C++引用计数

//reference count, copy-on-write
#include <stdio.h>
#include <iostream>
#include <../require.h>
#include <string>

using namespace std;

class Dog
{
    string nm;
    int refcount;
    Dog(const string& name)
        : nm(name), refcount(1)
    {
        cout << "creating Dog:" << *this << endl;
    }
    //prevent assignment:
    Dog& operator=(const Dog& rv);
public:
    //Dogs can only be  created on the heap
    static Dog* make(const string& name)
    {
        return new Dog(name);
    }
    Dog(const Dog& d)
        : nm(d.nm + "copy"), refcount(1)
    {
        cout << "Dog copy-constructor:" << *this << endl;
    }
    ~Dog()
    {
        cout << "Attached Dog:" << *this << endl;
    }
    void attach()
    {
        ++refcount;
        cout << "Attach Dog:" << *this << endl;
    }
    void  detach()
    {
        require(refcount != 0);
        cout << "Detaching Dog:" << *this << endl;
        //destroy object if no one is using it :
        if (--refcount == 0) delete this;
    }
    //conditionally copy this dog
    //call before modifying the dog,assign
    //resulting pointer to your Dog*
    Dog* unalias()
    {
        cout << "unaliasing dog:" << *this << endl;
        //not duplicate if not aliased;
        if (refcount == 1)
            return this;
        --refcount;
        //use copy-constructor to duplicate:
        return new Dog(*this);
    }
    void rename(const string& newName)
    {
        nm = newName;
        cout << "Dog renamed to:" << *this << endl;
    }
    friend ostream& operator<<(ostream& os, const Dog& d)
    {
        return os << "[" << d.nm << "],rc=" << d.refcount << endl;
    }
};

class DogHouse
{
    Dog* p;
    string houseName;
public:
    DogHouse(Dog* dog, const string& house)
        : p(dog), houseName(house)
    {
        cout << "created DogHouse:" << *this << endl;
    }
    DogHouse(const DogHouse& dh)
        : p(dh.p),
        houseName("copy-constructed" + dh.houseName)
    {
        p->attach();
        cout << "DogHouse copy-constructor:" << *this << endl;
    }
    DogHouse& operator=(const DogHouse& dh)
    {
        //check for self-assignment:
        if (&dh != this)
        {
            houseName = dh.houseName + "assigned";
            //clean up what you‘re using first:
            p->detach();
            p = dh.p;//like copy-constructor
            p->attach();
        }
        cout << "DogHouse operator=:" << *this << endl;
        return *this;
    }
    //decrement refcount, conditionally destroy
    ~DogHouse()
    {
        cout << "DogHouse destructor:" << *this << endl;
        p->detach();
    }
    void renameHouse(const string& newName)
    {
        houseName = newName;
    }
    void unalias()
    {
        p = p->unalias();
    }
    //copy-on-write.anytime you modify the
    //contents of the pointer you must first unalias it:
    void renameDog(const string& newName)
    {
        unalias();
        p->rename(newName);
    }
    //...or when you allow someone else access:
    Dog* getDog()
    {
        unalias();
        return p;
    }
    friend ostream& operator<<(ostream& os, const DogHouse& dh)
    {
        return os << "[" << dh.houseName << "] contains " << *dh.p;
    }
};

int main()
{
    DogHouse fidos(Dog::make("Fido"), "FidoHouse"),
    spots(Dog::make("Spot"), "SpotHouse");
    cout << "entering copy-construction" << endl;
    DogHouse bobs(fidos);
    cout << "After copy-constructing bobs" << endl;
    cout << "fidos:" << fidos << endl;
    cout << "spots:" << spots << endl;
    cout << "bobs:" << bobs << endl;
    cout << "Entering spots = fidos" << endl;
    spots = fidos;
    cout << "After spots=fidos" << endl;
    cout << "spots:" << spots << endl;
    cout << "entering self-assignment" << endl;
    bobs = bobs;
    cout << "After self-assignment" << endl;
    cout << "bobs:" << bobs << endl;
    //comment out the following lines
    cout << "entering rename(\"Bob\")" << endl;
    bobs.getDog()->rename("Bob");
    cout << "After rename(\"Bob\")" << endl;
}