std::find_first_of(3)

NAME
std::find_first_of - std::find_first_of

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class ForwardIt >
   InputIt find_first_of( InputIt first, InputIt last, (1) (constexpr since C++20)
                        ForwardIt s_first, ForwardIt
   s_last );
   template< class ExecutionPolicy, class ForwardIt1, class
   ForwardIt2 >
   ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 (2) (since C++17)
   last,
                        ForwardIt2 s_first, ForwardIt2
   s_last );
   template< class InputIt, class ForwardIt, class
   BinaryPred >
   InputIt find_first_of( InputIt first, InputIt last, (3) (constexpr since C++20)
                        ForwardIt s_first, ForwardIt
   s_last,
                        BinaryPred p );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2, class
   BinaryPred >
   ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt (4) (since C++17)
   last,
                        ForwardIt2 s_first, ForwardIt2
   s_last,
                        BinaryPred p );
   Searches the range [first, last) for any of the elements in the range
   [s_first, s_last).
   1) Elements are compared using operator==.
   3) Elements are compared using the given binary predicate p.
   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
   first, last - the range of elements to examine
   s_first, s_last - the range of elements to search for
   policy - the execution policy to use. See execution policy for details.
                     binary predicate which returns true if the elements should be
                     treated as equal.
                     The signature of the predicate function should be equivalent to
                     the following:
                      bool pred(const Type1 &a, const Type2 &b);
   p - 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 objects of types
                     ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly
                     converted to Type1 and Type2 respectively.

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   ForwardIt1 must meet the requirements of LegacyForwardIterator.
   -
   ForwardIt2 must meet the requirements of LegacyForwardIterator.
   -
   BinaryPred must meet the requirements of BinaryPredicate.

Return value
   Iterator to the first element in the range [first, last) that is equal to an element
   from the range [s_first, s_last).
   If [s_first, s_last) is empty or if no such element is found, last is returned.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last) and \(\scriptsize S\)S as
   std::distance(s_first, s_last):
   1,2) At most \(\scriptsize N\cdot S\)N·S comparisons using operator==.
   3,4) At most \(\scriptsize N\cdot S\)N·S applications of the predicate p.

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
                       find_first_of (1)
   template<class InputIt, class ForwardIt>
   InputIt find_first_of(InputIt first, InputIt last,
                        ForwardIt s_first, ForwardIt s_last)
   {
       for (; first != last; ++first)
           for (ForwardIt it = s_first; it != s_last; ++it)
               if (*first == *it)
                   return first;
       return last;
   }
                       find_first_of (3)
   template<class InputIt, class ForwardIt, class BinaryPred>
   InputIt find_first_of(InputIt first, InputIt last,
                        ForwardIt s_first, ForwardIt s_last,
                        BinaryPred p)
   {
       for (; first != last; ++first)
           for (ForwardIt it = s_first; it != s_last; ++it)
               if (p(*first, *it))
                   return first;
       return last;
   }

Example
   The following code searches for any of specified integers in a vector of integers: // Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 auto print_sequence = [](const auto id, const auto& seq, int pos = -1)
 {
     std::cout << id << "{ ";
     for (int i{}; auto const& e : seq)
     {
         const bool mark{i == pos};
         std::cout << (i++ ? ", " : "");
         std::cout << (mark ? "[ " : "") << e << (mark ? " ]" : "");
     }
     std::cout << " }\n";
 };
 int main()
 {
     const std::vector<int> v{0, 2, 3, 25, 5};
     const auto t1 = {19, 10, 3, 4};
     const auto t2 = {1, 6, 7, 9};
     auto find_any_of = [](const auto& v, const auto& t)
     {
         const auto result = std::find_first_of(v.begin(), v.end(),
                        t.begin(), t.end());
         if (result == v.end())
         {
             std::cout << "No elements of v are equal to any element of ";
             print_sequence("t = ", t);
             print_sequence("v = ", v);
         }
         else
         {
             const auto pos = std::distance(v.begin(), result);
             std::cout << "Found a match (" << *result << ") at position " << pos;
             print_sequence(", where t = ", t);
             print_sequence("v = ", v, pos);
         }
     };
     find_any_of(v, t1);
     find_any_of(v, t2);
 }

Output:
 Found a match (3) at position 2, where t = { 19, 10, 3, 4 }
 v = { 0, 2, [ 3 ], 25, 5 }
 No elements of v are equal to any element of t = { 1, 6, 7, 9 }
 v = { 0, 2, 3, 25, 5 }
   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 576 C++98 first and last needed to be they only need to be
                       LegacyForwardIterators LegacyInputIterators
   LWG 1205 C++98 the return value was unclear if returns last in this case
                       [s_first, s_last) is empty

See also
   find
   find_if finds the first element satisfying specific criteria
   find_if_not (function template)
   (C++11)
   ranges::find_first_of searches for any one of a set of elements
   (C++20) (niebloid)