std::max_element - cppreference.com

From cppreference.com

Defined in header `<algorithm>`
template< class ForwardIt > ForwardIt max_element( ForwardIt first, ForwardIt last );	(1)	(constexpr since C++17)
template< class ExecutionPolicy, class ForwardIt > ForwardIt max_element( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since C++17)
template< class ForwardIt, class Compare > ForwardIt max_element( ForwardIt first, ForwardIt last, Compare comp );	(3)	(constexpr since C++17)
template< class ExecutionPolicy, class ForwardIt, class Compare > ForwardIt max_element( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );	(4)	(since C++17)

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 the value of the following expression is true:

`std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>`	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>`	(since C++20)

Parameters

first, last	-	the pair of iterators defining the range of elements to examine
policy	-	the execution policy to use
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; }

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

#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.

DR	Applied to	Behavior as published	Correct behavior
LWG 212	C++98	the return value was not specified if `[first,` `last)` is empty	returns `last` in this case
LWG 2150	C++98	the iterator to the first non-smallest element was returned	corrected the return value

min_element	returns the smallest element in a range (function template & algorithm function object)[edit]
ranges::min_element (C++20)
minmax_element (C++11)	returns the smallest and the largest elements in a range (function template & algorithm function object)[edit]
ranges::minmax_element (C++20)
max	returns the greater of the given values (function template & algorithm function object)[edit]
ranges::max (C++20)
ranges::max_element (C++20)	returns the largest element in a range (algorithm function object)[edit]