Qt源码解析之QObject

省去大部分virtual和public方法后，Qobject主要剩下以下成员：

//qobject.h
class Q_CORE_EXPORT Qobject{Q_OBJECTQ_PROPERTY(QString objectName READ objectName WRITE setObjectName NOTIFY objectNameChanged)Q_DECLARE_PRIVATE(QObject)
public:Q_INVOKABLE explicit QObject(QObject *parent=nullptr);virtual ~QObject();//...
protected:QObject(QObjectPrivate &dd, QObject *parent = nullptr);//...
protected:QScopedPointer<QObjectData> d_ptr;static const QMetaObject staticQtMetaObject;//...
private:Q_DISABLE_COPY(QObject)//...
}

一、Q_OBJECT

#define Q_OBJECT \
public: \QT_WARNING_PUSH \Q_OBJECT_NO_OVERRIDE_WARNING \static const QMetaObject staticMetaObject; \virtual const QMetaObject *metaObject() const; \virtual void *qt_metacast(const char *); \virtual int qt_metacall(QMetaObject::Call, int, void **); \QT_TR_FUNCTIONS \
private: \Q_OBJECT_NO_ATTRIBUTES_WARNING \Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \QT_WARNING_POP \struct QPrivateSignal {}; \QT_ANNOTATE_CLASS(qt_qobject, "")1、宏 QT_WARNING_PUSH 和 QT_WARNING_POP用于保存和恢复编译器的警告状态，以便在宏定义内部做一些修改或设置，而不影响用户定义的警告状态。
2、Q_OBJECT_NO_OVERRIDE_WARNING和Q_OBJECT_NO_ATTRIBUTES_WARNING这两个宏用于控制是否发出关于未覆盖（override）的警告或者关于某些属性的警告。
3、QT_TR_FUNCTIONS这个宏用于启用Qt的国际化（internationalization）功能，使得文本可以被翻译为不同的语言。
4、Q_DECL_HIDDEN_STATIC_METACALL在qobjectdefs.h有定义：
# define Q_DECL_HIDDEN_STATIC_METACALL Q_DECL_HIDDEN
使用 Q_DECL_HIDDEN 可以将类或函数标记为在外部接口中隐藏的，从而使它们对库的用户不可见。这对于避免一些链接时的符号冲突和提高库的封装性很有帮助。这个宏可能会被翻译成 __attribute__((visibility("hidden")))。也就是说qt_static_metacall这个函数没用到，我们忽略。

去除和编译器相关的宏，Q_OBJECT剩下的关键部分：

//qobjectdefs.h
#define Q_OBJECT \
public: \static const QMetaObject staticMetaObject; \virtual const QMetaObject *metaObject() const; \virtual void *qt_metacast(const char *); \virtual int qt_metacall(QMetaObject::Call, int, void **); \
private: \Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); Q_OBJECT宏声明了1个QMetaObject变量和3个QMetaObject相关的虚函数。QMetaObject类非常重要，和元对象系统相关。

二、Q_PROPERTY

//qobjectdefs.h
#define Q_PROPERTY(...) QT_ANNOTATE_CLASS(qt_property, __VA_ARGS__)
#define QT_ANNOTATE_CLASS(type, ...)在 qobjectdefs.h 中我们并没有看到 Q_PROPERTY 的准确定义。很多Qt的宏和特殊功能是通过moc生成的代码而不是在头文件中显式定义的。C++编译器能够识别 Q_PROPERTY 宏，是因为moc编译时生成了相应的代码。
使用Q_PROPERTY后，相当于把属性纳入了元对象系统，而且给出了一段Q_PROPERTY更细致的声明：
Q_PROPERTY(type name(READ getFunction [WRITE setFunction] |MEMBER memberName [(READ getFunction | WRITE setFunction)])[RESET resetFunction][NOTIFY notifySignal][REVISION int | REVISION(int[, int])][DESIGNABLE bool][SCRIPTABLE bool][STORED bool][USER bool][BINDABLE bindableProperty][CONSTANT][FINAL][REQUIRED])

三、Q_DECLARE_PRIVATE

//qglobal.h
#define Q_DECLARE_PRIVATE(Class) \inline Class##Private* d_func() \{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<Class##Private *>(qGetPtrHelper(d_ptr));) } \inline const Class##Private* d_func() const \{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<const Class##Private *>(qGetPtrHelper(d_ptr));) } \friend class Class##Private;加入参数并翻译过后：
inline QObjectPrivate* d_func()
{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<QObjectPrivate *>(qGetPtrHelper(d_ptr));) }
inline const QObjectPrivate* d_func() const
{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<const QObjectPrivate *>(qGetPtrHelper(d_ptr));) }
friend class QObjectPrivate;qGetPtrHelper()方法的定义：
//qglobal.h
template <typename T> static inline T *qGetPtrHelper(T *ptr) { return ptr; }
template <typename Wrapper> static inline typename Wrapper::pointer qGetPtrHelper(const Wrapper &p) { return p.data(); }qGetPtrHelper是一个模板函数，其目的是为了获取指针或类似指针的数据。
Q_CAST_IGNORE_ALIGN用于禁用GCC编译器的 -Wcast-align 警告。Q_DECLARE_PRIVATE宏定义了2个函数和1个友元类。2个d_func只是签名不同，传入参数d_ptr，都返回一个QObjectPrivate*类型的指针，而且友元类的名称也是QObjectPrivate。

四、QObjectData和QObjectPrivate

关于变量QScopedPointer<QObjectData> d_ptr:QScopedPointer类是用于存储指向动态分配对象的指针，并在其销毁时删除它，确保指向的对象在当前作用域消失时将被删除。
所以QScopedPointer<QObjectData>是一个QObjectData的指针。QObjectData定义：
//qobject.h
class Q_CORE_EXPORT QObjectData {//防止对象拷贝Q_DISABLE_COPY(QObjectData)
public:QObjectData() = default;virtual ~QObjectData() = 0;QObject *q_ptr;QObject *parent;QObjectList children;uint isWidget : 1;uint blockSig : 1;uint wasDeleted : 1;uint isDeletingChildren : 1;uint sendChildEvents : 1;uint receiveChildEvents : 1;uint isWindow : 1; //for QWindowuint deleteLaterCalled : 1;uint unused : 24;int postedEvents;QDynamicMetaObjectData *metaObject;QMetaObject *dynamicMetaObject() const;
#ifdef QT_DEBUGenum { CheckForParentChildLoopsWarnDepth = 4096 };
#endif
};上面说到d_func函数传入参数d_ptr，返回的QObjectPrivate*类型的指针，而d_ptr是QObjectData，那也就是说QObjectPrivate是QObjectData的子类。我们且看QObjectPrivate的定义:
//qobject_p.h
class Q_CORE_EXPORT QObjectPrivate : public QObjectData
{Q_DECLARE_PUBLIC(QObject)
public:struct ExtraData{//...}；//和信号&槽相关struct ConnectionOrSignalVector{//...}；//和信号&槽相关struct Connection : public ConnectionOrSignalVector{//...};//和信号&槽相关struct Sender{//...};//和信号&槽相关struct ConnectionData{//...};QObjectPrivate(int version = QObjectPrivateVersion);virtual ~QObjectPrivate();
public:ExtraData *extraData;QAtomicPointer<QThreadData> threadData;using ConnectionDataPointer = QExplicitlySharedDataPointer<ConnectionData>;QAtomicPointer<ConnectionData> connections;union {QObject *currentChildBeingDeleted;QAbstractDeclarativeData *declarativeData;};QAtomicPointer<QtSharedPointer::ExternalRefCountData> sharedRefcount;  
}Q_DECLARE_PUBLIC(QObject)定义：
//qglobal.h
#define Q_DECLARE_PUBLIC(Class)                                    \inline Class* q_func() { return static_cast<Class *>(q_ptr); } \inline const Class* q_func() const { return static_cast<const Class *>(q_ptr); } \friend class Class;翻译过后：
inline QObject* q_func() { return static_cast<QObject *>(q_ptr); } 
inline const QObject* q_func() const { return static_cast<const QObject *>(q_ptr); } \
friend class QObject;
这个宏实际上定义了2个签名不一样的函数q_func()，返回q_ptr指针，声明了QObject是友元类。QObjectPrivate的构造器定义如下：
//qobject.cpp
QObjectPrivate::QObjectPrivate(int version): threadData(nullptr), currentChildBeingDeleted(nullptr)
{checkForIncompatibleLibraryVersion(version);// QObjectData initializationq_ptr = nullptr;parent = nullptr;                           // no parent yet. It is set by setParent()isWidget = false;                           // assume not a widget objectblockSig = false;                           // not blocking signalswasDeleted = false;                         // double-delete catcherisDeletingChildren = false;                 // set by deleteChildren()sendChildEvents = true;                     // if we should send ChildAdded and ChildRemoved events to parentreceiveChildEvents = true;postedEvents = 0;extraData = nullptr;metaObject = nullptr;isWindow = false;deleteLaterCalled = false;
}
基本上是对继承下来的变量和自身变量进行初始化。

五、QObject()

当实例化一个继承自QObject的对象时，首先会调用QObject的构造器，构造器开始构造对象模型的世界，我们且看QObject构造函数QObject()的定义：
//qobject.cpp
QObject::QObject(QObject *parent): QObject(*new QObjectPrivate, parent)
{
}//qobject.cpp
QObject::QObject(QObjectPrivate &dd, QObject *parent): d_ptr(&dd)
{Q_ASSERT_X(this != parent, Q_FUNC_INFO, "Cannot parent a QObject to itself");Q_D(QObject);d_ptr->q_ptr = this;auto threadData = (parent && !parent->thread()) ? parent->d_func()->threadData.loadRelaxed() : QThreadData::current();threadData->ref();d->threadData.storeRelaxed(
threadData);if (parent) {QT_TRY {if (!check_parent_thread(parent, parent ? parent->d_func()->threadData.loadRelaxed() : nullptr, 
threadData))parent = nullptr;if (d->isWidget) {if (parent) {d->parent = parent;d->parent->d_func()->children.append(
this);}// no events sent here, this is done at the end of the QWidget constructor} else {setParent(parent);}} QT_CATCH(...) {threadData->deref();QT_RETHROW;}}
#if QT_VERSION < 0x60000qt_addObject(this);
#endifif (Q_UNLIKELY(qtHookData[QHooks::AddQObject]))reinterpret_cast<QHooks::AddQObjectCallback>(qtHookData[QHooks::AddQObject])(this);Q_TRACE(QObject_ctor, this);
}public的构造函数实际上是调用了protected的构造函数。
默认新建了一个QObjectPrivate并作为构造函数参数传入，赋值给了d_ptr。变量QScopedPointer<QObjectData> d_ptr在构造函数里实际被赋值为其新建的子实例QObjectPrivate。Q_D(QObject)定义：
//qglobal.h
#define Q_D(Class) Class##Private * const d = d_func()
调用d_func()得到QObjectPrivate* 并赋值给d，此时d和d_ptr都指向前面实例化的QObjectPrivate。d_ptr->q_ptr = this;
将QObjectPrivate->q_ptr设置为自身。//qobject.cpp
auto threadData = (parent && !parent->thread()) ? parent->d_func()->threadData.loadRelaxed() : QThreadData::current();
threadData->ref();
d->threadData.storeRelaxed(
threadData);
检查 parent 是否非空且它所属的线程是否为空，如果都不空的话，获取parent的线程数据；否则获取当前的线程数据。将线程数据存储到对象内部的数据结构中。//qobject.cpp
if (!check_parent_thread(parent, parent ? parent->d_func()->threadData.loadRelaxed() : nullptr, threadData))parent = nullptr;
检查parent和当前对象是否在相同的线程中，如果不在相同线程中，将 parent 设置为 nullptr。//qobject.cpp
if (d->isWidget) {if (parent) {d->parent = parent;d->parent->d_func()->children.append(this);}
}else{//...
}
如果对象是一个QWidget，parent不空，则建立起对象和parent的联系，对象的父对象就是parent，parent的children添加该对象。//qobject.cpp
if (d->isWidget) {//...
} else {setParent(parent);
}
如果对象不是QWidget，通过setParent(parent)设置父对象。setParent()的定义：
//qobject.cpp
void QObject::setParent(QObject *parent)
{Q_D(QObject);Q_ASSERT(!d->isWidget);d->setParent_helper(parent);
}
继续调用d->setParent_helper(parent)。setParent_helper()的定义：
void QObjectPrivate::setParent_helper(QObject *o)
{Q_Q(QObject);Q_ASSERT_X(q != o, Q_FUNC_INFO, "Cannot parent a QObject to itself");
#ifdef QT_DEBUGconst auto checkForParentChildLoops = qScopeGuard(
[&](){int depth = 0;auto p = parent;while (p) {if (++depth == CheckForParentChildLoopsWarnDepth) {qWarning(
"QObject %p (class: '%s', object name: '%s') may have a loop in its parent-child chain; ""this is undefined behavior",q, q->metaObject()->className(), qPrintable(q->objectName()));}p = p->parent();}});
#endifif (o == parent)return;if (parent) {QObjectPrivate *parentD = parent->d_func();if (parentD->isDeletingChildren && wasDeleted&& parentD->currentChildBeingDeleted == q) {// don't do anything since QObjectPrivate::deleteChildren() already// cleared our entry in parentD->children.} else {const int index = parentD->children.indexOf(q);if (index < 0) {// we're probably recursing into setParent() from a ChildRemoved event, don't do anything} else if (parentD->isDeletingChildren) {parentD->children[index] = 0;} else {parentD->children.removeAt(index);if (sendChildEvents && parentD->receiveChildEvents) {QChildEvent e(QEvent::ChildRemoved, q);QCoreApplication::sendEvent(parent, &e);}}}}parent = o;if (parent) {// object hierarchies are constrained to a single threadif (threadData != parent->d_func()->threadData) {qWarning(
"QObject::setParent: Cannot set parent, new parent is in a different thread");parent = nullptr;return;}parent->d_func()->children.append(q);if(sendChildEvents && parent->d_func()->receiveChildEvents) {if (!isWidget) {QChildEvent e(QEvent::ChildAdded, q);QCoreApplication::sendEvent(parent, &e);}}}if (!wasDeleted && !isDeletingChildren && declarativeData && QAbstractDeclarativeData::parentChanged)QAbstractDeclarativeData::parentChanged(declarativeData, q, o);
}Q_Q(QObject)的定义：
//qglobal.h
#define Q_Q(Class) Class * const q = q_func()
通过q_func()获取QObjectPrivate的q_ptr，在上面我们知道q_ptr指向了QObject，所以q和q_ptr都指向QObject。#ifdef QT_DEBUGconst auto checkForParentChildLoops = qScopeGuard(
[&](){int depth = 0;auto p = parent;while (p) {if (++depth == CheckForParentChildLoopsWarnDepth) {qWarning(
"QObject %p (class: '%s', object name: '%s') may have a loop in its parent-child chain; ""this is undefined behavior",q, q->metaObject()->className(), qPrintable(q->objectName()));}p = p->parent();}});
#endif
这一段通过warning可以推断出是在检测父子关系链中是否存在循环，如果循环链深度超过阈值，则警告。if (o == parent)return;
如果已经设置过parent且没变，直接返回。//如果已经有parent
if (parent) {//获取父对象的QObjectPrivateQObjectPrivate *parentD = parent->d_func();//检查父对象是否正在删除其子对象，当前对象是否已经被删除，前对象是否是父对象正在删除的子对象。//如果这些条件都成立，就跳过后续的处理，因为在删除子对象的过程中已经做了清理工作。if (parentD->isDeletingChildren && wasDeleted&& parentD->currentChildBeingDeleted == q) {// don't do anything since QObjectPrivate::deleteChildren() already// cleared our entry in parentD->children.} else {//获取当前对象在其父对象的子对象列表中的索引const int index = parentD->children.indexOf(q);//如果索引为负数，可能表示正在从 ChildRemoved 事件中递归到 setParent()，这时不执行任何操作。if (index < 0) {// we're probably recursing into setParent() from a ChildRemoved event, don't do anything} else if (parentD->isDeletingChildren) {//如果父对象正在删除其子对象，将相应的子对象指针更新为0。parentD->children[index] = 0;} else {//否则，从父对象的子对象列表中移除当前对象parentD->children.removeAt(index);//发送一个 ChildRemoved 事件给父对象。if (sendChildEvents && parentD->receiveChildEvents) {QChildEvent e(QEvent::ChildRemoved, q);QCoreApplication::sendEvent(parent, &e);}}}
}
上面这一段是在已有perent的情况下，断开parent和当前对象的联系，并确保在移除子对象时做了适当的清理和事件通知。实际上是为下面刷新parent做准备。parent = o;//更新parent//parent赋值后
if (parent) {// object hierarchies are constrained to a single thread// 对象层次结构受限于单个线程// 比较当前对象的线程数据和父对象的线程数据，如果它们不一致if (threadData != parent->d_func()->threadData) {qWarning("QObject::setParent: Cannot set parent, new parent is in a different thread");//父对象置空parent = nullptr;//直接返回return;}//将当前对象添加到父对象的子对象列表中。parent->d_func()->children.append(q);if(sendChildEvents && parent->d_func()->receiveChildEvents) {if (!isWidget) {//将这个事件发送给父对象QChildEvent e(QEvent::ChildAdded, q);QCoreApplication::sendEvent(parent, &e);}}
}
上面这一段是在设置对象的父对象后进行一些检查，确保父对象线程数据和该对象的一致，否则将parent设为nullptr，随后发送相应的ChildAdded事件给parent。
setParent_helper函数主要做了两件事:
1）确保旧parent安全撤离。
2）确保新parent正确设置。简单概括一下构造函数QObject()的内容：
1）新建QObjectPrivate并赋值给d_ptr。
2）赋值d_ptr->q_ptr为对象本身。
3）初始化threadData。
4）检查当前对象和parent是否在同一线程.
5）为当前对象和parent设置关联.

六、Q_DISABLE_COPY()

//qglobal.h
#define Q_DISABLE_COPY(Class) \Class(const Class &) = delete;\Class &operator=(const Class &) = delete;这里删除了拷贝构造函数和拷贝赋值操作符，确保QObject不能被拷贝构造或赋值。

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