Функция, которая возвращает индекс первого элемента

#include <iostream>
#include <algorithm>
 
using namespace std;
 
int firstLess( const vector<int>& array, int value )
{
   auto it = find_if( array.begin(), array.end(), [&](int v) {return v<value;} );
   if( it == array.end() ) return -1;
   return distance( array.begin(), it );
}
 
int main() {
   vector<int> array = { 16, 13, 12, 8,5,4,1,-1,-2,-5};
   cout << firstLess( array, 0 ) << endl;
   return 0;
}

#include <iostream>
#include <array>
#include <functional>
#include <vector>
#include <algorithm>
#include <iterator>
 
template<typename T, typename U>
int index(const T* ar, const unsigned size, const U val)
{
    auto pos = std::find_if(ar, ar+size, std::bind(std::greater<T>(), val, std::placeholders::_1));
    return (pos == ar+size) ? -1 : std::distance(ar, pos);
}
 
int main() 
{
    const unsigned size = 5;
    
    const int N = 9;
    
    std::array<int, size> ar1 = {16,8,3,4,1};
    int ar2[size] =             {18,14,16,16,13};
    std::vector<double> v =     {12.3, 12.6, 13.1, 8.02};
    std::cout<<"in ar1: "<<index(ar1.data(), size, N)<<"\nin ar2: "<<index(ar2, size, N)<<"\n"
             <<"in v: "<<index(v.data(), v.size(), N)<<"\n";
    
    return 0;
}