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std::ranges...nstant_range(3) C++ Standard Libarystd::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)
       (exposition 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 ref-
       erence 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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