C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 16:
16.1 Does delete p delete the pointer p, or the pointed-to-data *p?
16.2 Is it safe to delete the same pointer twice?
16.3 Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()?
16.4 Benefits of new over malloc()?
16.5 Can I use realloc() on pointers allocated via new?
16.6 Checking for NULL after p = new Fred()?
16.7 How can I convince my (older) compiler to automatically check new to see if it returns NULL?
16.8 Checking for NULL before delete p?
16.9 What are the two steps that happen when I say delete p?
16.10 Does p = new Fred() leak memory if the ctor throws an exception?
16.11 How do I allocate / unallocate an array of things?
16.12 What if I forget the [] when deleteing an array allocated via new T[n]?
16.13 Can I drop the [] when deleteing an array of some built-in type (char, int, etc)?
16.14 After p = new Fred[n], how does the compiler know there are n objects to be destructed during delete[] p?
16.15 Is it legal (and moral) for a member function to say delete this?
16.16 How do I allocate multidimensional arrays using new?
16.17 How to simplify the Matrix code from the previous FAQ?
16.18 How to make the Matrix class generic?
16.19 What's another way to build a Matrix template?
16.20 Does C++ have arrays whose length can be specified at run-time?
16.21 Allocating all objects via new, not local/global/static?
16.22 How do I do simple reference counting?
16.23 How do I provide reference counting with copy-on-write semantics?
16.24 How do I provide reference counting with copy-on-write semantics for a hierarchy of classes?
16.25 Preventing people from subverting the reference counting mechanism?
16.26 Can I use a garbage collector in C++?
16.27 What are the two kinds of garbage collectors for C++?
16.28 Where can I get more info on garbage collectors for C++?
[16.24] How do I provide reference counting with copy-on-write semantics for a hierarchy of classes?

The previous FAQ presented a reference counting scheme that provided users with reference semantics, but did so for a single class rather than for a hierarchy of classes. This FAQ extends the previous technique to allow for a hierarchy of classes. The basic difference is that Fred::Data is now the root of a hierarchy of classes, which probably cause it to have some virtual functions. Note that class Fred itself will still not have any virtual functions.

The Virtual Constructor Idiom is used to make copies of the Fred::Data objects. To select which derived class to create, the sample code below uses the Named Constructor Idiom, but other techniques are possible (a switch statement in the constructor, etc). The sample code assumes two derived classes: Der1 and Der2. Methods in the derived classes are unaware of the reference counting. 

class Fred {
public:

  static Fred create1(std::string const& s, int i);
  static Fred create2(float x, float y);

  Fred(Fred const& f);
  Fred& operator= (Fred const& f);
 ~Fred();

  void sampleInspectorMethod() const;   // No changes to this object
  void sampleMutatorMethod();           // Change this object

  ...

private:

  class Data {
  public:
    Data() : count_(1) { }
    Data(Data const& d) : count_(1) { }              // Do NOT copy the 'count_' member!
    Data& operator= (Data const&) { return *this; }  // Do NOT copy the 'count_' member!
    virtual ~Data() { assert(count_ == 0); }         // A virtual destructor
    virtual Data* clone() const = 0;                 // A virtual constructor
    virtual void sampleInspectorMethod() const = 0;  // A pure virtual function
    virtual void sampleMutatorMethod() = 0;
  private:
    unsigned count_;   // count_ doesn't need to be protected
    friend class Fred; // Allow Fred to access count_
  };

  class Der1 : public Data {
  public:
    Der1(std::string const& s, int i);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual Data* clone() const;
    ...
  };

  class Der2 : public Data {
  public:
    Der2(float x, float y);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual Data* clone() const;
    ...
  };

  Fred(Data* data);
  // Creates a Fred smart-reference that owns *data
  // It is private to force users to use a createXXX() method
  // Requirement: data must not be NULL

  Data* data_;   // Invariant: data_ is never NULL
};

Fred::Fred(Data* data) : data_(data)  { assert(data != NULL); }

Fred Fred::create1(std::string const& s, int i) { return Fred(new Der1(s, i)); }
Fred Fred::create2(float x, float y)            { return Fred(new Der2(x, y)); }

Fred::Data* Fred::Der1::clone() const { return new Der1(*this); }
Fred::Data* Fred::Der2::clone() const { return new Der2(*this); }

Fred::Fred(Fred const& f)
  : data_(f.data_)
{
  ++data_->count_;
}

Fred& Fred::operator= (Fred const& f)
{
  // DO NOT CHANGE THE ORDER OF THESE STATEMENTS!
  // (This order properly handles self-assignment)
  // (This order also properly handles recursion, e.g., if a Fred::Data contains Freds)
  Data* const old = data_;
  data_ = f.data_;
  ++data_->count_;
  if (--old->count_ == 0) delete old;
  return *this;
}

Fred::~Fred()
{
  if (--data_->count_ == 0) delete data_;
}

void Fred::sampleInspectorMethod() const
{
  // This method promises ("const") not to change anything in *data_
  // Therefore we simply "pass the method through" to *data_:
  data_->sampleInspectorMethod();
}

void Fred::sampleMutatorMethod()
{
  // This method might need to change things in *data_
  // Thus it first checks if this is the only pointer to *data_
  if (data_->count_ > 1) {
    Data* d = data_->clone();   // The Virtual Constructor Idiom
    --data_->count_;
    data_ = d;
  }
  assert(data_->count_ == 1);

  // Now we "pass the method through" to *data_:
  data_->sampleMutatorMethod();
}
Naturally the constructors and sampleXXX methods for Fred::Der1 and Fred::Der2 will need to be implemented in whatever way is appropriate.