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std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not - cppreference.com

Call signature

(1) (since C++23)
(until C++26) template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> find_last( I first, S last, const T& value, Proj proj = {} ); (since C++26) (2) (since C++23)
(until C++26) (since C++26) (3) (since C++23) (4) (since C++23) (5) (since C++23) (6) (since C++23)

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

1) find_last searches for an element equal to value.

3) find_last_if searches for the last element in the range [first, last) for which predicate pred returns true.

5) find_last_if_not searches for the last element in the range [first, last) 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)

first

and

ranges::end(r)

last

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

[edit] Parameters first, last - the iterator-sentinel pair defining 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 [edit] Return value

1,3,5) Let i be the last iterator in the range [first, last) for which E is true.

[edit] Complexity

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

[edit] Notes

ranges::find_last, ranges::find_last_if, ranges::find_last_if_not have better efficiency on common implementations if I models bidirectional_iterator or (better) random_access_iterator.

[edit] Possible implementation

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

find_last (1,2)

struct find_last_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             class T = std::projected_value_t<iterator_t<R>, Proj>>
    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.
        std::optional<I> found;
        for (; first != last; ++first)
            if (std::invoke(proj, *first) == value)
                found = first;
 
        if (!found)
            return {first, first};
 
        return {*found, std::ranges::next(*found, last)};
    }
 
    template<ranges::forward_range R,
             class Proj = std::identity,
             class T = std::projected_value_t<iterator_t<R>, Proj>>
    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;

find_last_if (3,4)

struct find_last_if_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        // Note: if I is mere forward_iterator, we may only go from begin to end.
        std::optional<I> found;
        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first)))
                found = first;
 
        if (!found)
            return {first, first};
 
        return {*found, std::ranges::next(*found, last)};
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate
                 <std::projected<ranges::iterator_t<R>, Proj>> Pred>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return this->operator()(ranges::begin(r), ranges::end(r),
                                std::ref(pred), std::ref(proj));
    }
};
 
inline constexpr find_last_if_fn find_last_if;

find_last_if_not (5,6)

struct find_last_if_not_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        // Note: if I is mere forward_iterator, we may only go from begin to end.
        std::optional<I> found;
        for (; first != last; ++first)
            if (!std::invoke(pred, std::invoke(proj, *first)))
                found = first;
 
        if (!found)
            return {first, first};
 
        return {*found, std::ranges::next(*found, last)};
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate
                 <std::projected<ranges::iterator_t<R>, Proj>> Pred>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return this->operator()(ranges::begin(r), ranges::end(r),
                                std::ref(pred), std::ref(proj));
    }
};
 
inline constexpr find_last_if_not_fn find_last_if_not;

[edit] Example

#include <algorithm>
#include <cassert>
#include <forward_list>
#include <iomanip>
#include <iostream>
#include <string_view>
 
int main()
{
    namespace ranges = std::ranges;
 
    constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2};
 
    {
        constexpr auto i1 = ranges::find_last(v.begin(), v.end(), 3);
        constexpr auto i2 = ranges::find_last(v, 3);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        constexpr auto i1 = ranges::find_last(v.begin(), v.end(), -3);
        constexpr auto i2 = 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 = ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = ranges::find_last_if(v, pred, abs);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == -3; };
        constexpr auto i1 = ranges::find_last_if(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = 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 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = ranges::find_last_if_not(v, pred, abs);
        static_assert(ranges::distance(v.begin(), i1.begin()) == 5);
        static_assert(ranges::distance(v.begin(), i2.begin()) == 5);
    }
    {
        auto pred = [](int x) { return x == 1 or x == 2 or x == 3; };
        constexpr auto i1 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs);
        constexpr auto i2 = 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 = ranges::find_last_if(list, cmp_one, &P::first);
 
    std::cout << "The found element and the tail after it are:\n";
    for (P const& e : subrange)
        std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} ";
    std::cout << '\n';
 
#if __cpp_lib_algorithm_default_value_type
    const auto i3 = ranges::find_last(list, {"three", 3}); // (2) C++26
#else
    const auto i3 = ranges::find_last(list, P{"three", 3}); // (2) C++23
#endif
    assert(i3.begin()->first == "three" && i3.begin()->second == 3);
}

Output:

The found element and the tail after it are:
{"one", 4} {"two", 5} {"three", 6}

[edit] See also

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