value-initializes base_ via its default member initializer (= V()).
   2) Initializes base_ with std::move(base).

Parameters
   base - a view std::ranges::as_const_view::base
   constexpr V base() const& requires std::copy_constructible<V>; (1) (since C++23)
   constexpr V base() &&; (2) (since C++23)
   Returns the underlying view.
   1) Copy-constructs the result from the underlying view. Equivalent to return base_;.
   2) Move-constructs the result from the underlying view. Equivalent to return
   std::move(base_);. std::ranges::as_const_view::begin
   constexpr auto begin() requires (!__simple_view<V>); (1) (since C++23)
   constexpr auto begin() const requires ranges::range<const V>; (2) (since C++23)
   Returns the constant iterator of the view.
   1,2) Equivalent to return ranges::cbegin(base_); std::ranges::as_const_view::end
   constexpr auto end() requires (!__simple_view<V>); (1) (since C++23)
   constexpr auto end() const requires ranges::range<const V>; (2) (since C++23)
   Returns the constant sentinel of the view.
   1,2) Equivalent to return ranges::cend(base_); std::ranges::as_const_view::size
   constexpr auto size() requires ranges::sized_range<V>; (1) (since C++23)
   constexpr auto size() const requires ranges::sized_range<const V>; (2) (since C++23)
   Returns the size of the view if the view is bounded.
   1,2) Equivalent to return ranges::size(base_);
   Deduction guides
   template< class R > (since C++23)
   as_const_view( R&& ) -> as_const_view<views::all_t<R>>;
   Helper templates
   template< class T >
   inline constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> (since
   = C++23)
       std::ranges::enable_borrowed_range<T>;
   This specialization of std::ranges::enable_borrowed_range makes as_const_view
   satisfy borrowed_range when the underlying view satisfies it.

Notes
      Feature-test macro Value Std Feature
   __cpp_lib_ranges_as_const 202207L (C++23) std::ranges::as_const_view,
                        std::const_iterator

Example
// Run this code
 #include <cassert>
 #include <ranges>
 int main()
 {
     int x[]{1, 2, 3, 4, 5};
     auto v1 = x | std::views::drop(2);
     assert(v1.back() == 5);
     v1[0]++; // OK, can modify non-const element
     auto v2 = x | std::views::drop(2) | std::views::as_const;
     assert(v2.back() == 5);
     // v2[0]++; // Compile-time error, cannot modify read-only element
 }

See also
   ranges::as_rvalue_view a view of a sequence that casts each element to an rvalue
   views::as_rvalue (class template) (range adaptor object)
   (C++23)
   ranges::cbegin returns an iterator to the beginning of a read-only range
   (C++20) (customization point object)
   ranges::cend returns a sentinel indicating the end of a read-only range
   (C++20) (customization point object)
   as_const obtains a reference to const to its argument
   (C++17) (function template)
   basic_const_iterator iterator adaptor that converts an iterator into a constant
   (C++23) iterator
                        (class template)