std::ranges::any_of() algorithm

since C++20

Simplified

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

// (2)
constexpr bool any_of( 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>
• R - ranges::input_range
• Proj - (none)
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>

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 bool any_of( 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 bool any_of( R&& r, Pred pred, Proj proj = {} );

Checks if a predicate holds for any of the elements in a range.

• (1) Checks if unary predicate pred returns true for any elements in the range [first; last) (after projecting with the projection proj).

• (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.
proj	Projection to apply to the elements.
pred	Predicate to apply to projected elements.

Return value

true if std::invoke(pred, std::invoke(proj, *i)) != false for at least one iterator i in the range, false otherwise.

important

Returns true if the range is empty.

Complexity

At most last - first applications of predicate and projection.

Exceptions

(none)

Possible implementation

any_of(1) and any_of(2)

struct any_of_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 bool operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        return ranges::find_if(first, last, std::ref(pred), std::ref(proj)) != last;
    }

    template<ranges::input_range R, class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>,Proj>> Pred>
    constexpr bool operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return operator()(ranges::begin(r), ranges::end(r),
                          std::ref(pred), std::ref(proj));
    }
};

inline constexpr any_of_fn any_of;

Examples

Main.cpp

#include <algorithm>
#include <functional>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>

namespace ranges = std::ranges;

constexpr bool some_of(auto&& r, auto&& pred) // some but not any
{
    return not (ranges::any_of(r, pred) or ranges::none_of(r, pred));
}

constexpr auto w = { 1, 2, 3 };
static_assert(!some_of(w, [](int x) { return x < 1; }));
static_assert( some_of(w, [](int x) { return x < 2; }));
static_assert(!some_of(w, [](int x) { return x < 4; }));

int main()
{
    std::vector<int> v(10, 2);
    std::partial_sum(v.cbegin(), v.cend(), v.begin());
    std::cout << "Among the numbers: ";
    ranges::copy(v, std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';

    if (ranges::any_of(v.cbegin(), v.cend(), [](int i) { return i % 2 == 0; }))
        std::cout << "All numbers are even\n";

    if (ranges::none_of(v, std::bind(std::modulus<int>(), std::placeholders::_1, 2)))
        std::cout << "None of them are odd\n";

    auto DivisibleBy = [](int d)
    {
        return [d](int m) { return m % d == 0; };
    };

    if (ranges::any_of(v, DivisibleBy(7)))
        std::cout << "At least one number is divisible by 7\n";
}

Output

Among the numbers: 2 4 6 8 10 12 14 16 18 20
All numbers are even
None of them are odd
At least one number is divisible by 7