std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not

Since C++26

Header: <algorithm>

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

# Declarations

Call signature

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>
find_last( I first, S last, const T& value, Proj proj = {} );

(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)

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

(since C++23) (until C++26)

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

(since C++26)

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>
find_last_if( I first, S last, Pred pred, Proj proj = {} );

(since C++23)

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>
find_last_if( R&& r, Pred pred, Proj proj = {} );

(since C++23)

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>
find_last_if_not( I first, S last, Pred pred, Proj proj = {} );

(since C++23)

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>
find_last_if_not( R&& r, Pred pred, Proj proj = {} );

(since C++23)

# Parameters

# 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.

# 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);
}

# See also