std::min_element(3)

NAME
std::min_element - std::min_element

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >
   ForwardIt min_element( ForwardIt first, (until C++17)
   ForwardIt last );
   template< class ForwardIt >
   constexpr ForwardIt min_element( ForwardIt (since C++17)
   first, ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt
   >
                        (2) (since C++17)
   ForwardIt min_element( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last ); (1)
   template< class ForwardIt, class Compare >
   ForwardIt min_element( ForwardIt first, (until C++17)
   ForwardIt last, Compare comp );
   template< class ForwardIt, class Compare >
   constexpr ForwardIt min_element( ForwardIt (since C++17)
   first, ForwardIt last, Compare comp ); (3)
   template< class ExecutionPolicy, class
   ForwardIt, class Compare >
                        (4) (since C++17)
   ForwardIt min_element( ExecutionPolicy&& policy,
   ForwardIt first, ForwardIt last, Compare comp );
   Finds the smallest element in the range [first, last).
   1) Elements are compared using operator<.
   3) Elements are compared using the given binary comparison function comp.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   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) is true.

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 a is less than b.
                 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 smallest element in the range [first, last). If several elements in
   the range are equivalent to the smallest element, returns the iterator to the first
   such element. Returns last if the range is empty.

Complexity
   Exactly max(N-1,0) comparisons, where N = std::distance(first, last).

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

First version
   template<class ForwardIt>
   ForwardIt min_element(ForwardIt first, ForwardIt last)
   {
       if (first == last) return last;
       ForwardIt smallest = first;
       ++first;
       for (; first != last; ++first) {
           if (*first < *smallest) {
               smallest = first;
           }
       }
       return smallest;
   }

Second version
   template<class ForwardIt, class Compare>
   ForwardIt min_element(ForwardIt first, ForwardIt last, Compare comp)
   {
       if (first == last) return last;
       ForwardIt smallest = first;
       ++first;
       for (; first != last; ++first) {
           if (comp(*first, *smallest)) {
               smallest = first;
           }
       }
       return smallest;
   }

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 int main()
 {
     std::vector<int> v{3, 1, 4, 1, 5, 9};
     std::vector<int>::iterator result = std::min_element(v.begin(), v.end());
     std::cout << "min element at: " << std::distance(v.begin(), result);
 }

Output:
 min element at: 1

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