std::ranges::find() algorithm

od C++20

Simplified

// (1)
constexpr I
    find( I first, S last, const T& value, Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    find( R&& r, const T& value, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::input_iterator
• S - std::sentinel_for<I>
• T - (none)
• Proj - (none)
• (2) - R - std::ranges::input_range

The Proj template argument has a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>>
• (2) - indirect_binary_predicate<ranges::equal_to, projected<ranges::iterator_t<R>, Proj>, const T*>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
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
    find( I first, S last, const T& value, Proj proj = {} );

// (2)
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>
    find( R&& r, const T& value, Proj proj = {} );

Returns the last element in the range [first; last) that satisfies specific criteria:

• (1) Searches for an element equal to value (using operator==)

• (2) Same as (1), 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.

Parameters

first last	The range of elements to apply the function to.
r	The range of elements to apply the function to.
value	Value to compare the elements too.
proj	Projection to apply to the elements.

Return value

• (1) Let i be the last iterator in the range [first; last) for which *i == value is true. Returns a value of type ranges::subrange<I> initialized as follows:

  {
    i,
    last
  }

  or { last, last } if no such iterator is found.

• (2) Same as (1) but the return type is ranges::borrowed_subrange_t<I>.

Complexity

Exactly last - first applications of the predicate and projection.

Exceptions

(none)

Possible implementation

These implementations only show the slower algorithm used when I models forward_iterator.

find(1) and find(2)

struct find_last_fn
{
    template<std::forward_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 ranges::subrange<I>
        operator()(I first, S last, const T &value, Proj proj = {}) const
    {
        // Note: if I is mere forward_iterator, we may only go from begin to end.
        I found {};
        for (; first != last; ++first)
            if (std::invoke(proj, *first) == value)
                found = first;

        if (found == I {})
            return {first, first};

        return {found, std::ranges::next(found, last)};
    }

    template<ranges::forward_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_subrange_t<R>
        operator()(R&& r, const T &value, Proj proj = {}) const
    {
        return this->operator()(ranges::begin(r), ranges::end(r), value, std::ref(proj));
    }
};

inline constexpr find_last_fn find_last;

Notes

ranges::find_last has better efficiency on common implementations if I models bidirectional_iterator or (better) random_access_iterator.

Examples

Main.cpp

#include <algorithm>
#include <forward_list>
#include <iomanip>
#include <iostream>
#include <string_view>

int main()
{
  constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};

  {
    constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), 3);
    constexpr auto i2 = std::ranges::find_last(v, 3);
    static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
    static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
  }
  {
    constexpr auto i1 = std::ranges::find_last(v.begin(), v.end(), -3);
    constexpr auto i2 = std::ranges::find_last(v, -3);
    static_assert(i1.begin() == v.end());
    static_assert(i2.begin() == v.end());
  }

  auto abs = [](int x) { return x < 0 ? -x : x; };

  {
    auto pred = [](int x) { return x == 3; };
    constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
    constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
    static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
    static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
  }
  {
    auto pred = [](int x) { return x == -3; };
    constexpr auto i1 = std::ranges::find_last_if(v.begin(), v.end(), pred, abs);
    constexpr auto i2 = std::ranges::find_last_if(v, pred, abs);
    static_assert(i1.begin() == v.end());
    static_assert(i2.begin() == v.end());
  }

  {
    auto pred = [](int x) { return x == 1 or x == 2; };
    constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
    constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
    static_assert(std::ranges::distance(v.begin(), i1.begin()) == 5);
    static_assert(std::ranges::distance(v.begin(), i2.begin()) == 5);
  }
  {
    auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
    constexpr auto i1 = std::ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
    constexpr auto i2 = std::ranges::find_last_if_not(v, pred, abs);
    static_assert(i1.begin() == v.end());
    static_assert(i2.begin() == v.end());
  }

  using P = std::pair<std::string_view, int>;
  std::forward_list<P> list
  {
    {"one", 1}, {"two", 2}, {"three", 3},
      {"one", 4}, {"two", 5}, {"three", 6},
  };
  auto cmp_one = [](const std::string_view &s) { return s == "one"; };

  // find latest element that satisfy the comparator, and projecting pair::first
  const auto subrange = std::ranges::find_last_if(list, cmp_one, &P::first);

  // print the found element and the "tail" after it
  for (P const& e : subrange)
    std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
  std::cout << '\n';
}

Output

{"one", 4} {"two", 5} {"three", 6}