std::ranges::find_if() algorithm

since C++20

Simplified

// (1)
constexpr I
    find_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    find_if( R&& r, Pred pred, Proj proj = {} );

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

• I - std::input_iterator
• S - std::sentinel_for<I>
• Proj - (none)
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
• (2) - R - std::ranges::input_range

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

Detailed

// (1)
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
    find_if( I first, S last, Pred pred, Proj proj = {} );

// (2)
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>
    find_if( R&& r, Pred pred, Proj proj = {} );

Returns an iterator to the first element in the range satisfiying specific criteria (or last iterator if there is no such iterator):

• (1) Searches for an element for which predicate p returns true.

• (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 examine.
r	The range of elements to examine.
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.

Exceptions

(none)

Possible implementation

find_if(1)

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;

Examples

Main.cpp

#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