template<class T> // template class for smart
class RCPtr { // pointers-to-T objects; T
public: // must inherit from RCObject
RCPtr(T* realPtr = 0);
RCPtr(const RCPtr& rhs);
~RCPtr();
RCPtr& operator=(const RCPtr& rhs);
T* operator->() const;
T& operator*() const;
private:
T *pointee;
void init();
};
class RCObject { // base class for reference-
public: // counted objects
void addReference();
void removeReference();
void markUnshareable();
bool isShareable() const;
bool isShared() const;
protected:
RCObject();
RCObject(const RCObject& rhs);
RCObject& operator=(const RCObject& rhs);
virtual ~RCObject() = 0;
private:
int refCount;
bool shareable;
};
class String { // class to be used by
public: // application developers
String(const char *value = "");
const char& operator[](int index) const;
char& operator[](int index);
private:
// class representing string values
struct StringValue: public RCObject {
char *data;
StringValue(const char *initValue);
StringValue(const StringValue& rhs);
void init(const char *initValue);
~StringValue();
};
RCPtr<StringValue> value;
};
RCObject::RCObject()
: refCount(0), shareable(true) {}
RCObject::RCObject(const RCObject&)
: refCount(0), shareable(true) {}
RCObject& RCObject::operator=(const RCObject&)
{ return *this; }
RCObject::~RCObject() {}
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; }
这是RCPtr的实现：
template<class T>
void RCPtr<T>::init()
{
if (pointee == 0) return;
if (pointee->isShareable() == false) {
pointee = new T(*pointee);
}
pointee->addReference();
}
template<class T>
RCPtr<T>::RCPtr(T* realPtr)
: pointee(realPtr)
{ init(); }
template<class T>
RCPtr<T>::RCPtr(const RCPtr& rhs)
: pointee(rhs.pointee)
{ init(); }
template<class T>
RCPtr<T>::~RCPtr()
{ if (pointee)pointee->removeReference(); }
template<class T>
RCPtr<T>& RCPtr<T>::operator=(const RCPtr& rhs)
{
if (pointee != rhs.pointee) {
if (pointee) pointee->removeReference();
pointee = rhs.pointee;
init();
}
return *this;
}
template<class T>
T* RCPtr<T>::operator->() const { return pointee; }
template<class T>
T& RCPtr<T>::operator*() const { return *pointee; }
这是String::StringValue的实现：
void String::StringValue::init(const char *initValue)
{
data = new char[strlen(initValue) + 1];
strcpy(data, initValue);
}
String::StringValue::StringValue(const char *initValue)
{ init(initValue); }
String::StringValue::StringValue(const StringValue& rhs)
{ init(rhs.data); }
String::StringValue::~StringValue()
{ delete [] data; }
最后，归结到String，它的实现是：
String::String(const char *initValue)
: value(new StringValue(initValue)) {}
const char& String::operator[](int index) const
{ return value->data[index]; }
char& String::operator[](int index)
{
if (value->isShared()) {
value = new StringValue(value->data);
}
value->markUnshareable();
return value->data[index];
}
什么情况下使用引用计数：
少量的值被大量的对象共享。这样的共享通常通过调用赋值操作和拷贝构造而发生。对象/值的比例越高，越是适宜使用引用计数。
