More Effective C++之引用计数
Reference counting让我想起了Java，当假如想用C++来实现Java的能力的话，那Reference counting必不可少。Reference counting可以节省程序的运行成本，大量的构造、析构、分配、释放和拷贝的代价被省略。
实现
classRCObject
{
public:
RCObject():refCount(0),shareable(true){}
RCObject(constRCObject&):refCount(0),shareable(true){}
RCObject& operator=(constRCObject& rhs){return *this;}
virtual ~RCObject()=0;
void AddReference(){++refCount;}
void RemoveReference(){if (--refCount == 0) deletethis;}
void markUnshareable(){shareable = false;}
bool isShareable() const{returnshareable;}
bool isShared() const {returnrefCount > 1;}
private:
int refCount;
bool shareable;
};
RCObject::~RCObject(){}
template <classT>
class RCPtr
{
public:
RCPtr(T* realPtr = 0):pointee(realPtr){init();}
RCPtr(constRCPtr& rhs):pointee(rhs.pointee){init();}
~RCPtr(){if (pointee) pointee->RemoveReference();}
RCPtr& operator = (constRCPtr& rhs)
{
if (pointee!=rhs.pointee)
{
if (pointee)
pointee->RemoveReference();
pointee = rhs.pointee;
init();
}
return *this;
}
T* operator->() const { returnpointee;}
T& operator*() const{return *pointee;}
private:
T* pointee;
void init()
{
if (pointee == 0)
return;
if (pointee->isShareable() == false)
pointee = newT(*pointee);
pointee->AddReference();
}
};
class String
{
public:
String(const char* value = ""):value(newStringValue(value)){}
const char& operator[](intnIndex) const
{
return value->data[nIndex];
}
char& operator[](intnIndex)
{
if (value->isShared())
value = newStringValue(value->data);
value->markUnshareable();
returnvalue->data[nIndex];
}
protected:
private:
strUCt StringValue:publicRCObject
{
char* data;
String Value(constchar* initValue)
{
init(initValue);
}
String Value(constStringValue& rhs)
{
init(rhs.data);
}
void init(constchar * initValue)
{
data = newchar[strlen(initValue) + 1];
strcpy(data,initValue);
}
~String Value()
{
delete [] data;
}
};
RCPtr<StringValue> value;
};
这是Meyers给出的String的实现，然而我的观点是假如没有非凡的必要的话，对stirng最好不要使用引用计数，因为在多线程程序中同步的代价要大于引用计数本身的好处，得不偿失。
假如StringValue是一个现成的类，无法修改它的实现，那怎么办？没关系可以用委托，下面是一个典型的实现：
classRCObject
{
public:
RCObject():refCount(0),shareable(true){}
RCObject(constRCObject&):refCount(0),shareable(true){}
RCObject& operator=(constRCObject& rhs){return *this;}
virtual ~RCObject()=0;
void AddReference(){++refCount;}
void RemoveReference(){if (--refCount == 0) deletethis;}
void markUnshareable(){shareable = false;}
bool isShareable() const{returnshareable;}
bool isShared() const {returnrefCount > 1;}
private:
int refCount;
bool shareable;
};
RCObject::~RCObject(){}
template<classT>
class RCIPtr
{
public:
RCIPtr(T* realPtr = 0):counter(new CountHolder)
{
counter->pointee = realPtr;
init();
}
RCIPtr(constRCIPtr& rhs):counter(rhs.counter)
{
init();
}
~RCIPtr()
{
counter->RemoveReference();
}
RCIPtr& operator = (constRCIPtr& rhs)
{
if (counter != rhs.counter)
{
counter->RemoveReference();
counter = rhs.counter;
init();
}
return *this;
}
constT* operator->()const
{
returncounter->pointee;
}
T* operator->()
{
makeCopy();
returncounter->pointee;
}
constT& operator*() const
{
return *(counter->pointee);
}
T& operator*()
{
makeCopy();
return *(counter->pointee);
}
private:
struct CountHolder:publicRCObject
{
~Count Holder(){deletepointee;}
T* pointee;
};
Count Holder* counter;
void init()
{
if (counter->isShareable() == false)
{
T* oldValue = counter->pointee;
counter = newCountHolder;
counter->pointee = newT(*oldValue);
}
counter->AddReference();
}
void makeCopy()
{
if (counter->isShared())
{
T* oldValue = counter->pointee;
counter->RemoveReference();
counter = newCountHolder;
counter->pointee = newT(*oldValue);
counter->AddReference();
}
}
};
class Widget
{
public:
Widget(intSize){}
Widget(constWidget& rhs){}
~Widget(){}
Widget operator=(const Widget& rhs){}
void doThis(){printf("doThis()\n");return;}
int showThat() const{printf("showThat()\n"); return 0;}
protected:
private:
inti;
};
class RCWidget
{
public:
RCWidget(intsize):value(newWidget(size)){}
void doThis(){value->doThis();}
int showThat()const {returnvalue->showThat();}
protected:
private:
RCIPtr<Widget> value;
};
评估
实现引用计数是需要有前提的，不是所有的情况下，使用引用计数都是合适的。适合情况如下：
相对多的对象共享相对少量的实值。
对象的实值产生或者销毁的成本很高，或者占用很多内存。
但是要记住，即使是Java也会有内存泄漏，不要指望小小的引用计数（上面简单的实现）不会产生同样的问题。
引用计数是一项很深奥的技术，想想Java，所以需要很谨慎的对待，但愿它能带来程序设计上的优化。