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

NAME
     std::ranges::view_interface::operatorbool	   -	std::ranges::view_inter-
     face::operatorbool

Synopsis
	constexpr explicit operator bool() requires /* see below  */;	     (1)
     (since C++20)
	constexpr  explicit  operator bool() const requires /* see below */; (2)
     (since C++20)

	The default implementation  of	operator  bool	member	function  checks
     whether the view
	is  non-empty.	It  makes  the	derived type contextually convertible to
     bool.

	1) Let derived be static_cast<D&>(*this).  The	expression  in	the  re-
     quires-clause is
	equal  to requires { ranges::empty(derived); }, and the function body is
     equivalent
	to return !ranges::empty(derived);.
	2) Same as (1), except that derived is static_cast<const D&>(*this).

Parameters
	(none)

Return value
	false if  the  value  of  the  derived	type  is  empty  (determined  by
     std::ranges::empty),
	true otherwise.

Notes
	In  C++20, no type derived from std::ranges::view_interface in the stan-
     dard library
	provides their own operator bool. Almost all of these types use the  de-
     fault
	implementation.

	A notable exception is std::ranges::basic_istream_view. For its iterator
     type never
	satisfies  forward_iterator,  the view cannot use the inherited operator
     bool.

Example
     // Run this code

      #include <array>
      #include <iostream>
      #include <ranges>

      int main()
      {
	  const std::array ints {0, 1, 2, 3, 4};
	  auto odds = ints | std::views::filter([](int i) { return 0 != i  %  2;
     });
	  auto negs = ints | std::views::filter([](int i) { return i < 0; });
	  std::cout << std::boolalpha
		    << "Has odd numbers: " << (!!odds) << ' ' << '\n'
		    << "Has negative numbers: " << (!!negs) << ' ' << '\n';
      }

Output:
      Has odd numbers: true
      Has negative numbers: false

See also
	ranges::empty checks whether a range is empty
	(C++20)       (customization point object)
	empty	       returns whether the derived view is empty. Provided if it
     satisfies
	(C++20)       sized_range or forward_range.
		      (public member function)
	empty	      checks whether the container is empty
	(C++17)       (function template)

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