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Qt 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++:

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?

Important Classes

These classes form the basis of the Qt Object Model.

QMetaClassInfoAdditional information about a class
QMetaEnumMeta-data about an enumerator
QMetaMethodMeta-data about a member function
QMetaObjectContains meta-information about Qt objects
QMetaPropertyMeta-data about a property
QMetaTypeManages named types in the meta-object system
QObjectThe base class of all Qt objects
QObjectCleanupHandlerWatches the lifetime of multiple QObjects
QPointerTemplate class that provides guarded pointers to QObjects
QSignalMapperBundles signals from identifiable senders
QVariantActs like a union for the most common Qt data types

Qt Objects: Identity vs Value

Some of the added features listed above for the Qt Object Model, require that we think of Qt Objects as identities, not values. Values are copied or assigned; identities are cloned. Cloning means to create a new identity, not an exact copy of the old one. For example, twins have different identities. They may look identical, but they have different names, different locations, and may have completely different social networks.

Then cloning an identity is a more complex operation than copying or assigning a value. We can see what this means in the Qt Object Model.

A Qt Object...

For these reasons, Qt Objects should be treated as identities, not as values. Identities are cloned, not copied or assigned, and cloning an identity is a more complex operation than copying or assigning a value. Therefore, QObject and all subclasses of QObject (direct or indirect) have their copy constructor and assignment operator disabled.


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Qt 4.6.3