std::find_first_of() algorithm

od C++20

// (1)
template< class InputIt, class ForwardIt >
constexpr InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last );

// (2)
template< class ForwardIt1, class ForwardIt2 >
constexpr ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt1 last,
                          ForwardIt2 s_first, ForwardIt2 s_last );

// (4)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt last,
                          ForwardIt2 s_first, ForwardIt2 s_last,
                          BinaryPredicate p );

od C++17

// (1)
template< class InputIt, class ForwardIt >
InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last );

// (2)
template< class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt1 last,
                          ForwardIt2 s_first, ForwardIt2 s_last );

// (4)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt last,
                          ForwardIt2 s_first, ForwardIt2 s_last,
                          BinaryPredicate p );

do C++17

// (1)
template< class InputIt, class ForwardIt >
InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last );

// (2)
template< class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_end( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last );

Searches the range [first; last) for any of the elements in the range [s_first; s_last).

• (1) Compares elements with operator==.

• (2) Compares elements with the given binary predicate p.

• (3 - 4) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first last	The range of elements to examine.
s_first s_last	The range of elements to search for.
policy	The execution policy to use. See execution policy for details.
p	Binary predicate which returns true if the elements should be treated as equal. The signature of the function should be equivalent to the following: bool fun(const Type1& a, const Type2& b); The signature does not need to have const&. The function must not modify the objects passed to it. Must accept all values of type (possibly const) Type1 and Type2, regardless of value category (so Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (od C++11)) The types Type1 and Type2 must be such that an object of type ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to them.

Type requirements

InputIt	LegacyInputIterator
ForwardIt ForwardIt1 ForwardIt2	LegacyForwardIterator
BinaryPredicate	BinaryPredicate

Return value

Iterator to the first element in the range [first; last) that is equal to an element from the range [s_first; s_last).

If [s_first; s_last) is empty or if no such element is found, last is returned.

Complexity

Given S as std::distance(s_first, s_last) and N as std::distance(first, last):

• (1, 3) - At most N * S comparisons.
• (2, 4) - At most N * S applications of p.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

find_first_of (1)

template<class InputIt, class ForwardIt>
InputIt find_first_of(InputIt first, InputIt last,
                      ForwardIt s_first, ForwardIt s_last)
{
    for (; first != last; ++first)
        for (ForwardIt it = s_first; it != s_last; ++it)
            if (*first == *it)
                return first;
    return last;
}

find_first_of (2)

template<class InputIt, class ForwardIt, class BinaryPredicate>
InputIt find_first_of(InputIt first, InputIt last,
                      ForwardIt s_first, ForwardIt s_last,
                      BinaryPredicate p)
{
    for (; first != last; ++first)
        for (ForwardIt it = s_first; it != s_last; ++it)
            if (p(*first, *it))
                return first;
    return last;
}

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

auto print_sequence = [](auto const id, auto const& seq, int pos = -1)
{
    std::cout << id << "{ ";
    for (int i{}; auto const& e : seq)
    {
        const bool mark{i == pos};
        std::cout << (i++ ? ", " : "");
        std::cout << (mark ? "[ " : "") << e << (mark ? " ]" : "");
    }
    std::cout << " }\n";
};

int main()
{
    const std::vector<int> v{0, 2, 3, 25, 5};
    const auto t1 = {19, 10, 3, 4};
    const auto t2 = {1, 6, 7, 9};

    auto find_any_of = [](const auto& v, const auto& t)
    {
        const auto result = std::find_first_of(v.begin(), v.end(),
                                               t.begin(), t.end());
        if (result == v.end())
        {
            std::cout << "No elements of v are equal to any element of ";
            print_sequence("t = ", t);
            print_sequence("v = ", v);
        }
        else
        {
            const auto pos = std::distance(v.begin(), result);
            std::cout << "Found a match (" << *result << ") at position " << pos;
            print_sequence(", where t = ", t);
            print_sequence("v = ", v, pos);
        }
    };

    find_any_of(v, t1);
    find_any_of(v, t2);
}

Output

Found a match (3) at position 2, where t = { 19, 10, 3, 4 }
v = { 0, 2, [ 3 ], 25, 5 }
No elements of v are equal to any element of t = { 1, 6, 7, 9 }
v = { 0, 2, 3, 25, 5 }