std::max_element(3)

💖 std::max_element 💖

Okay so welcome to the std::max_element(3) page from the C++ Standard Libary manual (their spelling, not mine 😭). This is the C++ Standard Library manual, and this function is lowkey a whole main character: it finds the biggest thing in a range. Let's get into it, bestie.

🌸 NAME 🌸

std::max_element — std::max_element. Yes bestie, the name is the name. Self-referential icon behavior.

✨ Synopsis ✨

Defined in header <algorithm>.

• (1) — (constexpr since C++17):

template< class ForwardIt >
ForwardIt max_element( ForwardIt first, ForwardIt last );

• (2) — (since C++17):

template< class ExecutionPolicy, class ForwardIt >
ForwardIt max_element( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last );

• (3) — (constexpr since C++17):

template< class ForwardIt, class Compare >
ForwardIt max_element( ForwardIt first, ForwardIt last,
                       Compare comp );

• (4) — (since C++17):

template< class ExecutionPolicy, class ForwardIt, class Compare >
ForwardIt max_element( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last,
                       Compare comp );

Finds the greatest element in the range [first, last).

• 1) — Elements are compared using operator< (until C++20) / std::less{} (since C++20).
• 3) — Elements are compared using the comparison function comp.
• 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 — forward iterators defining the range to examine.
• policy — the execution policy to use. See execution policy for details.
• comp — comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(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 an object of type ForwardIt can be dereferenced and then implicitly converted to both of them.

🦋 Type requirements 🦋

• ForwardIt must meet the requirements of LegacyForwardIterator.

⭐ Return value ⭐

Iterator to the greatest element in the range [first, last).

If several elements in the range are equivalent to the greatest element, returns the iterator to the first such element. Returns last if the range is empty.

🔥 Complexity 🔥

Given N as std::distance(first, last):

• 1,2) — Exactly max(N-1,0) comparisons using operator< (until C++20) / std::less{} (since C++20).
• 3,4) — Exactly max(N-1,0) applications of the comparison function comp.

🌈 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 💅

max_element (1)

template<class ForwardIt>
ForwardIt max_element(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return last;

    ForwardIt largest = first;

    while (++first != last)
        if (*largest < *first)
            largest = first;

    return largest;
}

max_element (3)

template<class ForwardIt, class Compare>
ForwardIt max_element(ForwardIt first, ForwardIt last, Compare comp)
{
    if (first == last)
        return last;

    ForwardIt largest = first;

    while(++first != last)
        if (comp(*largest, *first))
            largest = first;

    return largest;
}

🎀 Example 🎀

The page also says // Run this code — so run it, bestie:

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

int main()
{
    std::vector<int> v{3, 1, -14, 1, 5, 9, -14, 9};
    std::vector<int>::iterator result;

    result = std::max_element(v.begin(), v.end());
    std::cout << "Max element found at index "
              << std::distance(v.begin(), result)
              << " has value " << *result << '\n';

    result = std::max_element(v.begin(), v.end(), [](int a, int b)
    {
        return std::abs(a) < std::abs(b);
    });
    std::cout << "Absolute max element found at index "
              << std::distance(v.begin(), result)
              << " has value " << *result << '\n';
}

Output

Max element found at index 5 has value 9
Absolute max element found at index 2 has value -14

🌟 Defect reports 🌟

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

• LWG 212 — applied to C++98 — behavior as published: the return value was not specified if [first, last) is empty — corrected behavior: returns last in this case.
• LWG 2150 — applied to C++98 — behavior as published: the iterator to the first non-smallest element was returned — corrected behavior: corrected the return value.

👑 See also 👑

• min_element — returns the smallest element in a range (function template).
• minmax_element (C++11) — returns the smallest and the largest elements in a range (function template).
• max — returns the greater of the given values (function template).
• ranges::max_element (C++20) — returns the largest element in a range (niebloid).

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Source/version/date check, bestie: http://cppreference.com, 2024.06.10, std::max_element(3) — that's the footer, periodt. 💅