std::find_first_of

Since C++17

Header: <algorithm>

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

# Declarations

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

(constexpr since C++20)

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

(since C++17)

template< class InputIt, class ForwardIt, class BinaryPred >
InputIt find_first_of( InputIt first, InputIt last,
ForwardIt s_first, ForwardIt s_last,
BinaryPred p );

(constexpr since C++20)

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

(since C++17)

# 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
p: binary predicate which returns true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively.

# 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).

# Example

#include <algorithm>
#include <iostream>
#include <vector>
 
auto print_sequence = [](const auto id, const auto& 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);
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 576	C++98	first and last needed to be LegacyForwardIterators	they only need to beLegacyInputIterators
LWG 1205	C++98	the return value was unclear if [s_first, s_last) is empty	returns last in this case