Feeds

Cross platform development for Windows and Mac OS X

Killing two birds with one stone

SANS - Survey on application security programs

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.

3 Big data security analytics techniques

More from The Register

next story
OpenBSD founder wants to bin buggy OpenSSL library, launches fork
One Heartbleed vuln was too many for Theo de Raadt
Got Windows 8.1 Update yet? Get ready for YET ANOTHER ONE – rumor
Leaker claims big release due this fall as Microsoft herds us into the CLOUD
This time it's 'Personal': new Office 365 sub covers just two devices
Redmond also brings Office into Google's back yard
Ubuntu 14.04 LTS: Great changes, but sssh don't mention the...
Why HELLO Amazon! You weren't here last time
Patch iOS, OS X now: PDFs, JPEGs, URLs, web pages can pwn your kit
Plus: iThings and desktops at risk of NEW SSL attack flaw
Next Windows obsolescence panic is 450 days from … NOW!
The clock is ticking louder for Windows Server 2003 R2 users
Batten down the hatches, Ubuntu 14.04 LTS due in TWO DAYS
Admins dab straining server brows in advance of Trusty Tahr's long-term support landing
Red Hat to ship RHEL 7 release candidate with a taste of container tech
Grab 'near-final' version of next Enterprise Linux next week
Apple inaugurates free OS X beta program for world+dog
Prerelease software now open to anyone, not just developers – as long as you keep quiet
prev story

Whitepapers

Securing web applications made simple and scalable
In this whitepaper learn how automated security testing can provide a simple and scalable way to protect your web applications.
3 Big data security analytics techniques
Applying these Big Data security analytics techniques can help you make your business safer by detecting attacks early, before significant damage is done.
The benefits of software based PBX
Why you should break free from your proprietary PBX and how to leverage your existing server hardware.
Mainstay ROI - Does application security pay?
In this whitepaper learn how you and your enterprise might benefit from better software security.
Combat fraud and increase customer satisfaction
Based on their experience using HP ArcSight Enterprise Security Manager for IT security operations, Finansbank moved to HP ArcSight ESM for fraud management.