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

using namespace std;

int main()
{
    string prefix = "foo";
    string suffix = "bar.txt";

    for (int i = 0; i < 10; i++) {
        stringstream ss; // An empty stringstream
        ss << prefix << i << suffix;

        cout << "i = " << i << ": name = " << ss.str() << endl; //ss.str() is a C++ std::string

        ofstream outstream(ss.str().c_str()); // File name is a C-style "string" (pointer to char)
        if (!outstream) {
            cerr << "Can't open file " << ss.str() << " for writing" << endl;
            exit(EXIT_FAILURE);
        }
        //
        // Do whatever you want with this file.
        // Then close it so that outstream can be reused the next time through the loop.
        //
        outstream.close();
    }
    return 0;
}