/* Just a snippet for the OP - same basic method as DaWei, but 
    with streams as opposed to sprintf */

std::string fileName;
std::stringstream ss;

for (unsigned i=0; i < desiredFiles; ++i)
{
   ss << baseName << i << baseExt << std::endl;
      /* "ss" works just like "cin" or "cout" - but instead of passing the data 
         directly to the console, the data is stored inside the stream. */

   std::getline(ss, fileName);
      /* Now fileName contains the full name of the file, eg myfile1.txt
         using exactly the same method as you would with "cin" */

   myFile.open (fileName.c_str(), ios::binary | ios::out);
   if (!myFile.good ()) return uhOh ("Ouput file didn't open");
   myFile << "This is file " << i << endl;
   myFile.close ();
   myFile.open (fileName.c_str(), ios::binary || ios::in);
   if (!myFile.good ()) return uhOh ("Input file didn't open");
   cout << myFile.rdbuf ();
   myFile.close ();
      /* This code is exactly the same as DaWei's - I just copy & pasted it
         The only difference is the ".c_str()" after "fileName" */
}

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>

using namespace std;

int main()
{
  stringstream ss;
  string fileName, baseName, baseExt;
  int i, num;
  fstream outPut;
  
  cout << "Enter File name: ";
  getline(cin, baseName);
  cout << "Enter File EXT: ";
  getline(cin, baseExt);
  cout << "Enter Amount Of File: ";
  cin >> num;
  
  while (num > 0)
  {
      num--;
      i++;          
      ss << baseName << i << baseExt << endl;
      getline(ss, fileName);
      outPut.open (fileName.c_str(), ios::binary | ios::out);
      outPut << "Hello";
      outPut.close ();
      cout << "File " << i << " created" << endl;
      }
  cout << "\n All files created thank you, press 'Enter' to exit" << endl;
  
  cin.sync();
  cin.get();
  return 0;
}