std::max_element() algorithm

od C++17

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

// (2)
template< class ForwardIt, class Compare >
ForwardIt max_element( ForwardIt first, ForwardIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt max_element( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class Compare >
ForwardIt max_element( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last, Compare comp );

do C++17

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

// (2)
template< class ForwardIt, class Compare >
ForwardIt max_element( ForwardIt first, ForwardIt last, Compare comp );

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

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

Parameters

first last	The range to find the largest element in.
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). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type 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 ForwardIt can be implicitly converted to both of them.

Type requirements

RandomIt

LegacyRandomAccessIterator

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):

Exactly max(N - 1, 0) comparisons.

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;
    ++first;

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

    return largest;
}

max_element(2)

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

    ForwardIt largest = first;
    ++first;

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

    return largest;
}

Examples

Main.cpp

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

int main()
{
    std::vector<int> v {3, 1, -14, 1, 5, 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