Program for Joining Strings in C++

#include <string>
#include <iostream>

const static int num = 4;

template<typename T>
void join(T* a[], tsize n, T c, std::basic_string<T>& s) {
   s.clear( );

   for (int i = 0; i < n; ++i) {
      if (a[i] != NULL)
         s += a[i];
      if (i < n-1)
         s += c;
   }
}

int main( ) {
   std::wstring ws;
   wchar_t* a[num];
   a[0] = L"Did";
   a[1] = L"you";
   a[2] = L"win";
   a[3] = L"???";
   join(a, num, L'/', ws);
}