std::max_element(3)

NAME
std::max_element - std::max_element

Synopsis
   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, (2) (since C++17)
                        ForwardIt first, ForwardIt last
   );
   template< class ForwardIt, class Compare >
   ForwardIt max_element( ForwardIt first, ForwardIt last, (3) (constexpr since C++17)
                        Compare comp );
   template< class ExecutionPolicy, class ForwardIt, class
   Compare >
   ForwardIt max_element( ExecutionPolicy&& policy, (4) (since C++17)
                        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.
                 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);
   comp -
                 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 \(\scriptsize N\)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.

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
// 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
   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 returns last in this
                       [first, last) is empty case
   LWG 2150 C++98 the iterator to the first non-smallest corrected the return
                       element was returned value

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