Object Model

The standard C++ object model provides very efficient runtime support for the object paradigm. But its static nature is inflexibile in certain problem domains. Graphical user interface programming is a domain that requires both runtime efficiency and a high level of flexibility. Qt provides this, by combining the speed of C++ with the flexibility of the Qt Object Model.

Qt adds these features to C++:

• a very powerful mechanism for seamless object communication called signals and slots
• queryable and designable object properties
• powerful events and event filters
• contextual string translation for internationalization
• sophisticated interval driven timers that make it possible to elegantly integrate many tasks in an event-driven GUI
• hierarchical and queryable object trees that organize object ownership in a natural way
• guarded pointers (QPointer) that are automatically set to 0 when the referenced object is destroyed, unlike normal C++ pointers which become dangling pointers when their objects are destroyed
• a dynamic cast that works across library boundaries.

Many of these Qt features are implemented with standard C++ techniques, based on inheritance from QObject. Others, like the object communication mechanism and the dynamic property system, require the Meta-Object System provided by Qt's own Meta-Object Compiler (moc).

The meta-object system is a C++ extension that makes the language better suited to true component GUI programming. Although templates can be used to extend C++, the meta-object system provides benefits using standard C++ that cannot be achieved with templates; see Why Doesn't Qt Use Templates for Signals and Slots?