Applications built for different QT versions

**Asperamanca** · 10th July 2009, 09:12

...but of course on the same machine.

If I already have a QT 4.0.1 DLL installed on a machine, can I safely replace it with a QT 4.5.2 DLL? Is compatibility assured, or a matter of how lucky you are?

**nish** · 10th July 2009, 09:32

4.0 is not compatible with 4.5 as far as i remember.

**wysota** · 10th July 2009, 09:34

The easiest way is to simply try

In general it should work although the application may behave differently due to changes in the libraries.

**Asperamanca** · 10th July 2009, 13:03

Originally Posted by wysota

The easiest way is to simply try

In general it should work although the application may behave differently due to changes in the libraries.

Unfortunately, it's not as simple as that. Oh, as long as I am just a developer, tinkering around, it is. But once I start to think about deployment, things could get ugly.

Consider Application Suite A, written by company X with QT 4.0
Consider Application Suite B, written by company Y with QT 4.5

If I deploy my App Suite B, I can't just ruin App Suite A by installing the new DLLs, especially when both applications are expected to work together. I also can't just "hope" that things will work with the new DLL version, this would have to be verified with (probably) weeks of testing.

If the DLLs are not compatible, Trolltech (or QT Software or whatever name they have right now) would do well to give the DLLs different names, so you can use them in parallel.

But I see this is something to discuss with them directly prior to obtaining a commercial license.

**wysota** · 10th July 2009, 13:32

Both applications can have their own Qt libs. Just copy the version you want to be used to the directory containing the application binary and it will use it. But anyway 4.0 and 4.5 should be binary compatible (of course it won't work the other way round).

**Asperamanca** · 10th July 2009, 14:17

Originally Posted by wysota

(of course it won't work the other way round).

Already noticed that the hard way!

**wysota** · 10th July 2009, 14:44

Well... obviously classes introduced in Qt 4.5 were not present in Qt 4.0 dlls

**rexi** · 10th July 2009, 14:59

Originally Posted by MrDeath

4.0 is not compatible with 4.5 as far as i remember.

Aren't Qt releases with the same major version number supposed to be compatible? So it should be possible to replace an older release with a newer one (if not, I would consider this a bug...)

**Asperamanca** · 10th July 2009, 15:17

Originally Posted by rexi

Aren't Qt releases with the same major version number supposed to be compatible? So it should be possible to replace an older release with a newer one (if not, I would consider this a bug...)

That's what I hope.

**wysota** · 10th July 2009, 15:26

Originally Posted by rexi

Aren't Qt releases with the same major version number supposed to be compatible?

No, that's true about minor version. They are binary compatible (both ways).

So it should be possible to replace an older release with a newer one (if not, I would consider this a bug...)

Yes, it's possible this way (at least should be) as Qt keeps backward binary compatibility across minor versions too.

**rexi** · 10th July 2009, 15:36

Originally Posted by wysota

Yes, it's possible this way (at least should be) as Qt keeps backward binary compatibility across minor versions too.

Ok, I did express myself wrong

This was what I actually meant. I just forgot the little word "backward". Replacing an old version with a newer one should work, the other way round obviously does not, because of added features in newer versions, as you already stated.