I want to create a text file in cpp using ofstream

Question

I want to create a file qbc.txt. I know how to create it, but I want to create a program that, if a file already exists with the same name, it would rename it to qbc(1).txt.

Answer 1

In C++17, boost's filesystem library was standardized as std::filesystem

It comes with a convenient std::filesystem::exists function.

It accepts a std::filesystem::path object, but fortunately those can be constructed with a std::string, making our program trivially easy:

std::string prefix = "qbc";
std::string extension = ".txt";
std::filesystem::path filename{prefix + extension};
int i = 0;
while (std::filesystem::exists(filename)){
    filename = prefix + "(" + std::to_string(++i) + ")" + extension;
}

// now filename is "qbc(1)" or "qbc(2)" etc.

Unfortunately no compiler has full support for it at the time of this writing!

Answer 2

Here is a simple solution. The file_exists() function came from @Raviprakash in his response. I've added how to change the filename and try again until success. I've done an approach similar to this before in Python.

If you know that your program is the only one that will create or remove these files, then you can cache the last created one and simply create the next one instead of looping over all of the created ones every time. But this kind of optimization would only make sense if you plan to make hundreds of thousands of files this way.

#include <fstream>
#include <string>

bool file_exists(const std::string &filename) {
  std::ifstream in(filename);
  return in.good();
}

std::ofstream& open_new(std::ofstream &out, std::string prefix,
                        std::string suffix)
{
  std::string filename = prefix + suffix;
  unsigned int index = 0;
  while (file_exists(filename)) {
    index++;
    filename = prefix + "(" + std::to_string(index) + ")" + suffix;
  }
  out.rdbuf()->open(filename, std::ios_base::out);
  return out;
}

int main() {
  std::string prefix = "qbc";
  std::string suffix = ".txt";
  std::ofstream out;
  open_new(out, prefix, suffix);
  out << "hello world!\n";
  return 0;
}

Answer 3

I know the program needs some improvements but the general idea is here:

    #include <fstream>
    #include <string>

    using namespace std;

    inline bool file_exists(const std::string& name)
    {
        ifstream f(name.c_str());
        return f.good();
    }

    int main()
    {
        string filename, name;
        name = "qbc";
        filename = name;

        int counter = 1;
        while (file_exists(filename+".txt")) {
            string str = to_string(counter);
            filename = name+ "(" + str + ")";
            counter++;
        }

        filename += ".txt";

        ofstream out(filename.c_str());

        return 0;
    }

Answer 4

I don't think this can be entirely solved using just the standard libraries. You can certainly keep picking a new file name until you find one that's unused and then create the new file (as the other answers have shown).

But there's an inherent race condition in that approach. What if another process creates a file between the time your program decides the name is available and the time it actually creates the file? Imagine two copies of your program both trying to write out files.

What you need is an atomic way to check for the file's existence and also to create the file. The normal way to do that is to first just try to create the file and then see if you succeeded or not. Unfortunately, I don't think the standard C++ or C libraries give you enough tools to do that. (I'd be happy to be proven wrong about that.)

Operating systems often provide APIs for doing just that. For example, Windows has GetTempFileName, which just keeps trying to create a new file until it succeeds. The key is that, once it succeeds, it keeps the file open so that it knows no other process can steal the name that's been selected.

If you tell us which OS you're using, we might be able to provide a more detailed answer.