Cross platform development for Windows and Mac OS X

Killing two birds with one stone

Build a business case: developing custom apps

Slotting It All Together

Having delved around under the hood, how do things look from the perspective of an application developer? Well firstly, the plumbing mentioned above is pretty much hidden from the developer. If you use Qt’s qmake build utility, moc is invoked behind the scenes, and automatically takes care of handling the moc output. This gets linked into the executable or #include’d with the class implementation file (depending on whether the original class declaration was in a header or .cpp file respectively).

To raise a signal, you just use the emit keyword, like this:

emit mySignal();

The pre-processor converts this into the appropriate call to MyClass::qt_metacall. Signals and slots can take one or more arguments, just like ‘regular’ C++ methods. However, a signal can only be connected to a slot with a compatible signature, thus ensuring a type-safe connection. E.g.:

myButton = new QPushButton(this);
connect (myButton, SIGNAL(clicked()), this, SLOT(buttonClicked()));

This creates a new push button and connects the clicked signal of the button (first two parameters) to the buttonClicked slot of the current class (second two parameters). Interestingly, a signal that provides – for example – three arguments can also be connected to a slot that expects only two, one or zero. As long as the arguments expected by the slot are type compatible with what’s provided by the signal, the connection can be made; extra arguments are simply ignored. As with the .NET multicast delegates, a single slot can be connected to multiple signals and you can likewise connect a single signal to multiple slots.

Figure 2: screenshot Qt Designer running.

Qt comes with a visual design tool called…ummm…Designer. This can be used to lay out a Qt form complete with your choice of user interface widgets. As you’d expect from a cross-platform programming tool, life isn’t quite as simple as with (say) Delphi or .NET because you need to use additional layout components (vertical layout, horizontal layout and grid layout) to put together your form in a way that will look good regardless of platform and also cope with changes in form size. This will hold no surprises for Java developers.

Once done, Designer can connect up the various widgets with the appropriate signals and slots. Figure 2 shows part of Designer running on my iMac. Notice the way in which the clicked signals emitted by the two pushbuttons (OK and Cancel) on the right are automatically connected to the dialog window’s accept and reject slots (Cunningly marked with electrical ‘Earth’ symbols to show the slot is on the form). Hitting Return in the ‘UserName’ edit box emits a returnPressed signal which is linked to the OK button’s click slot.

The lower window shows the Signal/Slot editor. (When you’re not in Signal/Slot edit mode, you don’t have the connections overlaying the design-time form). As you can see, this is laid out in (Sender-Signal)(Receiver-Slot) order, just like the QObject::connect call’s arguments.

Gartner critical capabilities for enterprise endpoint backup

More from The Register

next story
Why has the web gone to hell? Market chaos and HUMAN NATURE
Tim Berners-Lee isn't happy, but we should be
Microsoft boots 1,500 dodgy apps from the Windows Store
DEVELOPERS! DEVELOPERS! DEVELOPERS! Naughty, misleading developers!
'Stop dissing Google or quit': OK, I quit, says Code Club co-founder
And now a message from our sponsors: 'STFU or else'
Apple promises to lift Curse of the Drained iPhone 5 Battery
Have you tried turning it off and...? Never mind, here's a replacement
Linux turns 23 and Linus Torvalds celebrates as only he can
No, not with swearing, but by controlling the release cycle
Scratched PC-dispatch patch patched, hatched in batch rematch
Windows security update fixed after triggering blue screens (and screams) of death
This is how I set about making a fortune with my own startup
Would you leave your well-paid job to chase your dream?
prev story


Top 10 endpoint backup mistakes
Avoid the ten endpoint backup mistakes to ensure that your critical corporate data is protected and end user productivity is improved.
Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Backing up distributed data
Eliminating the redundant use of bandwidth and storage capacity and application consolidation in the modern data center.
The essential guide to IT transformation
ServiceNow discusses three IT transformations that can help CIOs automate IT services to transform IT and the enterprise
Next gen security for virtualised datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.