How usable is Qt without it's preprocessing step?

Question

I think it's unreasonable for a library to require preprocessing of my source code with a special tool. That said, several have recommended the Qt library to me for cross platform GUI development.

How usable is Qt without the preprocessing step?

EDIT: Okay people; I'm not meaning this question as a rip on Qt -- too many Qt fanboys are treating it as if it is. I don't want to discuss the merits of the fact that Qt came up with this preprocessing tool. I understand why the tool is there, and I understand why there are large parts of Qt's design that are built upon the idea of preprocessing.

I've never used Qt; ergo I am in no position to rip on Qt. But I would much rather pay in writing a small amount of boilerplate myself and not depend on ripping apart my entire build process. I won't use Flex and Bison in my current project for the same reason; if I won't use those tools, I'm definitely not going to use another kind of preprocessing.

So, please don't take me as ripping on Qt. I cannot comment on how nice or not nice it is; I have not used it. I just want to know if it is possible to use it without moc.

Answer 1

Qt doesn't require the use of moc just to use it, it requires that usage if you create a subclass of QObject, and to declare signals and slots in your custom classes.

It's not unreasonable, moc provides features that C++ doesn't have, signals/slots, introspection, etc.

So, to do something minimally advanced, you WILL have to use the moc preprocessor. Your either love it, or hate it.

Answer 2

I don't consider it unreasonable that Qt requires a special pre-processing tool, considering how large and comprehensive of a library it is.

Other similarly comprehensive libraries such as Boost and GLib don't require special pre-processing tools but do make extensive use of the standard C preprocessor. Qt could have been implemented using only the C preprocessor, but by using its own special preprocessing tool, it can provide a cleaner syntax and avoid many of the pitfalls associated with C preprocessor macros.

As has been answered already, though, you can use Qt without moc, just not anything that requires signals and slots. Yes, this does include all of the GUI stuff, but Qt is not by any means just a GUI library.

Answer 3

I really can't think of anything so unique and useful with Qt without using QObjects. I wish they worked their way around that pre-compilation step.

Answer 4

Is it possible? As long as you're not doing any gui programming, probably. Personally I mostly run with PyQt these days, so it's not a big concern for me.

Why you shouldn't care: Given the nature of the "precompilation" if you're using cmake or qmake, it's not really a big deal in terms of inconvenience. If you're doing anything with a GUI these days, you should be using a graphical designer for most of the work anyway, so you're already adding some "pre-compilation" steps.

Regarding why they did it: You might be interested to read Qt's explanation: http://doc.qt.nokia.com/4.6/templates.html

It boils down to:

• The template solution lacks properties and overloading
• The moc solution prevents signals from breaking binary compatability
• Runtime analysis and modification are possible with the non-templated signals/slots mechanism

On a side note, multithreading signals/slots are also an advantage of their system.

Answer 5

Using Qt while avoiding moc will be more difficult than just using them together as intended. You will also sacrifice most of the interesting features that motivated others to recommend Qt.

Without moc you can't

• Use signals & slots (which are all but required for UI)
• Use the dynamic property system (needed to write plugins, among other things)
• Use the internationalization features
• Expect to get help from anybody when nothing works

If you want to use Qt, use moc. In fact, don't even worry about moc -- just use QMake. You can write a QMake .pro file that looks like this:

TARGET = myApp

FORMS += MainWindow.ui

HEADERS += MainWindow.h

SOURCES += MainWindow.cpp
SOURCES += main.cpp

Everything will be taken care of automatically. Or you can spend all your time trying to figure out how to avoid moc.

See http://doc.qt.nokia.com/4.6/metaobjects.html and http://doc.qt.nokia.com/4.6/moc.html#moc

Answer 6

I don't have a full answer, but as I understand it, moc mainly (or perhaps only) generates additional C++ code. So potentially there's nothing it does that you couldn't also do yourself manually. However, I have no idea how tedious that might be nor how much study it might take to understand all the necessary concepts and details that go into that code.

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Also, as I side note: In my opinion, the reason you're getting as much defense of Qt and moc is because you started your question with the strongly worded "I think it's unreasonable" which is easily interpreted to mean that you don't think moc should ever have existed. This distracts from your actual question. I think it would have been better just to say "moc doesn't fit into my build system" or simply "I have my own reasons for not wanting to use it".

Answer 7

You can use Qt without the moc, but then you lose certain functions, especially those that make Qt interesting in the first place (such as most of the GUI stuff, signals and slots, and string translation). But it's still a nice general purpose library, even without moc.