14.4.1.4 Opening a File Using a File Descriptor

If you know a file descriptor, such as the integer 1 for standard output, you can open it like this:

ofstream outfile;
outfile.attach(1);

When you open a file by providing its name to one of the fstream constructors or by using the open function, the file is automatically closed when the fstream is destroyed (by a delete or when it goes out of scope). When you attach a file to an fstream, it is not automatically closed.