Please read carefully, if you want to answer...

I have a weird problem.

I try to check if I can create a file. To avoid not defined behaviour,
the code should throw an exception. Actually it throws an exception, but
creates the file anyway. How is this??

Can someone help with usefull information on this thing?

The code would look somewhat like this

It does create the file, but still throws the exception, which makes the code
useless.

Note that you can enable exceptions for an IO stream object: http://www.cplusplus.com/reference/ios/ios/exceptions/. That will automatically throw if there's a problem. If you wish to use exception handling for file IO in C++ that may be the "right" way, albeit C++ exceptions leave much to be desired. Alternatively, you can rely on return values and explicit checks... In this particular case you're throwing regardless of the result of opening the file so it's no wonder the exception occurs.

Append:

If you try to open a file for writing that doesn't exist the system will typically create it for you assuming you have sufficient privileges in the file system to do it. If you want to only open an existing file then you will need to specify the correct incantation of parameters when opening the file for the underlying system calls to require the file to exist before opening it. Since you're concealing the exact code it's impossible for us to further reason about what trouble you're having...

The method I would use to determine this is as follows :

1. Check if the containing folder of the file you want to create exists already.
2. Check the permissions of that folder.
3. Check if the file exists already.
4. Check the file's permissions if necessary.

You can do all this with Allegro 5.

See al_create_fs_entry, and al_get_fs_entry_mode. There's no need for exceptions at all. But you can still throw one if you like.

Because this way you know why it failed. Catching a vague iostream exception won't tell you that. As for file corruption or changes mid-code, the user needs to take care of any synchronization problems themself. If you execute the steps I laid out, and then try to open the file and the bad or fail bit is set or thrown you know someone else is using the file.

This is all moot anyway, as using fopen would either succeed or fail to create or open a file and you would know the result right away. (edit - well, you might want to know whether it exists first, see below).

Edit
You might also be interested in al_filename_exists, or al_create_fs_entry and al_fs_entry_exists.