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

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

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S,
   class T, class Proj = std::identity >
   requires std::indirect_binary_predicate<ranges::equal_to, (1) (since
   std::projected<I, Proj>, C++20)
   const T*>
   constexpr I find( I first, S last, const T& value, Proj proj = {} );
   template< ranges::input_range R, class T, class Proj = std::identity >
   requires std::indirect_binary_predicate<ranges::equal_to,
   std::projected<ranges::iterator_t<R>, Proj>, (2) (since
   const T*> C++20)
   constexpr ranges::borrowed_iterator_t<R> find( R&& r, const T& value,
   Proj proj = {} );
   template< std::input_iterator I, std::sentinel_for<I> S,
   class Proj = std::identity, (3) (since
   std::indirect_unary_predicate<std::projected<I, Proj>> Pred > C++20)
   constexpr I find_if( I first, S last, Pred pred, Proj proj = {} );
   template< ranges::input_range R, class Proj = std::identity,
   std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, (since
   Proj>> Pred > (4) C++20)
   constexpr ranges::borrowed_iterator_t<R>
   find_if( R&& r, Pred pred, Proj proj = {} );
   template< std::input_iterator I, std::sentinel_for<I> S,
   class Proj = std::identity, (5) (since
   std::indirect_unary_predicate<std::projected<I, Proj>> Pred > C++20)
   constexpr I find_if_not( I first, S last, Pred pred, Proj proj = {} );
   template< ranges::input_range R, class Proj = std::identity,
   std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, (since
   Proj>> Pred > (6) C++20)
   constexpr ranges::borrowed_iterator_t<R>
   find_if_not( R&& r, Pred pred, Proj proj = {} );
   Returns the first element in the range [first, last) that satisfies specific
   criteria:
   1) find searches for an element equal to value
   3) find_if searches for an element for which predicate pred returns true
   5) find_if_not searches for an element for which predicate pred returns false
   2,4,6) Same as (1,3,5), but uses r as the source range, as if using ranges::begin(r)
   as first and ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to examine
   r - the range of the elements to examine
   value - value to compare the elements to
   pred - predicate to apply to the projected elements
   proj - projection to apply to the elements

Return value
   Iterator to the first element satisfying the condition or iterator equal to last if
   no such element is found.

Complexity
   At most last - first applications of the predicate and projection.

Possible implementation

First version
struct find_fn {
  template< std::input_iterator I, std::sentinel_for<I> S,
            class T, class Proj = std::identity >
  requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
                        const T*>
  constexpr I operator()( I first, S last, const T& value, Proj proj = {} ) const
  {
      for (; first != last; ++first) {
          if (std::invoke(proj, *first) == value) {
              return first;
          }
      }
      return first;
  }
  template< ranges::input_range R, class T, class Proj = std::identity >
  requires std::indirect_binary_predicate<ranges::equal_to,
                        std::projected<ranges::iterator_t<R>, Proj>,
                        const T*>
  constexpr ranges::borrowed_iterator_t<R>
    operator()( R&& r, const T& value, Proj proj = {} ) const
  {
     return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj));
  } }; inline constexpr find_fn find;

Second version
struct find_if_fn {
  template< std::input_iterator I, std::sentinel_for<I> S,
            class Proj = std::identity,
            std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
  constexpr I operator()( I first, S last, Pred pred, Proj proj = {} ) const
  {
      for (; first != last; ++first) {
          if (std::invoke(pred, std::invoke(proj, *first))) {
              return first;
          }
      }
      return first;
  }
  template< ranges::input_range R, class Proj = std::identity,
            std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred >
  constexpr ranges::borrowed_iterator_t<R>
    operator()( R&& r, Pred pred, Proj proj = {} ) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
  } }; inline constexpr find_if_fn find_if;
                        Third version struct find_if_not_fn {
  template< std::input_iterator I, std::sentinel_for<I> S,
            class Proj = std::identity,
            std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
  constexpr I operator()( I first, S last, Pred pred, Proj proj = {} ) const
  {
      for (; first != last; ++first) {
          if (!std::invoke(pred, std::invoke(proj, *first))) {
              return first;
          }
      }
      return first;
  }
  template< ranges::input_range R, class Proj = std::identity,
            std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred >
  constexpr ranges::borrowed_iterator_t<R>
    operator()( R&& r, Pred pred, Proj proj = {} ) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
  } }; inline constexpr find_if_not_fn find_if_not;

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 int main()
 {
     namespace ranges = std::ranges;
     const int n1 = 3;
     const int n2 = 5;
     const auto v = {4, 1, 3, 2};
     if (ranges::find(v, n1) != v.end()) {
         std::cout << "v contains: " << n1 << '\n';
     } else {
         std::cout << "v does not contain: " << n1 << '\n';
     }
     if (ranges::find(v.begin(), v.end(), n2) != v.end()) {
         std::cout << "v contains: " << n2 << '\n';
     } else {
         std::cout << "v does not contain: " << n2 << '\n';
     }
     auto is_even = [](int x) { return x % 2 == 0; };
     if (auto result = ranges::find_if(v.begin(), v.end(), is_even); result != v.end()) {
       std::cout << "First even element in v: " << *result << '\n';
     } else {
       std::cout << "No even elements in v\n";
     }
     if (auto result = ranges::find_if_not(v, is_even); result != v.end()) {
       std::cout << "First odd element in v: " << *result << '\n';
     } else {
       std::cout << "No odd elements in v\n";
     }
     auto divides_13 = [](int x) { return x % 13 == 0; };
     if (auto result = ranges::find_if(v, divides_13); result != v.end()) {
       std::cout << "First element divisible by 13 in v: " << *result << '\n';
     } else {
       std::cout << "No elements in v are divisible by 13\n";
     }
     if (auto result = ranges::find_if_not(v.begin(), v.end(), divides_13); result != v.end()) {
       std::cout << "First element indivisible by 13 in v: " << *result << '\n';
     } else {
       std::cout << "All elements in v are divisible by 13\n";
     }
 }

Output:
 v contains: 3
 v does not contain: 5
 First even element in v: 4
 First odd element in v: 1
 No elements in v are divisible by 13
 First element indivisible by 13 in v: 4

See also
   ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
   (C++20) a given predicate)
                        (niebloid)
   ranges::find_end finds the last sequence of elements in a certain range
   (C++20) (niebloid)
   ranges::find_first_of searches for any one of a set of elements
   (C++20) (niebloid)
   ranges::mismatch finds the first position where two ranges differ
   (C++20) (niebloid)
   ranges::search searches for a range of elements
   (C++20) (niebloid)
   find
   find_if finds the first element satisfying specific criteria
   find_if_not (function template)
   (C++11)