1.QT中Q_Q和Q_D是什么？

• Q_Q可以得到外部可以访问的类指针
• Q_D可以得到内部封装，外部不可达的类指针

2. 怎么使用？

上代码

• APrivate.h

#pragma once
#include <QtCore>class A;class APrivate
{Q_DECLARE_PUBLIC(A)
public:APrivate();~APrivate();
public:void APrivateUseA();
private:friend A;A* q_ptr;int m_iA;
};

• A.h

#pragma once
#include <QtCore>
class APrivate;class A
{Q_DECLARE_PRIVATE(A)
public:A();~A();public://对外接口void setValue(int& iValue);int getValue();public:void APrivateCanUse();void APrivateUse();private:APrivate* d_ptr;
};

• A.cpp

#include "APrivate.h"
#include "A.h"
#include <iostream>A::A():d_ptr(new APrivate) 
{d_ptr->q_ptr = this;std::cout << "A()" << std::endl;
}
A::~A() 
{Q_D(A);delete d;std::cout << "~A()" << std::endl;
}void A::setValue(int& iValue)
{Q_D(A);d->m_iA = iValue;
}int A::getValue()
{Q_D(A);return d->m_iA;
}void A::APrivateCanUse()
{int iValue = 3;setValue(iValue);
}void A::APrivateUse()
{Q_D(A);d->APrivateUseA();
}APrivate::APrivate():m_iA(1)
{std::cout << "APrivate()" << std::endl;
}
APrivate::~APrivate() 
{std::cout << "~APrivate()" << std::endl;
}void APrivate::APrivateUseA()
{Q_Q(A);q->APrivateCanUse();
}

• main.cpp使用

#include <QtCore/QCoreApplication>
#include <iostream>
#include "A.h"
#include "APrivate.h"
#include <memory>int main(int argc, char* argv[])
{QCoreApplication a(argc, argv);{	//1std::unique_ptr<A> Aa = std::make_unique<A>();std::cout << Aa->getValue() << std::endl;//2int Value = 2;Aa->setValue(Value);std::cout << Aa->getValue() << std::endl;//3Aa->APrivateUse();std::cout << Aa->getValue() << std::endl;}return a.exec();
}

输出：

APrivate()
A()
1
2
3
~APrivate()
~A()

3.本质是什么？

• 详见QtCore文件夹下文的头文件qglobal.h
• 摘录主要部分如下：

template <typename T> inline T *qGetPtrHelper(T *ptr) { return ptr; }
template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) -> decltype(ptr.operator->()) { return ptr.operator->(); }// The body must be a statement:
#define Q_CAST_IGNORE_ALIGN(body) QT_WARNING_PUSH QT_WARNING_DISABLE_GCC("-Wcast-align") body QT_WARNING_POP
#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;#define Q_DECLARE_PRIVATE_D(Dptr, Class) \inline Class##Private* d_func() \{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<Class##Private *>(qGetPtrHelper(Dptr));) } \inline const Class##Private* d_func() const \{ Q_CAST_IGNORE_ALIGN(return reinterpret_cast<const Class##Private *>(qGetPtrHelper(Dptr));) } \friend class Class##Private;#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;#define Q_D(Class) Class##Private * const d = d_func()
#define Q_Q(Class) Class * const q = q_func()

• 为了方便理解裁剪代码如下:(对照步骤2如何使用来理解)

template <typename T> inline T *qGetPtrHelper(T *ptr) { return ptr; }
//A.h中的声明，Class为A
#define Q_DECLARE_PRIVATE(Class) \inline Class##Private* d_func() \{return reinterpret_cast<Class##Private *>(qGetPtrHelper(d_ptr)); } \inline const Class##Private* d_func() const \{return reinterpret_cast<const Class##Private *>(qGetPtrHelper(d_ptr));} \friend class Class##Private;
//APrivate.h中的声明，Class为A
#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;
//使用时的声明，取得临时指针
#define Q_D(Class) Class##Private * const d = d_func()
#define Q_Q(Class) Class * const q = q_func()

评论：

优：风格统一，使用便捷

劣：每次使用都要两个指针2*8=16个字节的额外开销

4.仿写

不用QT库，C++仿写

• BPrivate.h

#pragma once
class B;class BPrivate
{
public:BPrivate();~BPrivate();
public:void BPrivateUseB();
private:friend B;B* q_ptr;int m_iA;
};

• B.h

#pragma once
class BPrivate;class B
{
public:B();~B();public://对外接口void setValue(int& iValue);int getValue();public:void BPrivateCanUse();void BPrivateUse();private:BPrivate* d_ptr;
};

• B.cpp

#include "BPrivate.h"
#include "B.h"
#include <iostream>B::B() :d_ptr(new BPrivate)
{d_ptr->q_ptr = this;std::cout << "B()" << std::endl;
}
B::~B()
{delete d_ptr;std::cout << "~B()" << std::endl;
}void B::setValue(int& iValue)
{d_ptr->m_iA = iValue;
}int B::getValue()
{return d_ptr->m_iA;
}void B::BPrivateCanUse()
{int iValue = 3;setValue(iValue);
}void B::BPrivateUse()
{d_ptr->BPrivateUseB();
}BPrivate::BPrivate() :m_iA(1)
{std::cout << "BPrivate()" << std::endl;
}
BPrivate::~BPrivate()
{std::cout << "~BPrivate()" << std::endl;
}void BPrivate::BPrivateUseB()
{q_ptr->BPrivateCanUse();
}

• main.cpp

#include <QtCore/QCoreApplication>
#include <iostream>
#include "A.h"
#include "APrivate.h"
#include "B.h"
#include "BPrivate.h"
#include <memory>int main(int argc, char* argv[])
{QCoreApplication a(argc, argv);{	//1std::unique_ptr<A> Aa = std::make_unique<A>();std::cout << Aa->getValue() << std::endl;//2int Value = 2;Aa->setValue(Value);std::cout << Aa->getValue() << std::endl;//3Aa->APrivateUse();std::cout << Aa->getValue() << std::endl;}{//1std::unique_ptr<B> Bb = std::make_unique<B>();std::cout << Bb->getValue() << std::endl;//2int Value = 2;Bb->setValue(Value);std::cout << Bb->getValue() << std::endl;//3Bb->BPrivateUse();std::cout << Bb->getValue() << std::endl;}return a.exec();
}

• 输出（一样）：

APrivate()
A()
1
2
3
~APrivate()
~A()
BPrivate()
B()
1
2
3
~BPrivate()
~B()