std::find_first_of(3)

💖 std::find_first_of(3) 💖

Okay so this is the std::find_first_of(3) page from the C++ Standard Libary manual, and it's literally the algorithm for “do I recognize anyone in this range?” energy. No cap: it searches the range [first, last) for any of the elements in the range [s_first, s_last). It lives in the header <algorithm>. Let’s go, bestie 🫶.

---

🎀 NAME 🎀

std::find_first_of — std::find_first_of

---

✨ Synopsis ✨

Defined in header <algorithm>

template< class InputIt, class ForwardIt >
InputIt find_first_of( InputIt first, InputIt last,      (1)
                       (constexpr since C++20)
                       ForwardIt s_first, ForwardIt s_last );

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt1 last,   (2) (since C++17)
                          ForwardIt2 s_first, ForwardIt2 s_last );

template< class InputIt, class ForwardIt, class BinaryPred >
InputIt find_first_of( InputIt first, InputIt last,      (3)
                       (constexpr since C++20)
                       ForwardIt s_first, ForwardIt s_last,
                       BinaryPred p );

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first, ForwardIt1 last,   (4) (since C++17)
                          ForwardIt2 s_first, ForwardIt2 s_last,
                          BinaryPred p );

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

• (1) Elements are compared using operator==.
• (3) Elements are compared using the given binary predicate p.
• (2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if:

• std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until C++20)
• std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20)

---

🦋 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 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. So 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.

---

🌸 Type requirements 🌸

• InputIt must meet the requirements of LegacyInputIterator.
• ForwardIt must meet the requirements of LegacyForwardIterator.
• ForwardIt1 must meet the requirements of LegacyForwardIterator.
• ForwardIt2 must meet the requirements of LegacyForwardIterator.
• BinaryPred must meet the requirements of 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. That’s the “no match, bye” behavior 💅.

---

🔥 Complexity 🔥

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

• (1,2) At most N·S comparisons using operator==.
• (3,4) At most N·S applications of the predicate 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 (3):

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

---

🍓 Example 🍓

The following code searches for any of specified integers in a vector of integers:

// Run this code

#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);
}

🌈 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 }

---

💅 Defect reports 💅

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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

---

👑 See also 👑

• find
• find_if — finds the first element satisfying specific criteria.
• find_if_not — (function template) (C++11)
• ranges::find_first_of — searches for any one of a set of elements. (C++20) (niebloid)

---

Source/date straight from the footer, bestie: http://cppreference.com, 2024.06.10. Now go find your first of something ✨🦋💜.