std::includes(3)

NAME
std::includes - std::includes

Synopsis
   Defined in header <algorithm>
   template< class InputIt1, class InputIt2 >
   bool includes( InputIt1 first1, InputIt1 last1, (1) (constexpr since C++20)
                  InputIt2 first2, InputIt2 last2 );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2 >
   bool includes( ExecutionPolicy&& policy, (2) (since C++17)
                  ForwardIt1 first1, ForwardIt1 last1,
                  ForwardIt2 first2, ForwardIt2 last2 );
   template< class InputIt1, class InputIt2, class Compare
   >
   bool includes( InputIt1 first1, InputIt1 last1, (3) (constexpr since C++20)
                  InputIt2 first2, InputIt2 last2, Compare
   comp );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2, class
   Compare >
   bool includes( ExecutionPolicy&& policy, (4) (since C++17)
                  ForwardIt1 first1, ForwardIt1 last1,
                  ForwardIt2 first2, ForwardIt2 last2,
   Compare comp );
   Returns true if the sorted range [first2, last2) is a subsequence of the sorted
   range [first1, last1) (a subsequence need not be contiguous).
   1) If [first1, last1) or [first2, last2) is not sorted with respect to
   operator<
   (until C++20)
   std::less{}
   (since C++20), the behavior is undefined.
   3) If [first1, last1) or [first2, last2) is not sorted with respect to comp, the
   behavior is undefined.
   2,4) Same as (1,3), but executed according to policy.
   These overloads participate in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)

Parameters
   first1, last1 - the sorted range of elements to examine
   first2, last2 - the sorted range of elements to search for
   policy - the execution policy to use. See execution policy for details.
                   comparison function object (i.e. an object that satisfies the
                   requirements of Compare) which returns true if the first argument is
                   less than (i.e. is ordered before) the second.
                   The signature of the comparison function should be equivalent to the
                   following:
                   bool cmp(const Type1& a, const Type2& b);
   comp -
                   While the signature does not need to have const&, the function must
                   not modify the objects passed to it and must be able to accept all
                   values of type (possibly const) Type1 and Type2 regardless of value
                   category (thus, Type1& is not allowed
                   , nor is Type1 unless for Type1 a move is equivalent to a copy
                   (since C++11)).
                   The types Type1 and Type2 must be such that an object of type
                   InputIt can be dereferenced and then implicitly converted to both of
                   them.

Type requirements
   -
   InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
   -
   ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
   -
   Compare must meet the requirements of Compare.

Return value
   true if [first2, last2) is a subsequence of [first1, last1); otherwise false.
   An empty sequence is a subsequence of any sequence, so true is returned if
   [first2, last2) is empty.

Complexity
   Given \(\scriptsize N_1\)N
   1 as std::distance(first1, last1) and \(\scriptsize N_2\)N
   2 as std::distance(first2, last2):
   1,2) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N
   1+N
   2)-1 comparisons using
   operator<
   (until C++20)
   std::less{}
   (since C++20).
   3,4) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N
   1+N
   2)-1 applications of the comparison function comp.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
                        include (1)
   template<class InputIt1, class InputIt2>
   bool includes(InputIt1 first1, InputIt1 last1,
                 InputIt2 first2, InputIt2 last2)
   {
       for (; first2 != last2; ++first1)
       {
           if (first1 == last1 || *first2 < *first1)
               return false;
           if (!(*first1 < *first2))
               ++first2;
       }
       return true;
   }
                        include (3)
   template<class InputIt1, class InputIt2, class Compare>
   bool includes(InputIt1 first1, InputIt1 last1,
                 InputIt2 first2, InputIt2 last2, Compare comp)
   {
       for (; first2 != last2; ++first1)
       {
           if (first1 == last1 || comp(*first2, *first1))
               return false;
           if (!comp(*first1, *first2))
               ++first2;
       }
       return true;
   }

Example
// Run this code
 #include <algorithm>
 #include <cctype>
 #include <iostream>
 template<class Os, class Co>
 Os& operator<<(Os& os, const Co& v)
 {
     for (const auto& i : v)
         os << i << ' ';
     return os << '\t';
 }
 int main()
 {
     const auto
         v1 = {'a', 'b', 'c', 'f', 'h', 'x'},
         v2 = {'a', 'b', 'c'},
         v3 = {'a', 'c'},
         v4 = {'a', 'a', 'b'},
         v5 = {'g'},
         v6 = {'a', 'c', 'g'},
         v7 = {'A', 'B', 'C'};
     auto no_case = [](char a, char b) { return std::tolower(a) < std::tolower(b); };
     std::cout
     << v1 << "\nincludes:\n" << std::boolalpha
     << v2 << ": " << std::includes(v1.begin(), v1.end(), v2.begin(), v2.end()) << '\n'
     << v3 << ": " << std::includes(v1.begin(), v1.end(), v3.begin(), v3.end()) << '\n'
     << v4 << ": " << std::includes(v1.begin(), v1.end(), v4.begin(), v4.end()) << '\n'
     << v5 << ": " << std::includes(v1.begin(), v1.end(), v5.begin(), v5.end()) << '\n'
     << v6 << ": " << std::includes(v1.begin(), v1.end(), v6.begin(), v6.end()) << '\n'
     << v7 << ": " << std::includes(v1.begin(), v1.end(), v7.begin(), v7.end(), no_case)
           << " (case-insensitive)\n";
 }

Output:
 a b c f h x
 includes:
 a b c : true
 a c : true
 a a b : false
 g : false
 a c g : false
 A B C : true (case-insensitive)
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 1205 C++98 the return value was unclear if returns true in this
                       [first2, last2) is empty case

See also
   set_difference computes the difference between two sets
                    (function template)
   search searches for a range of elements
                    (function template)
   ranges::includes returns true if one sequence is a subsequence of another
   (C++20) (niebloid)