std::find,std::find_if,std::find_if_not(3)

NAME
std::find,std::find_if,std::find_if_not - std::find,std::find_if,std::find_if_not

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class T > (constexpr since
   InputIt find( InputIt first, InputIt last, C++20)
   const T& value ); (until C++26)
   template< class InputIt, class T =
   typename std::iterator_traits
                        (since C++26)
    <InputIt>::value_type >
   constexpr InputIt find( InputIt first,
   InputIt last, const T& value );
   template< class ExecutionPolicy, class
   ForwardIt, class T >
                        (since C++17)
   ForwardIt find( ExecutionPolicy&& policy, (until C++26)
                   ForwardIt first, ForwardIt
   last, const T& value );
   template< class ExecutionPolicy,
             class ForwardIt, class T =
   typename std::iterator_traits
                        (since C++26)
    <ForwardIt>::value_type >
   ForwardIt find( ExecutionPolicy&& policy, (1)
                   ForwardIt first, ForwardIt
   last, const T& value );
   template< class InputIt, class UnaryPred > (constexpr since
   InputIt find_if( InputIt first, InputIt (3) C++20)
   last, UnaryPred p ); (2)
   template< class ExecutionPolicy, class
   ForwardIt, class UnaryPred >
   ForwardIt find_if( ExecutionPolicy&& (4) (since C++17)
   policy,
                      ForwardIt first,
   ForwardIt last, UnaryPred p );
   template< class InputIt, class UnaryPred > (since C++11)
   InputIt find_if_not( InputIt first, (5) (constexpr since
   InputIt last, UnaryPred q ); C++20)
   template< class ExecutionPolicy, class
   ForwardIt, class UnaryPred >
   ForwardIt find_if_not( ExecutionPolicy&& (6) (since C++17)
   policy,
                        ForwardIt first,
   ForwardIt last, UnaryPred q );
   Returns an iterator to the first element in the range [first, last) that satisfies
   specific criteria (or last if there is no such iterator).
   1) find searches for an element equal to value (using operator==).
   3) find_if searches for an element for which predicate p returns true.
   5) find_if_not searches for an element for which predicate q returns false.
   2,4,6) Same as (1,3,5), 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
   value - value to compare the elements to
   policy - the execution policy to use. See execution policy for details.
                 unary predicate which returns true for the required element.
                 The expression p(v) must be convertible to bool for every argument v
   p - of type (possibly const) VT, where VT is the value type of InputIt,
                 regardless of value category, and must not modify v. Thus, a parameter
                 type of VT&is not allowed
                 , nor is VT unless for VT a move is equivalent to a copy
                 (since C++11).
                 unary predicate which returns false for the required element.
                 The expression q(v) must be convertible to bool for every argument v
   q - of type (possibly const) VT, where VT is the value type of InputIt,
                 regardless of value category, and must not modify v. Thus, a parameter
                 type of VT&is not allowed
                 , nor is VT unless for VT a move is equivalent to a copy
                 (since C++11).

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   UnaryPredicate must meet the requirements of Predicate.

Return value
   The first iterator it in the range [first, last) satisfying the following condition
   or last if there is no such iterator:
   1,2) *it == value is true.
   3,4) p(*it) is true.
   5,6) q(*it) is false.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1,2) At most \(\scriptsize N\)N comparisons with value using operator==.
   3,4) At most \(\scriptsize N\)N applications of the predicate p.
   5,6) At most \(\scriptsize N\)N applications of the predicate q.

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
   template<class InputIt, class T = typename std::iterator_traits<InputIt>::value_type>
   constexpr InputIt find(InputIt first, InputIt last, const T& value)
   {
       for (; first != last; ++first)
           if (*first == value)
               return first;
       return last;
   }
                        find_if
   template<class InputIt, class UnaryPred>
   constexpr InputIt find_if(InputIt first, InputIt last, UnaryPred p)
   {
       for (; first != last; ++first)
           if (p(*first))
               return first;
       return last;
   }
                        find_if_not
   template<class InputIt, class UnaryPred>
   constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
   {
       for (; first != last; ++first)
           if (!q(*first))
               return first;
       return last;
   }

Notes
   If C++11 is not available, an equivalent to std::find_if_not is to use std::find_if
   with the negated predicate.
   template<class InputIt, class UnaryPred>
   InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
   {
       return std::find_if(first, last, std::not1(q));
   }
             Feature-test macro Value Std Feature
   __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
                        algorithms (1,2)

Example
   The following example finds numbers in given sequences. // Run this code
 #include <algorithm>
 #include <array>
 #include <cassert>
 #include <complex>
 #include <iostream>
 #include <vector>
 int main()
 {
     const auto v = {1, 2, 3, 4};
     for (const int n : {3, 5})
         (std::find(v.begin(), v.end(), n) == std::end(v))
             ? std::cout << "v does not contain " << n << '\n'
             : std::cout << "v contains " << n << '\n';
     auto is_even = [](int i) { return i % 2 == 0; };
     for (const auto& w : {std::array{3, 1, 4}, {1, 3, 5}})
         if (auto it = std::find_if(begin(w), end(w), is_even); it != std::end(w))
             std::cout << "w contains an even number " << *it << '\n';
         else
             std::cout << "w does not contain even numbers\n";
     std::vector<std::complex<double>> nums{{4, 2}};
     #ifdef __cpp_lib_algorithm_default_value_type
         // T gets deduced making list-initialization possible
         const auto it = std::find(nums.begin(), nums.end(), {4, 2});
     #else
         const auto it = std::find(nums.begin(), nums.end(), std::complex<double>{4, 2});
     #endif
     assert(it == nums.begin());
 }

Output:
 v contains 3
 v does not contain 5
 w contains an even number 4
 w does not contain even numbers
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      T was required to be EqualityComparable,
   LWG 283 C++98 but removed the requirement
                      the value type of InputIt might not be T

See also
                       finds the first two adjacent items that are equal (or satisfy a
   adjacent_find given predicate)
                       (function template)
   find_end finds the last sequence of elements in a certain range
                       (function template)
   find_first_of searches for any one of a set of elements
                       (function template)
   mismatch finds the first position where two ranges differ
                       (function template)
   search searches for a range of elements
                       (function template)
   ranges::find
   ranges::find_if
   ranges::find_if_not finds the first element satisfying specific criteria
   (C++20) (niebloid)
   (C++20)
   (C++20)