对象模型

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 把这些特征添加到 C++：

a very powerful mechanism for seamless object communication called 信号和槽
queryable and designable 对象特性
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 对象树 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.
support for custom type creation.

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 元对象系统 provided by Qt's own MOC (元对象编译器) .

The meta-object system is a C++ extension that makes the language better suited to true component GUI programming.

重要类

These classes form the basis of the Qt Object Model.

QMetaClassInfo	有关类的额外信息
QMetaEnum	有关枚举器的元数据
QMetaMethod	关于成员函数的元数据
QMetaObject	包含有关 Qt 对象的元信息
QMetaProperty	关于特性的元数据
QMetaSequence	允许类型擦除对顺序容器的访问
QMetaType	在元对象系统中管理命名类型
QObject	所有 Qt 对象的基类
QObjectCleanupHandler	看守多个 QObject 的寿命
QPointer	提供指向 QObject 守卫指针的模板类
QSignalBlocker	围绕 QObject::blockSignals() 的异常安全包裹器
QSignalMapper	捆绑来自可识别发送器的信号
QVariant	举动像最常见 Qt 数据类型的并集

Qt 对象：标识 vs 值

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.

Qt 对象 ...

might have a unique QObject::objectName (). If we copy a Qt Object, what name should we give the copy?
has a location in an object hierarchy . If we copy a Qt Object, where should the copy be located?
can be connected to other Qt Objects to emit signals to them or to receive signals emitted by them. If we copy a Qt Object, how should we transfer these connections to the copy?
can have new properties added to it at runtime that are not declared in the C++ class. If we copy a Qt Object, should the copy include the properties that were added to the original?

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.