Fstream fails to create new file

Question

I\'m using a FileManager for a project so that reading and writing is less of a hassle for me. Or would be, if I didn\'t spend all this time debugging it. So, this comfort-c

Answer 1

Best method:

void FileManager::open(std::string const& filename)
{
    using std::ios_base;
    if( stream_.is_open() )
        stream_.close();

    stream_.open( filename.c_str() ); // ...try existing file
    if( !stream_.is_open() ) // ...else, create new file...
        stream_.open(filename.c_str(), ios_base::in | ios_base::out | ios_base::trunc);
}

So the code tests for an existing file and, if not, creates it.

Answer 2

Just get your function to open

void FileManager::open(std::string const& filename)
{
    using std::ios_base;
    if(stream_.is_open())
        stream_.close();

    stream_.open(filename.c_str(), ios_base::in | ios_base::out | ios_base::trunc);
}

if that is the mode you require.

There is no magic way to open a file for read/write creating it if it does not exist but not truncating it (removing its content) if it does. You have to do that in multiple steps.

Answer 3

How can I accomplish this?

std::ofstream file("file.txt");
file << data;

Isn't that simpler?

Answer 4

You have to call fstream::open with an explicit openmode argument of

ios_base::in | ios_base::out | ios_base::trunc

Otherwise open will fail due to ENOENT.

Table 95 of the draft C++ standard lists possible file open modes and their equivalent in stdio. The default, ios_base::out | ios_base::in is r+. The one I listed above is equivalent to w+. ios_base::out | ios_base::app is equivalent to a. All other combinations involving ios_base::app are invalid.

(At the risk of being scoffed at: you could switch to stdio and use the file mode a+, which reads from the start and appends at the end.)

Answer 5

That's the way the library works: std::ios::in | std::ios::out opens it in the equivalent of stdio's "r+", that is it will only open an existing file. I don't believe there's a mode that will do what you are wanting, you'll have to us an OS-specific call or check the file for existence (again, using an OS-specific call) and open it in one mode or the other depending on whether it already exists.

Edit: I assumed that you didn't want the file truncated if it already exists. As other people have noted, if you're happy to truncate any existing file then in | out | trunc is an option.

Answer 6

You cannot use std::ios::in on a non-existing file. Use

std::ios::in | std::ios::out | std::ios::trunc

instead (but make sure it doesn't exist or it will be truncated to zero bytes).