How to create a file in a different directory in C++?

Question

#include <iostream>
#include <fstream>
#include <cstdlib>

int main() {
    std::fstream f1("/tmp/test");
    if (!f1) {
        std::cerr << "f1 failed\n";
    } else {
        std::cerr << "f1 success\n";
    }
    FILE *f2 = fopen("/tmp/test", "w+");
    if (!f2) {
        std::cerr << "f2 failed\n";
    } else {
        std::cerr << "f2 success\n";
    }
}

Creating a file in /tmp/ doesn't work for me using fstreams but it does with fopen. What could be the problem? (I get f1 failed and f2 success when /tmp/test doesn't already exist)

Answer 1

You have to tell the fstream you are opening the file for output, like this

std::fstream fs("/tmp/test", std::ios::out);

Or use ofstream instead of fstream, that opens the file for output by default:

std::ofstream fs("/tmp/test");

Answer 2

Your first method call does not automatically create a file: see fstream.

If you want your first method call to create a file, use:

std::fstream f1("/tmp/test", fstream::out);

Answer 3

I don't know which is the default mode for the fstream constructor, I tried with this and it worked

std::fstream f1("/tmp/test", std::fstream::in | std::fstream::out);

It creates a file for input and output, you should check the fstream documentation here