C++: Which is the best method of checking for file existence on windows platform [duplicate]

Answer 1

I would prefer the first. The second checks if the file can be opened, while a file might exist without you having the rights to open it.

You can use the INVALID_FILE_ATTRIBUTES constant.

Answer 2

I also like to confirm that the string isn't a directory:

DWORD attr = GetFileAttributes(file);
if(attr == INVALID_FILE_ATTRIBUTES || (attr & FILE_ATTRIBUTE_DIRECTORY))
    return false;   //  not a file
return true;

Answer 3

There are two things to consider here:

1. Checking if the file exists via its attributes is potentially many orders of magnitude faster - If a file exists on a 'slow' file system - tape, network storage, cd etc then opening the file will involve an actual round trip to the files location. The files attributes on the other hand are queried and cached by the filesystem drivers when the directory is queried, so probing file attributes involves a once off directory enumeration cost - meaning far fewer round trips - which can be a significant saving if multiple "slow" files are being checked.

2. However, the files attributes merely indicate that the file existed at the time the call was made. The file can be deleted, or you might not haver permissions to access it. If you are about to try and open the file anyway, it would be better to dispense with the file attributes check and actually try and open the file.

Answer 4

The only robust way to check whether a file exists is to try to open it, and see if it succeeds or fails.

Any other method is a potential race condition. For example, GetFileAttributes can tell you if a file existed when you called the function, but that's not the same as whether it exists when you try to open it. It might have been deleted (or created) in the meantime.

Answer 5

Note that GetFileAttributes() may fail for other reasons than lack of existence (for example permissions issues). I would add a check on the error code for robustness:

GetFileAttributes("C:\\MyFile.txt"); // from winbase.h
if(INVALID_FILE_ATTRIBUTES == GetFileAttributes("C:\\MyFile.txt") && GetLastError()==ERROR_FILE_NOT_FOUND)
{
    //File not found
}

Answer 6

If you're targeting only Windows, option #1 is clearly the better of the two. Also, the constant you're looking for already exists in the Windows headers -- it's called INVALID_FILE_ATTRIBUTES.