CODEWITHSUNDEEP
c++qtfile-copying
sashoalm
sashoalm

Reputation: 79595

Copy file even when destination exists (in Qt)

In the QFile::copy documentation it says

If a file with the name newName already exists, copy() returns false (i.e., QFile will not overwrite it).

But I need to copy a file even if the destination exists. Any workaround available in Qt for that?

Deleting the file is the obvious solution but it invites a race condition...

Upvotes: 27

Views: 24348

Answers (4)

Riho
Riho

Reputation: 4593

Just call remove() before calling copy()

Upvotes: 4

karlphillip
karlphillip

Reputation: 93458

if (QFile::exists("/home/user/dst.txt"))
{
    QFile::remove("/home/user/dst.txt");
}

QFile::copy("/home/user/src.txt", "/home/user/dst.txt");

Upvotes: 43

LovesTha
LovesTha

Reputation: 1683

The simplest retrying I can think of is:

while !QFile::copy("/home/user/src.txt", "/home/user/dst.txt")
{
    QFile::remove("/home/user/dst.txt");
}

But this still isn't a real solution as some of the race conditions are things that don't block remove.

I'm currently hunting for a way to handle writing a web page as an output but without the auto refresh ever catching between the remove and the copy.

Upvotes: 6

unwind
unwind

Reputation: 399949

The obvious solution is of course to delete the file if it exists, before doing the copy.

Note however that doing so opens up the code to a classic race condition, since on a typical multitasking operating system a different process could re-create the file between your applications' delete and copy calls. That would cause the copy to still fail, so you need to be prepared (and perhaps re-try the delete, but that might introduce a need for count so you don't spend forever attempting, and on and on).

Upvotes: 18

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