QT源码分析: qobject_cast

QT API里关于qobject_cast的描述：

T qobject_cast ( QObject * object )

Returns the given object cast to type T if the object is of type T (or of a subclass); otherwise returns 0. If object is 0 then it will also return 0.

The class T must inherit (directly or indirectly) QObject and be declared with the Q_OBJECT macro.

A class is considered to inherit itself.

Example:

 QObject *obj = new QTimer;          // QTimer inherits QObject

 QTimer *timer = qobject_cast<QTimer *>(obj);
 // timer == (QObject *)obj

 QAbstractButton *button = qobject_cast<QAbstractButton *>(obj);
 // button == 0

 

The qobject_cast() function behaves similarly to the standard C++ dynamic_cast(), with the advantages that it doesn't require RTTI support and it works across dynamic library boundaries.

Warning: If T isn't declared with the Q_OBJECT macro, this function's return value is undefined.

大体意思就鼓励大家使用qobject_cast而非dynamic_cast。xxx_cast<TA>(TB)，转换是否成功主要看TA/TB是否在一个类继承体系中，TB是否是TA相同类型或者子类。dynamic_cast依赖RTTI类判断，而qobject_cast依赖QMetaObject信息来判断。下面来看看源代码吧：

// */src/corelib/kernel/qmetaobject.cpp

 

template <class T>
inline T qobject_cast(QObject *object)
{
#if !defined(QT_NO_MEMBER_TEMPLATES) && !defined(QT_NO_QOBJECT_CHECK)

    reinterpret_cast<T>(0)->qt_check_for_QOBJECT_macro(*reinterpret_cast<T>(object));

    //首先检查是类T定义中是否使用了宏Q_OBJECT。如果使用了，那么qt_check_for_QOBJECT_macro就有了定义，

    //但是操作实际上是没有任何意义；如果没用到这个宏，那么就出现编译错误，因为Q_OBJECT的展开中

    //才有这个函数的定义

#endif
    return static_cast<T>(reinterpret_cast<T>(0)->staticMetaObject.cast(object));

 //真正检查对象的元信息的代码，具体看下边的QObject::cast(QObject*)。这里staticMetaObject

 //是static变量，所以可以用强制转换0地址为对象指针的方法来调用。又是一个0地址的用处哦

}

template <class T>
inline T qobject_cast(const QObject *object)
{
    // this will cause a compilation error if T is not const

    register T ptr = static_cast<T>(object);

    Q_UNUSED(ptr);

#if !defined(QT_NO_MEMBER_TEMPLATES) && !defined(QT_NO_QOBJECT_CHECK)

    reinterpret_cast<T>(0)->qt_check_for_QOBJECT_macro(*reinterpret_cast<T>(const_cast<QObject *>(object)));

#endif

    return static_cast<T>(const_cast<QObject *>(reinterpret_cast<T>(0)->staticMetaObject.cast(const_cast<QObject *>(object))));
}

// */src/corelib/kernel/qobjectdefs.h

/* This is a compile time check that ensures that any class cast with qobject_cast
   actually contains a Q_OBJECT macro. Note: qobject_cast will fail if a QObject
   subclass doesn't contain Q_OBJECT.

   In qt_check_for_QOBJECT_macro, we call a dummy templated function with two
   parameters, the first being "this" and the other the target of the qobject
   cast. If the types are not identical, we know that a Q_OBJECT macro is missing.

   If you get a compiler error here, make sure that the class you are casting
   to contains a Q_OBJECT macro.
*/

/* tmake ignore Q_OBJECT */
#define Q_OBJECT_CHECK \
    template <typename T> inline void qt_check_for_QOBJECT_macro(const T &_q_argument) const \
    { int i = qYouForgotTheQ_OBJECT_Macro(this, &_q_argument); i = i; }
  //这个"i=i;“是多余之笔吗？如果谁看出门道来告诉我，好不好？
//上边注释中第二段说的不是废话吗？能进到这个函数里就说明Q_OBJECT已经有使用了，已经达到目的了
template <typename T>
inline int qYouForgotTheQ_OBJECT_Macro(T, T) { return 0; }
//没用啊

template <typename T1, typename T2>
inline void qYouForgotTheQ_OBJECT_Macro(T1, T2) {}

// */src/corelib/kernel/qobjectdefs.h 

#define Q_OBJECT \
public: \
    Q_OBJECT_CHECK \  //上边几个函数就此展开
    static const QMetaObject staticMetaObject; \
    Q_OBJECT_GETSTATICMETAOBJECT \
    virtual const QMetaObject *metaObject() const; \
    virtual void *qt_metacast(const char *); \
    QT_TR_FUNCTIONS \
    virtual int qt_metacall(QMetaObject::Call, int, void **); \
private:

// */src/corelib/kernel/qmetaobject.cpp

/*!
    \internal
    Returns \a obj if object \a obj inherits from this
    meta-object; otherwise returns 0.
*/
QObject *QMetaObject::cast(QObject *obj) const
{
    if (obj) {
        const QMetaObject *m = obj->metaObject();
        do {
            if (m == this)
                return const_cast<QObject*>(obj);
        } while ((m = m->d.superdata));  //通过MetaObject信息比对来决定返回的内容
    }
    return 0; //如果在继承体系中没找到相应的metaobject信息，那么返回0
}

 最后说一句，上边说qobject_cast可以跨动态库的。网上没找到有人解释这句话，我猜想，可能意思是支持不同动态库的对象的转换，例如QObject和QWidget，隐含的条件当然是“被转换的对象，都从QObject这个根上继承下来的”，这个就好理解了。

以上就是qobject_cast()相关的代码。读一读然后写一些自己的理解和不懂的地方，收获还是很大的。有问题也请指正。