温酒读Qt：QObject中篇2 ——欲遮还羞的 QObjectPrivate

《妙法莲华经》曰：“佛道长远，久受勤苦，乃可得成。” 世事修炼，莫不如是，日拱一卒无有尽，功不唐捐终入海。

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《温酒读Qt：QObject 序篇》
《温酒读Qt：QObject中篇1—— Q_OBJECT的隐秘角落》

1、QObjectPrivate class

先贴源码，然后我们挑重点进行分析；

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**
** Copyright (C) 2019 The Qt Company Ltd.
** Copyright (C) 2013 Olivier Goffart <ogoffart@woboq.com>
** Contact: https://www.qt.io/licensing/
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#ifndef QOBJECT_P_H
#define QOBJECT_P_H

//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API.  It exists for the convenience
// of qapplication_*.cpp, qwidget*.cpp and qfiledialog.cpp.  This header
// file may change from version to version without notice, or even be removed.
//
// We mean it.
//

#include <QtCore/private/qglobal_p.h>
#include "QtCore/qobject.h"
#include "QtCore/qpointer.h"
#include "QtCore/qsharedpointer.h"
#include "QtCore/qcoreevent.h"
#include "QtCore/qlist.h"
#include "QtCore/qvector.h"
#include "QtCore/qvariant.h"
#include "QtCore/qreadwritelock.h"

QT_BEGIN_NAMESPACE

class QVariant;
class QThreadData;
class QObjectConnectionListVector;
namespace QtSharedPointer { struct ExternalRefCountData; }

/* for Qt Test */
struct QSignalSpyCallbackSet
{
    typedef void (*BeginCallback)(QObject *caller, int signal_or_method_index, void **argv);
    typedef void (*EndCallback)(QObject *caller, int signal_or_method_index);
    BeginCallback signal_begin_callback,
                    slot_begin_callback;
    EndCallback signal_end_callback,
                slot_end_callback;
};
void Q_CORE_EXPORT qt_register_signal_spy_callbacks(QSignalSpyCallbackSet *callback_set);

extern Q_CORE_EXPORT QBasicAtomicPointer<QSignalSpyCallbackSet> qt_signal_spy_callback_set;

enum { QObjectPrivateVersion = QT_VERSION };

class Q_CORE_EXPORT QAbstractDeclarativeData
{
public:
    static void (*destroyed)(QAbstractDeclarativeData *, QObject *);
    static void (*destroyed_qml1)(QAbstractDeclarativeData *, QObject *);
    static void (*parentChanged)(QAbstractDeclarativeData *, QObject *, QObject *);
    static void (*signalEmitted)(QAbstractDeclarativeData *, QObject *, int, void **);
    static int  (*receivers)(QAbstractDeclarativeData *, const QObject *, int);
    static bool (*isSignalConnected)(QAbstractDeclarativeData *, const QObject *, int);
    static void (*setWidgetParent)(QObject *, QObject *); // Used by the QML engine to specify parents for widgets. Set by QtWidgets.
};

// This is an implementation of QAbstractDeclarativeData that is identical with
// the implementation in QtDeclarative and QtQml for the first bit
struct QAbstractDeclarativeDataImpl : public QAbstractDeclarativeData
{
    quint32 ownedByQml1:1;
    quint32 unused: 31;
};

class Q_CORE_EXPORT QObjectPrivate : public QObjectData
{
    Q_DECLARE_PUBLIC(QObject)

public:
    struct ExtraData
    {
        ExtraData() {}
    #ifndef QT_NO_USERDATA
        QVector<QObjectUserData *> userData;
    #endif
        QList<QByteArray> propertyNames;
        QVector<QVariant> propertyValues;
        QVector<int> runningTimers;
        QList<QPointer<QObject> > eventFilters;
        QString objectName;
    };

    typedef void (*StaticMetaCallFunction)(QObject *, QMetaObject::Call, int, void **);
    struct Connection;
    struct SignalVector;

    struct ConnectionOrSignalVector {
        union {
            // linked list of orphaned connections that need cleaning up
            ConnectionOrSignalVector *nextInOrphanList;
            // linked list of connections connected to slots in this object
            Connection *next;
        };

        static SignalVector *asSignalVector(ConnectionOrSignalVector *c) {
            if (reinterpret_cast<quintptr>(c) & 1)
                return reinterpret_cast<SignalVector *>(reinterpret_cast<quintptr>(c) & ~quintptr(1u));
            return nullptr;
        }
        static Connection *fromSignalVector(SignalVector *v) {
            return reinterpret_cast<Connection *>(reinterpret_cast<quintptr>(v) | quintptr(1u));
        }
    };

    struct Connection : public ConnectionOrSignalVector
    {
        // linked list of connections connected to slots in this object, next is in base class
        Connection **prev;
        // linked list of connections connected to signals in this object
        QAtomicPointer<Connection> nextConnectionList;
        Connection *prevConnectionList;

        QObject *sender;
        QAtomicPointer<QObject> receiver;
        QAtomicPointer<QThreadData> receiverThreadData;
        union {
            StaticMetaCallFunction callFunction;
            QtPrivate::QSlotObjectBase *slotObj;
        };
        QAtomicPointer<const int> argumentTypes;
        QAtomicInt ref_;
        uint id = 0;
        ushort method_offset;
        ushort method_relative;
        int signal_index : 27; // In signal range (see QObjectPrivate::signalIndex())
        ushort connectionType : 3; // 0 == auto, 1 == direct, 2 == queued, 4 == blocking
        ushort isSlotObject : 1;
        ushort ownArgumentTypes : 1;
        Connection() : ref_(2), ownArgumentTypes(true) {
            //ref_ is 2 for the use in the internal lists, and for the use in QMetaObject::Connection
        }
        ~Connection();
        int method() const { Q_ASSERT(!isSlotObject); return method_offset + method_relative; }
        void ref() { ref_.ref(); }
        void freeSlotObject()
        {
            if (isSlotObject) {
                slotObj->destroyIfLastRef();
                isSlotObject = false;
            }
        }
        void deref() {
            if (!ref_.deref()) {
                Q_ASSERT(!receiver.loadRelaxed());
                Q_ASSERT(!isSlotObject);
                delete this;
            }
        }
    };
    // ConnectionList is a singly-linked list
    struct ConnectionList {
        QAtomicPointer<Connection> first;
        QAtomicPointer<Connection> last;
    };

    struct Sender
    {
        Sender(QObject *receiver, QObject *sender, int signal)
            : receiver(receiver), sender(sender), signal(signal)
        {
            if (receiver) {
                ConnectionData *cd = receiver->d_func()->connections.loadRelaxed();
                previous = cd->currentSender;
                cd->currentSender = this;
            }
        }
        ~Sender()
        {
            if (receiver)
                receiver->d_func()->connections.loadRelaxed()->currentSender = previous;
        }
        void receiverDeleted()
        {
            Sender *s = this;
            while (s) {
                s->receiver = nullptr;
                s = s->previous;
            }
        }
        Sender *previous;
        QObject *receiver;
        QObject *sender;
        int signal;
    };

    struct SignalVector : public ConnectionOrSignalVector {
        quintptr allocated;
        // ConnectionList signals[]
        ConnectionList &at(int i)
        {
            return reinterpret_cast<ConnectionList *>(this + 1)[i + 1];
        }
        const ConnectionList &at(int i) const
        {
            return reinterpret_cast<const ConnectionList *>(this + 1)[i + 1];
        }
        int count() const { return static_cast<int>(allocated); }
    };



    /*
        This contains the all connections from and to an object.

        The signalVector contains the lists of connections for a given signal. The index in the vector correspond
        to the signal index. The signal index is the one returned by QObjectPrivate::signalIndex (not
        QMetaObject::indexOfSignal). allsignals contains a list of special connections that will get invoked on
        any signal emission. This is done by connecting to signal index -1.

        This vector is protected by the object mutex (signalSlotLock())

        Each Connection is also part of a 'senders' linked list. This one contains all connections connected
        to a slot in this object. The mutex of the receiver must be locked when touching the pointers of this
        linked list.
    */
    struct ConnectionData {
        // the id below is used to avoid activating new connections. When the object gets
        // deleted it's set to 0, so that signal emission stops
        QAtomicInteger<uint> currentConnectionId;
        QAtomicInt ref;
        QAtomicPointer<SignalVector> signalVector;
        Connection *senders = nullptr;
        Sender *currentSender = nullptr;   // object currently activating the object
        QAtomicPointer<Connection> orphaned;

        ~ConnectionData()
        {
            deleteOrphaned(orphaned.loadRelaxed());
            SignalVector *v = signalVector.loadRelaxed();
            if (v)
                free(v);
        }

        // must be called on the senders connection data
        // assumes the senders and receivers lock are held
        void removeConnection(Connection *c);
        void cleanOrphanedConnections(QObject *sender)
        {
            if (orphaned.loadRelaxed() && ref.loadAcquire() == 1)
                cleanOrphanedConnectionsImpl(sender);
        }
        void cleanOrphanedConnectionsImpl(QObject *sender);

        ConnectionList &connectionsForSignal(int signal)
        {
            return signalVector.loadRelaxed()->at(signal);
        }

        void resizeSignalVector(uint size) {
            SignalVector *vector = this->signalVector.loadRelaxed();
            if (vector && vector->allocated > size)
                return;
            size = (size + 7) & ~7;
            SignalVector *newVector = reinterpret_cast<SignalVector *>(malloc(sizeof(SignalVector) + (size + 1) * sizeof(ConnectionList)));
            int start = -1;
            if (vector) {
                memcpy(newVector, vector, sizeof(SignalVector) + (vector->allocated + 1) * sizeof(ConnectionList));
                start = vector->count();
            }
            for (int i = start; i < int(size); ++i)
                newVector->at(i) = ConnectionList();
            newVector->next = nullptr;
            newVector->allocated = size;

            signalVector.storeRelaxed(newVector);
            if (vector) {
                vector->nextInOrphanList = orphaned.loadRelaxed();
                orphaned.storeRelaxed(ConnectionOrSignalVector::fromSignalVector(vector));
            }
        }
        int signalVectorCount() const {
            return  signalVector.loadAcquire() ? signalVector.loadRelaxed()->count() : -1;
        }

        static void deleteOrphaned(ConnectionOrSignalVector *c);
    };

    QObjectPrivate(int version = QObjectPrivateVersion);
    virtual ~QObjectPrivate();
    void deleteChildren();

    void setParent_helper(QObject *);
    void moveToThread_helper();
    void setThreadData_helper(QThreadData *currentData, QThreadData *targetData);
    void _q_reregisterTimers(void *pointer);

    bool isSender(const QObject *receiver, const char *signal) const;
    QObjectList receiverList(const char *signal) const;
    QObjectList senderList() const;

    void addConnection(int signal, Connection *c);

    static QObjectPrivate *get(QObject *o) {
        return o->d_func();
    }
    static const QObjectPrivate *get(const QObject *o) { return o->d_func(); }

    int signalIndex(const char *signalName, const QMetaObject **meta = nullptr) const;
    bool isSignalConnected(uint signalIdx, bool checkDeclarative = true) const;
    bool maybeSignalConnected(uint signalIndex) const;
    inline bool isDeclarativeSignalConnected(uint signalIdx) const;

    // To allow abitrary objects to call connectNotify()/disconnectNotify() without making
    // the API public in QObject. This is used by QQmlNotifierEndpoint.
    inline void connectNotify(const QMetaMethod &signal);
    inline void disconnectNotify(const QMetaMethod &signal);

    template <typename Func1, typename Func2>
    static inline QMetaObject::Connection connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
                                                  const typename QtPrivate::FunctionPointer<Func2>::Object *receiverPrivate, Func2 slot,
                                                  Qt::ConnectionType type = Qt::AutoConnection);

    template <typename Func1, typename Func2>
    static inline bool disconnect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
                                  const typename QtPrivate::FunctionPointer<Func2>::Object *receiverPrivate, Func2 slot);

    static QMetaObject::Connection connectImpl(const QObject *sender, int signal_index,
                                               const QObject *receiver, void **slot,
                                               QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type,
                                               const int *types, const QMetaObject *senderMetaObject);
    static QMetaObject::Connection connect(const QObject *sender, int signal_index, QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type);
    static bool disconnect(const QObject *sender, int signal_index, void **slot);

    void ensureConnectionData()
    {
        if (connections.loadRelaxed())
            return;
        ConnectionData *cd = new ConnectionData;
        cd->ref.ref();
        connections.storeRelaxed(cd);
    }
public:
    ExtraData *extraData;    // extra data set by the user
    QThreadData *threadData; // id of the thread that owns the object

    using ConnectionDataPointer = QExplicitlySharedDataPointer<ConnectionData>;
    QAtomicPointer<ConnectionData> connections;

    union {
        QObject *currentChildBeingDeleted; // should only be used when QObjectData::isDeletingChildren is set
        QAbstractDeclarativeData *declarativeData; //extra data used by the declarative module
    };

    // these objects are all used to indicate that a QObject was deleted
    // plus QPointer, which keeps a separate list
    QAtomicPointer<QtSharedPointer::ExternalRefCountData> sharedRefcount;
};

Q_DECLARE_TYPEINFO(QObjectPrivate::ConnectionList, Q_MOVABLE_TYPE);

inline bool QObjectPrivate::isDeclarativeSignalConnected(uint signal_index) const
{
    return declarativeData && QAbstractDeclarativeData::isSignalConnected
            && QAbstractDeclarativeData::isSignalConnected(declarativeData, q_func(), signal_index);
}

inline void QObjectPrivate::connectNotify(const QMetaMethod &signal)
{
    q_ptr->connectNotify(signal);
}

inline void QObjectPrivate::disconnectNotify(const QMetaMethod &signal)
{
    q_ptr->disconnectNotify(signal);
}

namespace QtPrivate {
template<typename Func, typename Args, typename R> class QPrivateSlotObject : public QSlotObjectBase
{
    typedef QtPrivate::FunctionPointer<Func> FuncType;
    Func function;
    static void impl(int which, QSlotObjectBase *this_, QObject *r, void **a, bool *ret)
    {
        switch (which) {
            case Destroy:
                delete static_cast<QPrivateSlotObject*>(this_);
                break;
            case Call:
                FuncType::template call<Args, R>(static_cast<QPrivateSlotObject*>(this_)->function,
                                                 static_cast<typename FuncType::Object *>(QObjectPrivate::get(r)), a);
                break;
            case Compare:
                *ret = *reinterpret_cast<Func *>(a) == static_cast<QPrivateSlotObject*>(this_)->function;
                break;
            case NumOperations: ;
        }
    }
public:
    explicit QPrivateSlotObject(Func f) : QSlotObjectBase(&impl), function(f) {}
};
} //namespace QtPrivate

template <typename Func1, typename Func2>
inline QMetaObject::Connection QObjectPrivate::connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
                                                       const typename QtPrivate::FunctionPointer<Func2>::Object *receiverPrivate, Func2 slot,
                                                       Qt::ConnectionType type)
{
    typedef QtPrivate::FunctionPointer<Func1> SignalType;
    typedef QtPrivate::FunctionPointer<Func2> SlotType;
    Q_STATIC_ASSERT_X(QtPrivate::HasQ_OBJECT_Macro<typename SignalType::Object>::Value,
                      "No Q_OBJECT in the class with the signal");

    //compilation error if the arguments does not match.
    Q_STATIC_ASSERT_X(int(SignalType::ArgumentCount) >= int(SlotType::ArgumentCount),
                      "The slot requires more arguments than the signal provides.");
    Q_STATIC_ASSERT_X((QtPrivate::CheckCompatibleArguments<typename SignalType::Arguments, typename SlotType::Arguments>::value),
                      "Signal and slot arguments are not compatible.");
    Q_STATIC_ASSERT_X((QtPrivate::AreArgumentsCompatible<typename SlotType::ReturnType, typename SignalType::ReturnType>::value),
                      "Return type of the slot is not compatible with the return type of the signal.");

    const int *types = nullptr;
    if (type == Qt::QueuedConnection || type == Qt::BlockingQueuedConnection)
        types = QtPrivate::ConnectionTypes<typename SignalType::Arguments>::types();

    return QObject::connectImpl(sender, reinterpret_cast<void **>(&signal),
        receiverPrivate->q_ptr, reinterpret_cast<void **>(&slot),
        new QtPrivate::QPrivateSlotObject<Func2, typename QtPrivate::List_Left<typename SignalType::Arguments, SlotType::ArgumentCount>::Value,
                                        typename SignalType::ReturnType>(slot),
        type, types, &SignalType::Object::staticMetaObject);
}

template <typename Func1, typename Func2>
bool QObjectPrivate::disconnect(const typename QtPrivate::FunctionPointer< Func1 >::Object* sender, Func1 signal,
                                const typename QtPrivate::FunctionPointer< Func2 >::Object* receiverPrivate, Func2 slot)
{
    typedef QtPrivate::FunctionPointer<Func1> SignalType;
    typedef QtPrivate::FunctionPointer<Func2> SlotType;
    Q_STATIC_ASSERT_X(QtPrivate::HasQ_OBJECT_Macro<typename SignalType::Object>::Value,
                      "No Q_OBJECT in the class with the signal");
    //compilation error if the arguments does not match.
    Q_STATIC_ASSERT_X((QtPrivate::CheckCompatibleArguments<typename SignalType::Arguments, typename SlotType::Arguments>::value),
                      "Signal and slot arguments are not compatible.");
    return QObject::disconnectImpl(sender, reinterpret_cast<void **>(&signal),
                          receiverPrivate->q_ptr, reinterpret_cast<void **>(&slot),
                          &SignalType::Object::staticMetaObject);
}

Q_DECLARE_TYPEINFO(QObjectPrivate::Connection, Q_MOVABLE_TYPE);
Q_DECLARE_TYPEINFO(QObjectPrivate::Sender, Q_MOVABLE_TYPE);

class QSemaphore;
class Q_CORE_EXPORT QAbstractMetaCallEvent : public QEvent
{
public:
    QAbstractMetaCallEvent(const QObject *sender, int signalId, QSemaphore *semaphore = nullptr)
        : QEvent(MetaCall), signalId_(signalId), sender_(sender)
#if QT_CONFIG(thread)
        , semaphore_(semaphore)
#endif
    { Q_UNUSED(semaphore); }
    ~QAbstractMetaCallEvent();

    virtual void placeMetaCall(QObject *object) = 0;

    inline const QObject *sender() const { return sender_; }
    inline int signalId() const { return signalId_; }

private:
    int signalId_;
    const QObject *sender_;
#if QT_CONFIG(thread)
    QSemaphore *semaphore_;
#endif
};

class Q_CORE_EXPORT QMetaCallEvent : public QAbstractMetaCallEvent
{
public:
    // blocking queued with semaphore - args always owned by caller
    QMetaCallEvent(ushort method_offset, ushort method_relative,
                   QObjectPrivate::StaticMetaCallFunction callFunction,
                   const QObject *sender, int signalId,
                   void **args, QSemaphore *semaphore);
    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj,
                   const QObject *sender, int signalId,
                   void **args, QSemaphore *semaphore);

    // queued - args allocated by event, copied by caller
    QMetaCallEvent(ushort method_offset, ushort method_relative,
                   QObjectPrivate::StaticMetaCallFunction callFunction,
                   const QObject *sender, int signalId,
                   int nargs);
    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj,
                   const QObject *sender, int signalId,
                   int nargs);

    ~QMetaCallEvent() override;

    inline int id() const { return d.method_offset_ + d.method_relative_; }
    inline const void * const* args() const { return d.args_; }
    inline void ** args() { return d.args_; }
    inline const int *types() const { return reinterpret_cast<int*>(d.args_ + d.nargs_); }
    inline int *types() { return reinterpret_cast<int*>(d.args_ + d.nargs_); }

    virtual void placeMetaCall(QObject *object) override;

private:
    inline void allocArgs();

    struct Data {
        QtPrivate::QSlotObjectBase *slotObj_;
        void **args_;
        QObjectPrivate::StaticMetaCallFunction callFunction_;
        int nargs_;
        ushort method_offset_;
        ushort method_relative_;
    } d;
    // preallocate enough space for three arguments
    char prealloc_[3*(sizeof(void*) + sizeof(int))];
};

class QBoolBlocker
{
    Q_DISABLE_COPY_MOVE(QBoolBlocker)
public:
    explicit inline QBoolBlocker(bool &b, bool value=true):block(b), reset(b){block = value;}
    inline ~QBoolBlocker(){block = reset; }
private:
    bool &block;
    bool reset;
};

void Q_CORE_EXPORT qDeleteInEventHandler(QObject *o);

struct QAbstractDynamicMetaObject;
struct Q_CORE_EXPORT QDynamicMetaObjectData
{
    virtual ~QDynamicMetaObjectData();
    virtual void objectDestroyed(QObject *) { delete this; }

    virtual QAbstractDynamicMetaObject *toDynamicMetaObject(QObject *) = 0;
    virtual int metaCall(QObject *, QMetaObject::Call, int _id, void **) = 0;
};

struct Q_CORE_EXPORT QAbstractDynamicMetaObject : public QDynamicMetaObjectData, public QMetaObject
{
    ~QAbstractDynamicMetaObject();

    QAbstractDynamicMetaObject *toDynamicMetaObject(QObject *) override { return this; }
    virtual int createProperty(const char *, const char *) { return -1; }
    int metaCall(QObject *, QMetaObject::Call c, int _id, void **a) override
    { return metaCall(c, _id, a); }
    virtual int metaCall(QMetaObject::Call, int _id, void **) { return _id; } // Compat overload
};

QT_END_NAMESPACE

#endif // QOBJECT_P_H

代码量也不多，不过很多朋友对于看源码这件事情，刚开始看到这一坨代码可能内心莫名的就会生出反感的情绪来。这里说说博主的方法哈。
在这里插入图片描述
我们基于Qt的框架开发，特别是在刚接触的情况下，最有效和快速准确的熟悉Qt模块/class/API等的方法，肯定是查看帮助手册。而且我也不止一次的表扬过Qt的Assistant, 写的是真的好，条理清晰，一目了然。 只不过，我们现在调试源码，你在帮助手册中去查看QObjectPrivate,这定然是无法查找到的。但是，帮助手册中的目录却值得借鉴。

首先，明确我们当前目标为 QObject 的私有类QObjectPrivate，我们重点要关注以下几点

  • 1、QObjectPrivate 是什么，主要有什么作用？
  • 2、QObjectPrivate 的继承关系？(这个很重要，能帮你在后面快速厘清成员变量和成员函数，以及哪些是重写的函数，以及哪些是自己特有的函数)
  • 3、它包含哪些成员变量和成员函数？大概起到了什么作用？

从这个角度来梳理，那我们就相对比较容易了。
让大家觉得不容易的，可能就是在查看源码的过程中包含了各种语法糖，各种看着就很高级的，不知如何下手。不过反过来思考，你暂时看着、理解着难受的，不都是你未知，并可以前进的方向么，这是一件很开心的事情啊。博主奉行的就是“终生学习”的理念，对于未知常常会充满兴趣，并决心去愉快的探索。

2、继承关系

QObjectPrivate -> QObjectData

继承的深度只有一层，舒服，简单搂一眼QObjectData

class Q_CORE_EXPORT QObjectData {
    Q_DISABLE_COPY(QObjectData)
public:
    QObjectData() = default;
    virtual ~QObjectData() = 0; // 这个接口表明了QObjectData是一个接口类，只能被继承
    QObject *q_ptr;				// 原来q指针刺客在这里申明的
    QObject *parent;			// 每个QObject的对象或者子类对象都有记住父类的对象指针，那么在查找某个对象的子类对象时就轻而易举了
    QObjectList children;   	// typedef QList<QObject*> QObjectList;  这里更直接，直接记录了该对象的所有子类对象；每个子类对象展开又包含有同样的结构，说说看这是什么，可不就是一个嵌套的链式结构么

    uint isWidget : 1;			// 位域的知识不多说
    uint blockSig : 1;
    uint wasDeleted : 1;
    uint isDeletingChildren : 1;
    uint sendChildEvents : 1;
    uint receiveChildEvents : 1;
    uint isWindow : 1; //for QWindow
    uint deleteLaterCalled : 1;
    uint unused : 24;
    
    int postedEvents;
    
    QDynamicMetaObjectData *metaObject;
    QMetaObject *dynamicMetaObject() const;

#ifdef QT_DEBUG
    enum { CheckForParentChildLoopsWarnDepth = 4096 };
#endif
};

看完了QObjectData 的数据结构，其实我们有一个很有趣的问题引出来。

我们都知道Qt中所有的对象都继承自QObject,譬如QWidget也是继承自QObject 的。我们在编码的过程中，为什么实例化QObject 的子类对象，我们只说在堆上为对象申请内存的情况，为何却不需要自己编码去显示调用对象内存释放函数，来销毁对象实例呢？

问题抛出来，我们会在循序渐进的过程中解答这个问题~ 当然，如果是已经看到这儿的朋友，你怀有强烈的好奇心，也不妨自己动动手去寻找答案（注释中其实已经给了提示了哈）

3、成员变量

从代码中找成员变量，有时候代码行数比较多，且成员变量的定义比较分散的时候，确实不宜直接查看。这时候我们得想想办法。
在最近的 《2024了，我不允许你还不会：Qt查看与调试源码》一文中，同大家分享了Qt源码调试环境的搭建技巧。这里我们也是写一段简单的代码，通过调试器来辅助我们查看源码：

调试代码：

#include <QCoreApplication>
#include <QObject>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QObject o1,o2;
    QObject::connect(&o1,&QObject::destroyed, &o2,&QObject::destroyed);

    return a.exec();
}

我们始终要调试的时QObject 对吧，我们也说了 QObject 与 QObjectPrivate的关系，对吧。
好的，那我们如下打两个断点。然后开始愉快的探索之旅。

在这里插入图片描述

如上，我们实例化了两个QObject的对象，分别是o1和o2;
在这里插入图片描述
我们看到，QObject的实例化对象o1中包含了4个成员变量,分别是vptr 、d_ptr 以及staticMetaObject和staticQtMetaObject .

  • vptr 从何而来？ 我们回归下C++的基础知识，QObject 是一个虚类，当实例化对象时，编译器会自动为实例化对象添加一个内置的成员变量 vptr(即我们常说的虚表指针，指向QObject 类在编译时生成的虚函数表)。
  • d_ptr 是啥？ 我们在源码中很快可以找到其定义：QScopedPointer<QObjectData> d_ptr; 指向 QObjectData数据成员变量的智能指针。
  • staticQtMetaObject 是怎么定义的？ static const QMetaObject staticQtMetaObject; 即元对象类型的静态常成员变量。
  • staticMetaObject 呢？同上。

我们回到本章主题，在来看看 QObjectPrivate.
前面我们已经说过，QObjectPrivate 公有继承自 QObjectData,在QObject类中定义了QScopedPointer<QObjectData> d_ptr;
QObject的构造函数里 ，定义了d_ptr;

QObject::QObject(QObject *parent)
    : d_ptr(new QObjectPrivate)
{
  // ...
}

所以，这里就是我们常说的“向上造型”，用基类（QObjectData）指针 指向子类(QObjectPrivate)对象.

我们再借助调试器来看看 QObjectPrivate实例化对象的布局：
在这里插入图片描述
因为 QObjectPrivate 共有继承自QObjectData,所以，它可以访问 QObjectData的所有成员变量（QObjectData 中所有成员变量都是 public属性的）。
其次，它还含有如下几个成员变量：

  • currentChildBeingDeleted :
  • declarativeData
    这俩哥们被定义在一个联合体中，即两个存在互斥关系。
union {
   QObject *currentChildBeingDeleted; // should only be used when QObjectData::isDeletingChildren is set
   QAbstractDeclarativeData *declarativeData; //extra data used by the declarative module
};
  • extraData : extra data set by the user
  • shareRefcount : these objects are all used to indicate that a QObject was deleted plus QPointer, which keeps a separate list
QAtomicPointer<QtSharedPointer::ExternalRefCountData> sharedRefcount;

  • threadData : id of the thread that owns the object

  • connections : 记录信号和槽的连接信息

QAtomicPointer<ConnectionData> connections;

我特意把 connections 放到下面来说，因为这里非常有意思。继续往下挖，就到了Qt信号和槽机制的根了,黑大粗那种。看看 ConnectionData 类：

struct ConnectionData {
        // the id below is used to avoid activating new connections. When the object gets
        // deleted it's set to 0, so that signal emission stops
        QAtomicInteger<uint> currentConnectionId;
        
        QAtomicInt ref;  
        
        QAtomicPointer<SignalVector> signalVector;  /// 信号容器
        Connection *senders = nullptr;
        Sender *currentSender = nullptr;   // object currently activating the object
        QAtomicPointer<Connection> orphaned;

        ~ConnectionData()
        {
            deleteOrphaned(orphaned.loadRelaxed());
            SignalVector *v = signalVector.loadRelaxed();
            if (v)
                free(v);
        }

        // must be called on the senders connection data
        // assumes the senders and receivers lock are held
        void removeConnection(Connection *c);
        void cleanOrphanedConnections(QObject *sender)
        {
            if (orphaned.loadRelaxed() && ref.loadAcquire() == 1)
                cleanOrphanedConnectionsImpl(sender);
        }
        void cleanOrphanedConnectionsImpl(QObject *sender);

        ConnectionList &connectionsForSignal(int signal)
        {
            return signalVector.loadRelaxed()->at(signal);
        }

        void resizeSignalVector(uint size) {
            SignalVector *vector = this->signalVector.loadRelaxed();
            if (vector && vector->allocated > size)
                return;
            size = (size + 7) & ~7;
            SignalVector *newVector = reinterpret_cast<SignalVector *>(malloc(sizeof(SignalVector) + (size + 1) * sizeof(ConnectionList)));
            int start = -1;
            if (vector) {
                memcpy(newVector, vector, sizeof(SignalVector) + (vector->allocated + 1) * sizeof(ConnectionList));
                start = vector->count();
            }
            for (int i = start; i < int(size); ++i)
                newVector->at(i) = ConnectionList();
            newVector->next = nullptr;
            newVector->allocated = size;

            signalVector.storeRelaxed(newVector);
            if (vector) {
                vector->nextInOrphanList = orphaned.loadRelaxed();
                orphaned.storeRelaxed(ConnectionOrSignalVector::fromSignalVector(vector));
            }
        }
        int signalVectorCount() const {
            return  signalVector.loadAcquire() ? signalVector.loadRelaxed()->count() : -1;
        }

        static void deleteOrphaned(ConnectionOrSignalVector *c);
};

struct ConnectionOrSignalVector {
        union {
            // linked list of orphaned connections that need cleaning up
            // 需要清理的孤立连接的链接列表
            ConnectionOrSignalVector *nextInOrphanList;
            // linked list of connections connected to slots in this object
            // 连接到此对象中插槽的连接的链接列表
            Connection *next;
        };

        static SignalVector *asSignalVector(ConnectionOrSignalVector *c) {
            if (reinterpret_cast<quintptr>(c) & 1)
                return reinterpret_cast<SignalVector *>(reinterpret_cast<quintptr>(c) & ~quintptr(1u));
            return nullptr;
        }
        static Connection *fromSignalVector(SignalVector *v) {
            return reinterpret_cast<Connection *>(reinterpret_cast<quintptr>(v) | quintptr(1u));
        }
    };



struct SignalVector : public ConnectionOrSignalVector {
        quintptr allocated;
        // ConnectionList signals[]
        ConnectionList &at(int i)
        {
            return reinterpret_cast<ConnectionList *>(this + 1)[i + 1];
        }
        const ConnectionList &at(int i) const
        {
            return reinterpret_cast<const ConnectionList *>(this + 1)[i + 1];
        }
        int count() const { return static_cast<int>(allocated); }
};

 // ConnectionList is a singly-linked list
    struct ConnectionList {
        QAtomicPointer<Connection> first;
        QAtomicPointer<Connection> last;
    };

ConnectionList 是一个单链表；每个节点都是一个Connection 对象。

 struct Connection : public ConnectionOrSignalVector
 {
        // linked list of connections connected to slots in this object, next is in base class
        Connection **prev;
        // linked list of connections connected to signals in this object
        QAtomicPointer<Connection> nextConnectionList;
        Connection *prevConnectionList;

        QObject *sender;    // 发送者对象
        QAtomicPointer<QObject> receiver; // 接收者对象
        QAtomicPointer<QThreadData> receiverThreadData; // 接收者线程id
	
		/**
		 typedef void (*StaticMetaCallFunction)(QObject *, QMetaObject::Call, int, void **);

		  // internal base class (interface) containing functions required to call a slot managed by a pointer to function.
    class QSlotObjectBase {
        QAtomicInt m_ref;
        // don't use virtual functions here; we don't want the
        // compiler to create tons of per-polymorphic-class stuff that
        // we'll never need. We just use one function pointer.
        typedef void (*ImplFn)(int which, QSlotObjectBase* this_, QObject *receiver, void **args, bool *ret);
        const ImplFn m_impl;
    protected:
        enum Operation {
            Destroy,
            Call,
            Compare,

            NumOperations
        };
    public:
        explicit QSlotObjectBase(ImplFn fn) : m_ref(1), m_impl(fn) {}

        inline int ref() noexcept { return m_ref.ref(); }
        inline void destroyIfLastRef() noexcept
        { if (!m_ref.deref()) m_impl(Destroy, this, nullptr, nullptr, nullptr); }

        inline bool compare(void **a) { bool ret = false; m_impl(Compare, this, nullptr, a, &ret); return ret; }
        inline void call(QObject *r, void **a)  { m_impl(Call,    this, r, a, nullptr); }
    protected:
        ~QSlotObjectBase() {}
    private:
        Q_DISABLE_COPY_MOVE(QSlotObjectBase)
    };

	*/

		/// 定义了连接绑定的槽函数
        union {
            StaticMetaCallFunction callFunction;
            QtPrivate::QSlotObjectBase *slotObj;
        };
        QAtomicPointer<const int> argumentTypes;
        QAtomicInt ref_;
        uint id = 0;
        
        
        ushort method_offset;
        ushort method_relative;
        
        int signal_index : 27; // In signal range (see QObjectPrivate::signalIndex())
        
        ushort connectionType : 3; // 0 == auto, 1 == direct, 2 == queued, 4 == blocking
        ushort isSlotObject : 1;
        ushort ownArgumentTypes : 1;
        Connection() : ref_(2), ownArgumentTypes(true) {
            //ref_ is 2 for the use in the internal lists, and for the use in QMetaObject::Connection
        }
        ~Connection();
        
        int method() const { 
        	Q_ASSERT(!isSlotObject); 
        	return method_offset + method_relative; 
        }
        void ref() { ref_.ref(); }
        void freeSlotObject()
        {
            if (isSlotObject) {
                slotObj->destroyIfLastRef();
                isSlotObject = false;
            }
        }
        void deref() {
            if (!ref_.deref()) {
                Q_ASSERT(!receiver.loadRelaxed());
                Q_ASSERT(!isSlotObject);
                delete this;
            }
        }
};

Connection 对象中定义了对象连接的信号和槽：

  • 一个对象既可以定义成信号的发送者，也可以定义成信号的接收者
  • 一个对象可以接收多个信号
  • 一个发送者对象可以绑定多个接收者信号及对应的槽函数

我们再回头看两个函数：

// Used by QAccessibleWidget
QObjectList QObjectPrivate::receiverList(const char *signal) const
{
    QObjectList returnValue;
    int signal_index = signalIndex(signal);
    ConnectionData *cd = connections.loadRelaxed();
    if (signal_index < 0 || !cd)
        return returnValue;
    if (signal_index < cd->signalVectorCount()) {
        const QObjectPrivate::Connection *c = cd->signalVector.loadRelaxed()->at(signal_index).first.loadRelaxed();

        while (c) {
            QObject *r = c->receiver.loadRelaxed();
            if (r)
                returnValue << r;
            c = c->nextConnectionList.loadRelaxed();
        }
    }
    return returnValue;
}

// Used by QAccessibleWidget
QObjectList QObjectPrivate::senderList() const
{
    QObjectList returnValue;
    ConnectionData *cd = connections.loadRelaxed();
    if (cd) {
        QBasicMutexLocker locker(signalSlotLock(q_func()));
        for (Connection *c = cd->senders; c; c = c->next)
            returnValue << c->sender;
    }
    return returnValue;
}

看到这里，我们对于QObject 中对象数据类 QObjectPrivate 有了一些认识了。

楼已高，起下篇~

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