原文https://blog.csdn.net/wanywhn/article/details/80293581

简说内省

所谓内省是指面向对象语言的一种在运行期间查询对象信息的能力， 比如如果该语具有运行期间检查对象型别的能力，那么我们称它是型别内省（type intropection）的，型别内省可以用来实施多态。

C++的内省比较有限，它仅支持上面所说的型别内省， C++的型别内省是通过运行时类型识别（RTTI）(Run-Time Type Information)中的typeid 以及dynamic_case关键字来实现的，举例说明。

  1. // rabbit 派生于 Animal, jump为虚函数  
  2. class Animal{
  3.         virtual void jump();
  4. };
  6. class rabbit: public Animal{
  7.       virtual void jump();
  8. };
  10. if ( rabbit *p == dynamic_case<Animal*>(obj))     
  11. {     
  12.    p->jump();     
  13. }     
  14. //我们还可以通过typeid萃取到对象的型别信息，比如对象的名称   
  15. std::cout << typeid(obj).name() << std::endl

Qt内省

Qt拓展了C++的内省机制，（实际上，它并没有采用C++的RTTI),而是提供了更为强大的元对象(meta object)机制，来实现内省。接下来，就让我们看看，Qt是如何扩展c++内省机制的。
要深刻理解Qt的内省机制，首先理解QObject，QObject类是整个Qt对象模型的心脏，Qt对象模型最为核心的功能是提供一种无缝的对象通讯机制，即就是我们所熟知的信号和槽。QObject主要有三大职责： 内存管理、内省(intropection)与事件处理。本文将集中在在内省的讨论。以下代码介绍了QObject类提供的内省方法:

  1. //每个对象可以通过QObject::setObjectName()和QObject::objectName()设置、取得类的实例的名字   
  2. rabbit obj;    
  3. obj.setObjectName("instanceName");    
  4. QString name1 = obj.objectName();   // return instanceName   
  6. //每个对象还可以通过它的元对象className方法得到类的名字   
  7. QString name2 = obj.metaObject()->className();  // return rabbit   
  9. //每个对象可以通过QObject::inherits方法来查询是否对前对象类派生于量一个类  
  10. QLabel obj;
  11. bool isherited =  obj.inherits("QObject");         // returns true   
  12. isherited =  obj.inherits("QWideget");         // returns true

再来一下QObject::inherits方法的底层实现：

  1. inline bool inherits(const char *classname) const   
  2.     { return const_cast<QObject *>(this)->qt_metacast(classname) != 0; }

原来，QObject::inherits是通过qt_metacast()这个虚函数实现的， 事实上每个QObject的派生类都必须实现metaObject()以及其他qt_metacall()方法，从而满足自省方法className, inherits等方法的调用（当然还有其他用途）。而所有有关派生从QObject的子类中的内省方法无须有用户实现，用户只要在类中声明宏Q_OBJECT即可，Qt的元对象编译器（moc)负责实现派生QObject的子类中的内省方法。

  1. /* tmake ignore Q_OBJECT */
  2. #define Q_OBJECT \
  3. public: \
  4.     Q_OBJECT_CHECK \
  5.     static const QMetaObject staticMetaObject; \
  6.     Q_OBJECT_GETSTATICMETAOBJECT \
  7.     virtual const QMetaObject *metaObject() const; \
  8.     virtual void *qt_metacast(const char *); \
  9.     QT_TR_FUNCTIONS \
  10.     virtual int qt_metacall(QMetaObject::Call, int, void **); \
  11. private: \
  12.     Q_DECL_HIDDEN static const QMetaObjectExtraData staticMetaObjectExtraData; \
  13.     Q_DECL_HIDDEN static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **);

此外，所有的Qt widgets类均继承自QObject, QObject所提供的isWidgetType自省方法可以很方便让QObject子对象查询自己是否是Wideget, 而且它会比 qobject_cast<QWidget *>(obj) 或者obj->inherits快很多。原因qobject_cast()和inherits()都是借助元对象系统来实现其功能的(详见红色注解)，isWidgetType()是QObject本身的标志位得以实现。

QMetaObject自省方法

更多自省方法定义在QMetaObject，以下是QMetaObject声明的源代码：

  2. struct Q_CORE_EXPORT QMetaObject
  3. {
  4.     const char *className() const;
  5.     const QMetaObject *superClass() const;
  7.     QObject *cast(QObject *obj) const;
  8.     const QObject *cast(const QObject *obj) const;
  10. #ifndef QT_NO_TRANSLATION
  11.     // ### Qt 4: Merge overloads
  12.     QString tr(const char *s, const char *c) const;
  13.     QString trUtf8(const char *s, const char *c) const;
  14.     QString tr(const char *s, const char *c, int n) const;
  15.     QString trUtf8(const char *s, const char *c, int n) const;
  16. #endif // QT_NO_TRANSLATION
  18.     int methodOffset() const;
  19.     int enumeratorOffset() const;
  20.     int propertyOffset() const;
  21.     int classInfoOffset() const;
  23.     int constructorCount() const;
  24.     int methodCount() const;
  25.     int enumeratorCount() const;
  26.     int propertyCount() const;
  27.     int classInfoCount() const;
  29.     int indexOfConstructor(const char *constructor) const;
  30.     int indexOfMethod(const char *method) const;
  31.     int indexOfSignal(const char *signal) const;
  32.     int indexOfSlot(const char *slot) const;
  33.     int indexOfEnumerator(const char *name) const;
  34.     int indexOfProperty(const char *name) const;
  35.     int indexOfClassInfo(const char *name) const;
  37.     QMetaMethod constructor(int index) const;
  38.     QMetaMethod method(int index) const;
  39.     QMetaEnum enumerator(int index) const;
  40.     QMetaProperty property(int index) const;
  41.     QMetaClassInfo classInfo(int index) const;
  42.     QMetaProperty userProperty() const;
  44.     static bool checkConnectArgs(const char *signal, const char *method);
  45.     static QByteArray normalizedSignature(const char *method);
  46.     static QByteArray normalizedType(const char *type);
  48.     // internal index-based connect
  49.     static bool connect(const QObject *sender, int signal_index,
  50.                         const QObject *receiver, int method_index,
  51.                         int type = 0, int *types = 0);
  52.     // internal index-based disconnect
  53.     static bool disconnect(const QObject *sender, int signal_index,
  54.                            const QObject *receiver, int method_index);
  55.     static bool disconnectOne(const QObject *sender, int signal_index,
  56.                               const QObject *receiver, int method_index);
  57.     // internal slot-name based connect
  58.     static void connectSlotsByName(QObject *o);
  60.     // internal index-based signal activation
  61.     static void activate(QObject *sender, int signal_index, void **argv);  //obsolete
  62.     static void activate(QObject *sender, int from_signal_index, int to_signal_index, void **argv); //obsolete
  63.     static void activate(QObject *sender, const QMetaObject *, int local_signal_index, void **argv);
  64.     static void activate(QObject *sender, const QMetaObject *, int from_local_signal_index, int to_local_signal_index, void **argv); //obsolete
  66.     // internal guarded pointers
  67.     static void addGuard(QObject **ptr);
  68.     static void removeGuard(QObject **ptr);
  69.     static void changeGuard(QObject **ptr, QObject *o);
  71.     static bool invokeMethod(QObject *obj, const char *member,
  72.                              Qt::ConnectionType,
  73.                              QGenericReturnArgument ret,
  74.                              QGenericArgument val0 = QGenericArgument(0),
  75.                              QGenericArgument val1 = QGenericArgument(),
  76.                              QGenericArgument val2 = QGenericArgument(),
  77.                              QGenericArgument val3 = QGenericArgument(),
  78.                              QGenericArgument val4 = QGenericArgument(),
  79.                              QGenericArgument val5 = QGenericArgument(),
  80.                              QGenericArgument val6 = QGenericArgument(),
  81.                              QGenericArgument val7 = QGenericArgument(),
  82.                              QGenericArgument val8 = QGenericArgument(),
  83.                              QGenericArgument val9 = QGenericArgument());
  85.     static inline bool invokeMethod(QObject *obj, const char *member,
  86.                              QGenericReturnArgument ret,
  87.                              QGenericArgument val0 = QGenericArgument(0),
  88.                              QGenericArgument val1 = QGenericArgument(),
  89.                              QGenericArgument val2 = QGenericArgument(),
  90.                              QGenericArgument val3 = QGenericArgument(),
  91.                              QGenericArgument val4 = QGenericArgument(),
  92.                              QGenericArgument val5 = QGenericArgument(),
  93.                              QGenericArgument val6 = QGenericArgument(),
  94.                              QGenericArgument val7 = QGenericArgument(),
  95.                              QGenericArgument val8 = QGenericArgument(),
  96.                              QGenericArgument val9 = QGenericArgument())
  97.     {
  98.         return invokeMethod(obj, member, Qt::AutoConnection, ret, val0, val1, val2, val3,
  99.                 val4, val5, val6, val7, val8, val9);
  100.     }
  102.     static inline bool invokeMethod(QObject *obj, const char *member,
  103.                              Qt::ConnectionType type,
  104.                              QGenericArgument val0 = QGenericArgument(0),
  105.                              QGenericArgument val1 = QGenericArgument(),
  106.                              QGenericArgument val2 = QGenericArgument(),
  107.                              QGenericArgument val3 = QGenericArgument(),
  108.                              QGenericArgument val4 = QGenericArgument(),
  109.                              QGenericArgument val5 = QGenericArgument(),
  110.                              QGenericArgument val6 = QGenericArgument(),
  111.                              QGenericArgument val7 = QGenericArgument(),
  112.                              QGenericArgument val8 = QGenericArgument(),
  113.                              QGenericArgument val9 = QGenericArgument())
  114.     {
  115.         return invokeMethod(obj, member, type, QGenericReturnArgument(), val0, val1, val2,
  116.                                  val3, val4, val5, val6, val7, val8, val9);
  117.     }
  119.     static inline bool invokeMethod(QObject *obj, const char *member,
  120.                              QGenericArgument val0 = QGenericArgument(0),
  121.                              QGenericArgument val1 = QGenericArgument(),
  122.                              QGenericArgument val2 = QGenericArgument(),
  123.                              QGenericArgument val3 = QGenericArgument(),
  124.                              QGenericArgument val4 = QGenericArgument(),
  125.                              QGenericArgument val5 = QGenericArgument(),
  126.                              QGenericArgument val6 = QGenericArgument(),
  127.                              QGenericArgument val7 = QGenericArgument(),
  128.                              QGenericArgument val8 = QGenericArgument(),
  129.                              QGenericArgument val9 = QGenericArgument())
  130.     {
  131.         return invokeMethod(obj, member, Qt::AutoConnection, QGenericReturnArgument(), val0,
  132.                 val1, val2, val3, val4, val5, val6, val7, val8, val9);
  133.     }
  135.     QObject *newInstance(QGenericArgument val0 = QGenericArgument(0),
  136.                          QGenericArgument val1 = QGenericArgument(),
  137.                          QGenericArgument val2 = QGenericArgument(),
  138.                          QGenericArgument val3 = QGenericArgument(),
  139.                          QGenericArgument val4 = QGenericArgument(),
  140.                          QGenericArgument val5 = QGenericArgument(),
  141.                          QGenericArgument val6 = QGenericArgument(),
  142.                          QGenericArgument val7 = QGenericArgument(),
  143.                          QGenericArgument val8 = QGenericArgument(),
  144.                          QGenericArgument val9 = QGenericArgument()) const;
  146.     enum Call {
  147.         InvokeMetaMethod,
  148.         ReadProperty,
  149.         WriteProperty,
  150.         ResetProperty,
  151.         QueryPropertyDesignable,
  152.         QueryPropertyScriptable,
  153.         QueryPropertyStored,
  154.         QueryPropertyEditable,
  155.         QueryPropertyUser,
  156.         CreateInstance
  157.     };
  159.     int static_metacall(Call, int, void **) const;
  160.     static int metacall(QObject *, Call, int, void **);
  162. #ifdef QT3_SUPPORT
  163.     QT3_SUPPORT const char *superClassName() const;
  164. #endif
  166.     struct { // private data
  167.         const QMetaObject *superdata;
  168.         const char *stringdata;
  169.         const uint *data;
  170.         const void *extradata;
  171.     } d;
  172. };

红色注解

注：红色下滑线部分，我不认同。我们可以来看看qobject_cast(obj)的实现源码：

  1. template <> inline QWidget *qobject_cast<QWidget*>(QObject *o)
  2. {
  3.     if (!o || !o->isWidgetType()) 
  4.       return 0;
  5.     return static_cast<QWidget*>(o);
  6. }
  7. template <> inline const QWidget *qobject_cast<const QWidget*>(const QObject *o)
  8. {
  9.     if (!o || !o->isWidgetType()) 
  10.       return 0;
  11.     return static_cast<const QWidget*>(o);
  12. }

通过以上源码可以看出，qobject_cast<QWidget *>(obj)的实现实际上也是通过isWidgetType()来进行判断的。我们来继续看看isWidgetType()的实现，代码如下：

  1. inline bool isWidgetType() const { return d_ptr->isWidget; }

而d_ptr是定义为：

  1.  QScopedPointer<QObjectData> d_ptr;

QObjectData是QObject的数据结构体，其数据为：

  1. QObjectData {
  2. public:
  3.     virtual ~QObjectData() = 0;
  4.     QObject *q_ptr;
  5.     QObject *parent;
  6.     QObjectList children;
  8.     uint isWidget : 1;
  9.     uint pendTimer : 1;
  10.     uint blockSig : 1;
  11.     uint wasDeleted : 1;
  12.     uint ownObjectName : 1;
  13.     uint sendChildEvents : 1;
  14.     uint receiveChildEvents : 1;
  15.     uint inEventHandler : 1; //only used if QT_JAMBI_BUILD
  16.     uint inThreadChangeEvent : 1;
  17.     uint hasGuards : 1; //true iff there is one or more QPointer attached to this object
  18.     uint unused : 22;
  19.     int postedEvents;
  20.     QMetaObject *metaObject; // assert dynamic
  21. };

从上可以看出，isWidgetType() 的实现的是从内部本身标识为识别，而qobject_cast<QWidget *>(obj)也是用isWidgetType()，何来快与不快之说。 而至于obj->inherits，其实现代码如下：

  1.  inline bool inherits(const char *classname) const
  2.         { return const_cast<QObject *>(this)->qt_metacast(classname) != 0; }

总结

  1. Qt是通过QObject、QMetaObject类实现其内省机制,
  2. QObject暴露给用户的共有自省方法有objectName(), inherits()， isWidgetType()等
  3. 大多数自省方法是QObject派发给QMetaObject实现 (e.g. QMetaObject::className)，元对象模型编译器moc负责自省方法的实现

4、更多自省方法定义在QMetaObject，是为了信号槽通讯、事件派发等机制