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std::ranges...nstant_range(3)  C++ Standard Libary std::ranges...nstant_range(3)

NAME
     std::ranges::constant_range - std::ranges::constant_range

Synopsis
	Defined in header <ranges>
	template< class T >

	concept constant_range =				      (1) (since
     C++23)
	    ranges::input_range<T> &&

	    /*constant-iterator*/<ranges::iterator_t<T>>;
	Helper concepts
	template< class T >

	concept /*constant-iterator*/ =
	    std::input_iterator<T> &&				      (2) (expo-
     sition only*)

	    std::same_as<std::iter_const_reference_t<T>,
	std::iter_reference_t<T>>;

	1)  The  constant_range  concept  is  a  refinement  of  range for which
     ranges::begin
	returns a constant iterator.
	2) The exposition-only	concept  /*constant-iterator*/<T>  is  satisfied
     when the result
	of  the  indirection operation of the input iterator is its const refer-
     ence type which
	implies read-only.

Example
     // Run this code

      #include <ranges>
      #include <span>
      #include <string_view>
      #include <vector>

      // mechanisms for ensuring the parameter is a constant range
      // 1) an overload set where the mutable one defers to the constant one
      template<std::ranges::constant_range R>
      void takes_any_range1(R&& r)
      {
	  // R is definitely a constant range
      }

      template<std::ranges::range R>
      void takes_any_range1(R&& r)
      {
	  takes_any_range1(std::views::as_const(std::forward<R>(r)));
      }

      // 2) one function template that shadows its parameter
      template<std::ranges::range R>
      void takes_any_range2(R&& _r)
      {
	  auto r = std::views::as_const(std::forward<R>(_r));

	  // r is definitely a constant range
	  // never use _r again
      }

      // 3) one function template that recursively invokes itself
      template<std::ranges::range R>
      void takes_any_range3(R&& r)
      {
	  if constexpr (std::ranges::constant_range<R>)
	  {
	      // R is definitely a constant range
	      // put implementation here
	  }
	  else
	      takes_any_range3(std::views::as_const(std::forward<R>(r)));
      }

      static_assert
      (
	      std::ranges::constant_range<const std::vector<int>> and
	  not std::ranges::constant_range<std::vector<int>> and
	      std::ranges::constant_range<std::string_view> and
	  not std::ranges::constant_range<std::span<int>> and
	      std::ranges::constant_range<std::span<const int>> and
	  not std::ranges::constant_range<const std::span<int>>
      );

      int main() {}

http://cppreference.com 	   2024.06.10	   std::ranges...nstant_range(3)
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