Функция max

 int max (int a, int b, int c)
      {
          return max( max(a,b), c);
      }

#include <cstdio>
#include <cstdarg>
 
int my_max( int n, ... ) {
    int m = 0;
    va_list l;
    if ( n > 0 ) {
        va_start( l, n );
        for ( ; n > 0; n-- ) {
            int v = va_arg( l, int );
            if ( v > m ) m = v;
        }
        va_end( l );
    }
    return m;
}
 
int main()
{
    printf( "%d\n", my_max( 3, 1,3,5 ) );
    printf( "%d\n", my_max( 5, 7,1,9,3,5 ) );
    return 0;
}

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;
}

#include <iostream>
#include <algorithm>
#include <iterator>
 
int main()
{
    std::cout<<"Enter numbers (any letter to stop):";
    std::cout<<"Max elem is: "
             <<*std::max_element(std::istream_iterator<int>(std::cin), std::istream_iterator<int>())<<"\n";
    
    std::cout<<"\n";
    return 0;
}

#include <iostream>
 
template < typename T >
inline const T& largest_of( const T& a, const T& b )
{
    return a > b ? a : b;
}
 
template < typename T, typename ...REST >
inline const T& largest_of( const T& first, const REST&... rest... )
{
    return largest_of( first, largest_of( rest... ) );
}
 
int main()
{
    std::cout << largest_of( 9, 1, 7 ) << '\n' ;
    std::cout << largest_of( 1, 9, 2, 8, 3, 7, 4, 6, 5 ) << '\n' ;
}

std::cout << std::max({1, 2, 4}) << std::endl;