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std::ranges::starts_with(3)    C++ Standard Libary   std::ranges::starts_with(3)

NAME
     std::ranges::starts_with - std::ranges::starts_with

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::input_iterator I1, std::sentinel_for<I1> S1,

		  std::input_iterator I2, std::sentinel_for<I2> S2,
		  class Pred = ranges::equal_to,
		  class  Proj1	=  std::identity,  class Proj2 = std::identity >
     (since
	requires  std::indirectly_comparable<I1,   I2,	 Pred,	 Proj1,   Proj2>
     (1) C++23)
	constexpr bool
	    starts_with( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred =
	{},

			 Proj1 proj1 = {}, Proj2 proj2 = {} );
	template< ranges::input_range R1, ranges::input_range R2,

		  class Pred = ranges::equal_to,
		  class Proj1 = std::identity, class Proj2 = std::identity >
	requires	      std::indirectly_comparable<ranges::iterator_t<R1>,
     (since
					    ranges::iterator_t<R2>,
     (2) C++23)
					    Pred, Proj1, Proj2>
	constexpr bool
	    starts_with( R1&& r1, R2&& r2, Pred pred = {},

			 Proj1 proj1 = {}, Proj2 proj2 = {} );

	Checks whether the second range matches the prefix of the first range.

	1) Let N1 and N2 denote the size of ranges [first1, last1) and	[first2,
     last2)
	respectively. If N1 < N2, returns false. Otherwise, returns true only if
     every
	element  in the range [first2, last2) is equal to the corresponding ele-
     ment in
	[first1, first1 + N2). Comparison is done by applying the binary  predi-
     cate pred to
	elements in two ranges projected by proj1 and proj2 respectively.
	2) Same as (1), but uses r1 and r2 as the source ranges, as if using
	ranges::begin(r1) as first1, ranges:begin(r2) as first2, ranges::end(r1)
     as last1,
	and ranges::end(r2) as last2.

	The  function-like  entities  described on this page are niebloids, that
     is:

	  * Explicit template argument lists cannot be	specified  when  calling
     any of them.
	  * None of them are visible to argument-dependent lookup.
	  *  When any of them are found by normal unqualified lookup as the name
     to the left
	    of the function-call operator, argument-dependent lookup  is  inhib-
     ited.

	In  practice,  they may be implemented as function objects, or with spe-
     cial compiler
	extensions.

Parameters
	first1, last1 - the range of elements to examine
	r1	      - the range of elements to examine
	first2, last2 - the range of elements to be used as the prefix
	r2	      - the range of elements to be used as the prefix
	pred	      - the binary predicate that compares  the  projected  ele-
     ments
	proj1	       - the projection to apply to the elements of the range to
     examine
	proj2	      - the projection to apply to the elements of the range  to
     be used as
			the prefix

Return value
	true  if  the  second range matches the prefix of the first range, false
     otherwise.

Complexity
	Linear: at most min(N1, N2) applications of the predicate and both  pro-
     jections.

Possible implementation
	struct starts_with_fn
	{
	    template<std::input_iterator I1, std::sentinel_for<I1> S1,
		     std::input_iterator I2, std::sentinel_for<I2> S2,
		     class Pred = ranges::equal_to,
		     class Proj1 = std::identity, class Proj2 = std::identity>
	    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
	    constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
				      Pred  pred  =  {}, Proj1 proj1 = {}, Proj2
     proj2 = {}) const
	    {
		return	      ranges::mismatch(std::move(first1),	  last1,
     std::move(first2), last2,
					std::move(pred),       std::move(proj1),
     std::move(proj2)
				       ).in2 == last2;
	    }

	    template<ranges::input_range R1, ranges::input_range R2,
		     class Pred = ranges::equal_to,
		     class Proj1 = std::identity, class Proj2 = std::identity>
	    requires std::indirectly_comparable<ranges::iterator_t<R1>,
						ranges::iterator_t<R2>,
						Pred, Proj1, Proj2>
	    constexpr bool operator()(R1&& r1, R2&& r2,
				      Pred pred = {}, Proj1 proj1  =  {},  Proj2
     proj2 = {}) const
	    {
		return (*this)(ranges::begin(r1), ranges::end(r1),
			       ranges::begin(r2), ranges::end(r2),
			       std::move(pred), 	       std::move(proj1),
     std::move(proj2));
	    }
	};

	inline constexpr starts_with_fn starts_with {};

Notes
	       Feature-test macro	   Value    Std 	       Feature
	__cpp_lib_ranges_starts_ends_with	     202106L		 (C++23)
     std::ranges::starts_with,
							  std::ranges::ends_with

Example
     // Run this code

      #include <algorithm>
      #include <iostream>
      #include <ranges>
      #include <string_view>

      int main()
      {
	  using namespace std::literals;

	  constexpr auto ascii_upper = [](char8_t c)
	  {
	      return u8'a' <= c && c <= u8'z' ? static_cast<char8_t>(c + u8'A' -
     u8'a') : c;
	  };

	  constexpr auto cmp_ignore_case = [=](char8_t x, char8_t y)
	  {
	      return ascii_upper(x) == ascii_upper(y);
	  };

	  static_assert(std::ranges::starts_with("const_cast", "const"sv));
	  static_assert(std::ranges::starts_with("constexpr", "const"sv));
	  static_assert(!std::ranges::starts_with("volatile", "const"sv));

	  std::cout << std::boolalpha
		    <<	 std::ranges::starts_with(u8"Constantinopolis",  u8"con-
     stant"sv,
						{}, ascii_upper, ascii_upper) <<
     ' '
		    << std::ranges::starts_with(u8"Istanbul", u8"constant"sv,
						{}, ascii_upper, ascii_upper) <<
     ' '
		    << std::ranges::starts_with(u8"Metropolis", u8"metro"sv,
						cmp_ignore_case) << ' '
		    << std::ranges::starts_with(u8"Acropolis", u8"metro"sv,
						cmp_ignore_case) << '\n';

	  constexpr static auto v = { 1, 3, 5, 7, 9 };
	  constexpr auto odd = [](int x) { return x % 2; };
	  static_assert(std::ranges::starts_with(v, std::views::iota(1)
						  | std::views::filter(odd)
						  | std::views::take(3)));
      }

Output:
      true false true false

See also
	ranges::ends_with checks whether a range ends with another range
	(C++23) 	  (niebloid)
	ranges::mismatch  finds the first position where two ranges differ
	(C++20) 	  (niebloid)
	starts_with	  checks if the string starts with the given prefix
	(C++20) 	  (public member function of
			  std::basic_string<CharT,Traits,Allocator>)
	starts_with	  checks if the string view starts with the given prefix
	(C++20) 	     (public	 member     function	 of	std::ba-
     sic_string_view<CharT,Traits>)

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