[Qt-interest] Configure QtDesigner ui.h

[Scott Aron Bloom]

To expand a bit further...

One of the very common mistakes I have seen (honestly EVERY consulting
gig I have done, has shown this problem) with using the d-pointer design
pattern and Qt, is frankly lack of memory management.. Why, everyone
gets SO used to Qt handling it with parent child relationships...

I see code with a private Ui::MyWidget * impl_, in the header, with the
proper namespace and class declaration.

In the constructor, they new gets done correctly, but no call to the
destructor, ever.

What I have been promoting with my clients, is to use std::auto_pointer.

You get the benefit of not having to include the header of the
implementation file into the .h + you don't have to worry about
destruction of its memory.

Scott

-----Original Message-----
From: qt-interest-bounces+scott.bloom=onshorecs.com at qt.nokia.com
[mailto:qt-interest-bounces+scott.bloom=onshorecs.com at qt.nokia.com] On
Behalf Of Bo Thorsen
Sent: Friday, September 02, 2011 9:58 AM
To: qt-interest at qt.nokia.com
Subject: Re: [Qt-interest] Configure QtDesigner ui.h

I hope you will allow me to elaborate a bit on this answer. It's all
correct, but a little more explanation could be helpful.

As John wrote, it's very convenient to inherit from the UI class
(private inheritance FTW).

But I almost always use the d-pointer idiom to get my class definitions
as small as possible. This is to avoid the recompile explosion from
changing stuff that is included by the header file. And if you are
building a library, then you should keep binary compatibility when
possible, and that includes the size in bytes of the class, so you can't
have the UI file change - the fix again is to use a pointer to the UI
contents.

I have tried letting my private class inherit the UI class, but I have
dropped it again. The reason is one that also works for the original
question:

I want to separate the names from the UI file from the names. Pollution
of namespaces and scopes is something I take seriously. And it's for
some reason a lot more difficult to stay disciplined with naming the
objects in a UI file.

Also, the OP said that it is bad form to do code in the .h file. I
actually agree with this, and including the ui_xyz.h file in the cpp
file fixes this problem.

If I'm doing code that's just a quick fire and forget problem, I don't
have any problems using private inheritance from the UI class - a book
example would qualify for this. But with all non-trivial code or for any
of the above reasons, I won't do it.

There isn't a single reason that says "this why you absolutely have to
avoid that", but put all the reasons together, and there is only a bit
of convenience in the pro list (which quickly isn't convenience because
you get used to writing d->ui->...) and a lot of good reasons against. 
To me that makes it an easy choice.

Bo.

Den 02-09-2011 18:15, Ian Thomson skrev:
> You can do it but it can be a bit of a pain if you do it that way.
>
> There a few reasons why but the best one is this: If you make a 
> library with a widget using ui through inheritance, when you include 
> the header for the library in another project that project also has to

> have access to the produced ui_whatever.h file to be able to compile. 
> As the ui_whatever.h file is merely a side-effect of the build 
> process, it should not be part of the library's headers. If that file 
> is cleaned up with a build-clean then your project which includes the 
> library header will no longer compile. That's not a very nice
side-effect.
>
> Ian.
>
> John Weeks wrote:
>> Sorry, Bo. I always forget to change the To: field to send to the
list...
>>
>> On Sep 1, 2011, at 10:27 PM, Bo Thorsen wrote:
>>
>>> Just remember to make the ui a pointer in the class and do a "new 
>>> MyUIThing" in the constructor. Don't let your class inherit the UI
class.
>>
>> This caught my eye. Why do you say that? C++ GUI Programming with Qt 
>> 4, second edition, page 27 shows their simple example dialog doing 
>> exactly that. I rather like the readability of simply naming the 
>> widgets instead of lots of "ui->..." stuff.
>>
>> Regards, John Weeks
>>
>>
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Bo Thorsen,
Fionia Software.

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