Archive

Posts Tagged ‘Qt Code Snippets’

How to center a window on the screen

January 26th, 2011 No comments
#include <QStyle>
#include <QDesktopWidget>

window->setGeometry(QStyle::alignedRect(Qt::LeftToRight,
                                        Qt::AlignCenter,
                                        window->size(),
                                        qApp->desktop()->availableGeometry() ));

Qt wiki

Using QString effectively

October 12th, 2010 No comments

Saving Window Size State

August 30th, 2010 No comments
class MainWindow : public QMainWindow
{
 Q_OBJECT;
public:
 MainWindow(QWidget *parent = 0) : QMainWindow(parent)
 {
   QSettings settings;
   restoreGeometry(settings.value("mainWindowGeometry").toByteArray());
   // create docs, toolbars, etc...
   restoreState(settings.value("mainWindowState").toByteArray());
 }

 void closeEvent(QCloseEvent *event)
 {
   QSettings settings;
   settings.setValue("mainWindowGeometry", saveGeometry());
   settings.setValue("mainWindowState", saveState());
 }
};

int main(int argc, char *argv[])
{
 QApplication a(argc, argv);
 QCoreApplication::setOrganizationDomain("OrgDomain");
 QCoreApplication::setOrganizationName("OrgName");
 QCoreApplication::setApplicationName("AppName");
 QCoreApplication::setApplicationVersion("1.0.0");

 MainWindow w;
 w.show();

 return a.exec();
}

The above code will save and restore window position, size, toolbar visibility, toolbar docking area, dock states, locations and sizes. It saves using QSettings which will store your settings in a platform correct way.

Source

Using QString effectively

June 22nd, 2010 No comments

QLatin1String : Avoid mallocs in operator ”==”

Creating a QString from a C-string involves a malloc. For example, there is a hidden-malloc cost in the following code.
if (fruit== "apple") { ... } // hidden malloc
In the code above, “apple” gets converted into a QString using QString::fromAscii(). This means that QString will allocate memory for the string “apple” and creates a deep copy of the C-style string. This deep copy is not really necessary and can be avoided by not creating a QString to start with and if the encoding of the C-string was somehow known. Since Latin-1 string comparisons are most common, Qt provides a special class called QLatin1String that just holds a pointer to the C-string provided in it’s constructor. In addition, QString provides a QString::operator==(const QLatin1String &) overload that has special code to compare without malloc+deep copy. We can make the above code fast by writing it instead as,
if (fruit== QLatin1String("apple")) { ... } // fast and mentions encoding

QStringRef : String manipulation without the malloc

QString has various methods for string manipulations like mid(), left(), right(). All of them create a new QString and hence a malloc/deep copy of data in an existing QString. Instead, one can use QString::midRef(), QString::leftRef() and QString::rightRef() to obtain a QStringRef. A QStringRef is a reference of a portion of a QString. QString also provides many overloads like QString::operator==(const QStringRef &) for optimizations with QStringRef.

QString::reserve and QString::squeeze

If you expect a QString to grow, it’s better to call QString::reserve to allocate extra memory in advance so that every call to QString::append() does not result in a malloc. Extra memory can be reclaimed using QString::squeeze.

QStringBuilder : Fast QString concatenation

The code below requires atleast 2 mallocs.
if (foo.startsWith("(" + type + ") 0x"))
Qt 4.6 introduces an internal class called QStringBuilder that “reserves” memory for a concatenation chain in a single shot. It does so by having each of the + operations above return a different class (not QString). This class keeps track of the string’s that are being appended and the required memory at each step. At the final step, where the concatenation operation gets converted into a QString it allocates memory in a single shot and copies all the strings in the chain one after another. This features can be enabled by using QT_USE_FAST_CONCATENATION. With this defined, one can use the operator ’%’ instead of ’+’. One would now write,
if (foo.startsWith("(" % type % ") 0x"))
If we want to use ’+’ itself instead of ’%’, one can also define QT_USE_FAST_OPERATOR_PLUS. See Fast concatenation for more details

Use QStringMatcher to match a string repetitively

If you are looking for a string repetitively in many strings or in the same string many times, you should use QStringMatcher. It uses Boyer-Moore string search algorithm to perform fast searches.

QtService

June 22nd, 2010 No comments

The QtService component is useful for developing Windows services and Unix daemons.

The project provides a QtService template class that can be used to implement service applications, and a QtServiceController class to control a service.

On Windows systems the implementation uses the Service Control Manager.

On Unix systems services are implemented as daemons.

Homepage

Single Application

June 19th, 2010 No comments

The QtSingleApplication component provides support for applications that can be only started once per user.

For some applications it is useful or even critical that they are started only once by any user. Future attempts to start the application should activate any already running instance, and possibly perform requested actions, e.g. loading a file, in that instance.

The QtSingleApplication class provides an interface to detect a running instance, and to send command strings to that instance.

For console (non-GUI) applications, the QtSingleCoreApplication variant is provided, which avoids dependency on QtGui.

Documentation

Download Open Source Edition (LGPL): qtsingleapplication-2.6_1-opensource.zip

QThread

June 18th, 2010 No comments

QThread was designed and is intended to be used as an interface or a control point to an operating system thread, not as a place to put code that you want to run in a thread. We object-oriented programmers subclass because we want to extend or specialize the base class functionality. The only valid reasons I can think of for subclassing QThread is to add functionality that QThread doesn’t have, e.g. perhaps providing a pointer to memory to use as the thread’s stack, or possibly adding real-time interfaces/support. Code to download a file, or to query a database, or to do any other kind of processing should not be added to a subclass of QThread; it should be encapsulated in an object of it’s own.

You’re doing it wrong…

// create the producer and consumer and plug them together
Producer producer;
Consumer consumer;

bool bOk = producer.connect(&consumer,
                            SIGNAL(consumed()),
                            SLOT(produce()));
Q_ASSERT(bOk);
bOk = consumer.connect(&producer,
                       SIGNAL(produced(QByteArray *)),
                       SLOT(consume(QByteArray *)));
Q_ASSERT(bOk);

// they both get their own thread
QThread producerThread;
producer.moveToThread(&producerThread);

QThread consumerThread;
consumer.moveToThread(&consumerThread);

// go!
producerThread.start();
consumerThread.start();

Reference: Threading without the headache or QThread’s no longer abstract (see attached file)