std::max_element

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.

[edit] 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.

[edit] Complexity

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

1,2) Exactly \(\scriptsize \max(N-1,0)\)max(N-1,0) comparisons using operator<(until C++20)std::less {}(since C++20).

3,4) Exactly \(\scriptsize \max(N-1,0)\)max(N-1,0) applications of the comparison function comp.

[edit] 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.

[edit] 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;
}

[edit] Example

Run this code

#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

[edit] 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