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More Effective 的 Reference Counting

發覺計算機很多東西都是相同的，記得操作系統時候學過這一概率 copy on write，在Reference Counted中徹底用到，代碼的設計確實精妙，可以在不修改客戶端得類，利用RCIPtr間接指針，對客戶端的類實現引用計數，太妙了，詳細見代碼吧，代碼中Widget為已有的客戶端的類，RCIPtr是一個間接指針，RCObject是引用計數的基類，所有需要引用計數的類都必須繼承他，換句話說，RCObject類將引用計數邏輯封裝了，而RCIPtr則是連接客戶端類的紐帶.
代碼如下:
RCIPtr.h

#ifndef RCIPTR_H

#define RCIPTR_H

#include "RCObject.h"

template<class T>

class RCIPtr {

public :

RCIPtr(T* realPtr = 0);

RCIPtr(const RCIPtr& rhs);

~RCIPtr();

RCIPtr& operator=(const RCIPtr& rhs);

const T* operator->() const; //const version ->

T* operator->();

const T& operator*() const; //const version *

T& operator*();

int getRefCount() const; //測試用，獲取指針所指向的對象的引用數

private :

struct CountHolder : public RCObject { //T的實際的指針保寸的地方

~CountHolder() {delete pointee;};

T* pointee;

};

CountHolder *counter;

void init(); //初始化公共代碼

void makeCopy(); //Copy On Write的調用函數

};

#include "RCIPtr.cpp"

#endif

RCIPtr.cpp

#ifdef RCIPTR_H

template<class T>

void RCIPtr<T>::init() {

if (counter->isShareable() == false) {

T* oldValue = counter->pointee;

counter = new CountHolder;

counter->pointee = new T(*oldValue);

}

counter->addReference();

}

template<class T>

void RCIPtr<T>::makeCopy() {

if (counter->isShared()) {

counter->removeReference();

T* oldValue = counter->pointee;

counter = new CountHolder;

counter->pointee = new T(*oldValue);

counter->addReference();

}

template<class T>

RCIPtr<T>::RCIPtr(T* realPtr) : counter(new CountHolder) {

counter->pointee = realPtr;

init();

}

template<class T>

RCIPtr<T>::RCIPtr(const RCIPtr& rhs) : counter(rhs.counter) {

init();

}

template<class T>

RCIPtr<T>::~RCIPtr() {

counter->removeReference();

}

template<class T>

RCIPtr<T>& RCIPtr<T>::operator=(const RCIPtr& rhs) {

if (counter != rhs.counter) {

counter->removeReference();

counter = rhs.counter;

init();

}

return *this;

}

template<class T>

const T* RCIPtr<T>::operator->() const {

return counter->pointee;

}

template<class T>

T* RCIPtr<T>::operator->() {

makeCopy();

return counter->pointee;

}

template<class T>

const T& RCIPtr<T>::operator*() const {

return *(counter->pointee);

}

template<class T>

T& RCIPtr<T>::operator*() {

makeCopy();

return *(counter->pointee);

}

template<class T>

int RCIPtr<T>::getRefCount() const {

return counter->getRefCount();

}

#endif

RCObject.h

#ifndef RCOBJECT_H

#define RCOBJECT_H

/**

* base class, 用于reference-counted objects

class RCObject {

public :

void addReference(); //添加reference

void removeReference(); //刪除reference

void markUnshareable(); //標記object不可共享

bool isShareable() const; //判斷object是否可被共享

bool isShared() const; //判斷object是否被共享中

int getRefCount() const; //獲取對象的引用數

protected :

RCObject(); //RCObject default-constructor

RCObject(const RCObject& rhs); //RCObject copy-constructor

RCObject& operator=(const RCObject& rhs); //RCObject assignment

virtual ~RCObject() = 0;

private :

int refCount; //引用計數

bool shareable; //可否共享標記

};

#endif

RCObject.cpp

#include "RCObject.h"

RCObject::RCObject() : refCount(0), shareable(true) {

}

RCObject::RCObject(const RCObject& rhs) : refCount(0), shareable(true) {

}

RCObject& RCObject::operator =(const RCObject& rhs) {

return *this;

}

void RCObject::addReference() {

refCount++;

}

void RCObject::removeReference() {

if (--refCount == 0) delete this;

}

void RCObject::markUnshareable() {

shareable = false;

}

bool RCObject::isShareable() const {

return shareable;

}

bool RCObject::isShared() const {

return refCount > 1;

}

int RCObject::getRefCount() const {

return refCount;

}

RCObject::~RCObject() {

// delete this;

}

Widget.h

#ifndef WIDGET_H

#define WIDGET_H

/**

* Widget測試類

class Widget {

public :

Widget(int size);

Widget(const Widget& rhs);

~Widget();

Widget& operator=(const Widget& rhs);

void setSize(int size);

int getSize() const;

void showSize() const;

private :

int mSize;

};

#endif

Widget.cpp

#include "Widget.h"

#include <iostream>

using namespace std;

Widget::Widget(int size) : mSize(size) {

}

Widget::Widget(const Widget& rhs) : mSize(rhs.mSize) {

}

Widget::~Widget() {

}

Widget& Widget::operator=(const Widget& rhs) {

if (this == &rhs) {

return *this;

}

mSize = rhs.getSize();

return *this;

}

void Widget::setSize(int size) {

mSize = size;

}

int Widget::getSize() const {

return mSize;

}

void Widget::showSize() const {

cout << "Size of this Widget is " << mSize << endl;

}

RCWidget.h

#ifndef RCWIDGET_H

#define RCWIDGET_H

#include "RCIPtr.h"

#include "Widget.h"

class RCWidget {

public :

RCWidget(int size);

void setSize(int size);

int getSize() const;

void showSize() const;

int getRefCount() const;

private :

RCIPtr<Widget> value;

};

#endif

RCWidget.cpp

#include "RCWidget.h"

RCWidget::RCWidget(int size) : value(new Widget(size)) {

}

void RCWidget::setSize(int size) {

value->setSize(size);

}

int RCWidget::getSize() const {

return value->getSize();

}

void RCWidget::showSize() const {

value->showSize();

}

int RCWidget::getRefCount() const {

return value.getRefCount();

}

test.cpp

#include "RCWidget.h"

#include <iostream>

using namespace std;

int main() {

RCWidget w(100);

RCWidget v(w);

v.showSize();

w.showSize();

cout << "v RCWidget RefCount is " << v.getRefCount() << endl;

cout << "w RCWidget RefCount is " << w.getRefCount() << endl;

v.setSize(20);

v.showSize();

w.showSize();

cout << "v RCWidget RefCount is " << v.getRefCount() << endl;

cout << "w RCWidget RefCount is " << w.getRefCount() << endl;

return 0;

}

Result :
Size of this Widget is 100
Size of this Widget is 100
v RCWidget RefCount is 2
w RCWidget RefCount is 2
Size of this Widget is 20
Size of this Widget is 100
v RCWidget RefCount is 1
w RCWidget RefCount is 1
Press any key to continue

只要每次寫一個類似的RC類，就不用修改客戶端已有的類，而又可以實現Reference-Counting了

posted on 2008-04-09 21:36 豪閱讀(550) 評論(0) 編輯收藏引用所屬分類: C++之夢

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