c++ string concatenation second time

I am a newbie, writing a c++ code to open and read from multiple files and then dump part of the data into other files. I want to generate file names in a for loop. But I can't concatenate string(numbering of file) and string literal(a file extension). The same line of code works at the very beginning of the program, but not at the later part.

int main(int argc, char *argv[])
{
   std::cout << std::string("9") + ".dat" << std::endl;

    // many more lines

    dump = 1;
    if (dump == 1){
        for (int ilevel=std::max(levelmin,lmin); ilevel < lmax + 1; ilevel++){
            std::cout << std::string("9") + ".dat" << std::endl; // crashes here!
            std::ofstream fout (std::string("9") + ".dat", std::ios::out | std::ios::binary);
            std::cout << grid[ilevel].cube[0] << std::endl;

            fout.write ((char*)&grid[ilevel].cube[0], grid[ilevel].cube.size() * sizeof(grid[ilevel].cube[0]));
            fout.close();
        }
    }
    ...
}

If I put std::cout << std::string("9") + ".dat" << std::endl; at the beginning, it works and prints "9.dat". but in the later loop, segmentation fault.

In between I call a function that uses stringstream to pad leading zeros to an integer. The function looks:

std::string int2str(const int n, const int m){
    std::stringstream ss;
    ss << std::setfill('0') << std::setw(m) << n;
    std::string s2(ss.str());
    ss.clear();
    return s2;
}

I don't have a clear understanding about string and stringstream in c++. But out of many things in my program, this function is the only thing I can think of being relevant. Other parts of codes does not deal with strings. It's mostly array manipulation code.

I've also tried std::string("9") + std::string(".dat") but had no luck. What is wrong?