Question

On MacOS with gcc4.2 should the following code create a new file if none exists?

#include <fstream>

void test () {
    std::fstream file ("myfile.txt", std::ios::in | std::ios::out);
}

By my logic it should, either open up an existing file for read/writing or create a new empty file for read/writing. But the behaviour I get is that it will not create a new file if ‘myfile.txt’ does not exist.

How do I get the same behavior as fopen(“myfile.txt”, “r+”); ?

Furthermore,

#include <fstream>

void test () {
    std::ofstream file ("myfile.txt", std::ios::in | std::ios::out);
}

Will always truncate an existing file…

Is this the standard behavior?

Answer 1

First of all, I have no idea why you think that fopen("r+") creates a file if it doesn’t exist – according to ISO C & C++, it does not, it just opens an existing file for read/write. If you want to create a file with fopen, you use "w+".

For streams, you just specify trunc:

std::ofstream file ("myfile.txt",
    std::ios::in | std::ios::out | std::ios::trunc);

However, both this and fopen("w+") will truncate the file. There’s no standard way to open the file without truncating if it exists, but create it if it does not exist in a single call. At best you can try to open, check for failure, and then try to create/truncate; but this may lead to a race condition if file is created by another process after the check but before truncation.

In POSIX, you can use open with O_CREAT and without O_TRUNC.