QObject source code analysis -d pointer and Q pointer

have a look QObject Source code ：

class Q_CORE_EXPORT QObject
{
    
	...
protected:
    QScopedPointer<QObjectData> d_ptr;
    ...
}

You can see QObject There is one protected member d_ptr, The type of QObjcetData,QScopedPointer Be similar to unique_ptr, have a look QObjectData Source code ：

class Q_CORE_EXPORT QObjectData {
    
public:
    virtual ~QObjectData() = 0;
    QObject *q_ptr;
	....
};

above d_ptr and q_ptr Namely d Pointers and q The pointer
have a look QObject Constructors ：

QObject::QObject(QObject *parent)
    : d_ptr(new QObjectPrivate)
{
    
    Q_D(QObject);
    d_ptr->q_ptr = this;
	....
}

You can see ,QObject Give it during construction d_ptr assignment QObjectPrivate, Look at the source code ：

class Q_CORE_EXPORT QObjectPrivate : public QObjectData
{
    
	...
}

You can see QObjectPrivate by QObjectData Subclasses of , from QObject You can see that ,QObject and QObjectPrivate Have each other's object pointers , that QObject Why do that ？
actually QObject All member variables of are placed in QObjectPrivate in , Why put member variables in QObjectPrivate And then in QObject Use in D The pointer d_ptr Well ？ In fact, it is to solve the problems of compilation and dependency .

• Compilation problems ： If there is 20 individual cpp file #include 了 qobject.h file , So once you get to this .h The data member variables of the file are slightly modified , This will happen the next time the project is compiled 10 individual cpp All files must be recompiled . This is the disadvantage of directly defining the data member variables of the type in the type itself .
• The second is dependency . Generally will dll When provided for use by others , It must also contain the corresponding header file , If it changes at this time QObject.h Members of , After modifying the data member variables of a type , This will cause the memory layout of object instances of type to change , Then all use this dll Must be recompiled to use , Instead of simply replacing the previous dll Just go . Save data members with pointers , Put the implementation of this pointer in cpp Can avoid header file changes , So as to directly use the new dll You don't have to use this dll All the places are recompiled .

Actually q Pointers and d Programming skills for pointer utilization c++ PIMPL, If you are interested, you can find relevant information .