Qt的q指针和d指针笔记

原因：只要Qt内部增加/删除/修改/改变顺序/改变类型任何一个私有成员，所有使用Qt的程序都必须重新编译，否则会出现未定义行为（内存错位、崩溃等）。
d指针指向的私有数据专门有相应头文件 qwidget_p.h(其中p:private)来定义。
1、类定义d指针

cpp 复制代码

class Q_CORE_EXPORT QObject
{
    Q_OBJECT
    Q_PROPERTY(QString objectName READ objectName WRITE setObjectName NOTIFY objectNameChanged)
    Q_DECLARE_PRIVATE(QObject)
...
    QScopedPointer<QObjectData> d_ptr;
...
}

1.1、Q_DECLARE_PRIVATE定义

cpp 复制代码

#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;

Q_DECLARE_PRIVATE(MyWidget)展开

cpp 复制代码

inline MyWidgetPrivate* d_func()
{ 
    Q_CAST_IGNORE_ALIGN(return reinterpret_cast<MyWidgetPrivate *>(qGetPtrHelper(d_ptr));) 
}

inline const MyWidgetPrivate* d_func() const
{ 
    Q_CAST_IGNORE_ALIGN(return reinterpret_cast<const MyWidgetPrivate *>(qGetPtrHelper(d_ptr));)
} 
friend class MyWidgetPrivate;

1.2、 QObjectData定义

cpp 复制代码

class Q_CORE_EXPORT QObjectData {  // 这是一个接口类
public:
    virtual ~QObjectData() = 0; //纯虚函数
    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 QWindow
    uint deleteLaterCalled : 1;
    uint unused : 24;
    int postedEvents;
    QDynamicMetaObjectData *metaObject;
    QMetaObject *dynamicMetaObject() const;
};

class Q_WIDGETS_EXPORT QWidgetPrivate : public QObjectPrivate qwidget_p.h(其中p:private，经常会变化的部分提取出来)
class Q_CORE_EXPORT QObjectPrivate : public QObjectData qobject_p.h(其中p:private，经常会变化的部分提取出来)
2、私有类定义q指针 （继承了 QObjectData中定义了 QObject *q_ptr）

cpp 复制代码

class Q_CORE_EXPORT QObjectPrivate : public QObjectData
{
    Q_DECLARE_PUBLIC(QObject)
public:
...

Q_DECLARE_PUBLIC定义

cpp 复制代码

#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;

Q_DECLARE_PUBLIC(QObject)宏展开

cpp 复制代码

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;