DESCRIPTION

Here is a convenient way to implement a true copy semantic, by using shallow copies and reference counting, in order to minimise memory copies. This concept is generally related to the smart pointer method for managing memory.

The true semantic copy is defined as follows: if an object A is assigned to B, such as A = B, every further modification on A or B does not modify the other.

Notice that this class differs from the boost::shared_ptr class that implements safe pointers without the true copy semantic.

CLONE VARIANT

The smart_pointer_clone variant uses a T* T::clone() const member function instead of the usual T::T() copy constructor for obtaining a true copy of the data. This variant is motivated as follows: when using hierarchies of derived classes (also known as polymorphic classes), the usual copy is not possible because c++ copy constructors cannot be virtual, so you cannot make a copy this way. This is a well-known problem with C++'s implementation of polymorphism.

We uses a solution to the non-virtual copy constructor problem which is suggested by Ellis and Stroustrup in "The Annotated LRM". The solution is to require the T class to provide a virtual clone method for every class which makes a copy using new and the correct copy constructor, returning the result as a pointer to the superclass T. Each subclass of T overloads this function with its own variant which copies its own type. Thus the copy operation is now virtual and furthermore is localised to the individual subclass.

NOCOPY VARIANT

This variant of the smart pointer is designed for use on objects that cannot (or must not) be copied. An example would be when managing an object that contains, say, a file handle. It is essential that this not be copied because then you get the problem of deciding which copy is responsible for closing the file. To avoid the problem, wrap the file handle in a class and then manage a unique instance of it using a smart_pointer_nocopy. This ensures that the file handle cannot be copied and is closed when the last alias is destroyed.

The interface to the nocopy variant is the same as smart_pointer but with all operations that perform copying forbidden. In fact, because all three variants are instances of a common superclass, the forbidden methods do exist but will cause an error and exit if they are called.

The following modifiers cannot be used because they use copying of the pointed-to object and will thereore cause an error:

    T* operator-> ();
    T& operator* ();
    T* pointer ();
    T& data ();

EFERENCES

 [1] A. Geron and F. Tawbi,
    Pour mieux developper avec C++ : design pattern, STL, RTTI et smart pointers,
    InterEditions, 1999. Page 118.
 [2] STLplus, http://stlplus.sourceforge.net/stlplus3/docs/smart_ptr.html
    for the clone and nocopy variants.

IMPLEMENTATION

template <class T, class C>
class smart_pointer_base {
public:
// allocators:
  smart_pointer_base (T* p = 0);
  smart_pointer_base (const smart_pointer_base<T,C>&);
  smart_pointer_base<T,C>& operator= (const smart_pointer_base<T,C>&);
  ~smart_pointer_base ();
// accessors:
  const T* pointer    () const;
  const T& data       () const;
  const T* operator-> () const;
  const T& operator*  () const;
// modifiers:
  T* pointer    ();
  T& data       ();
  T* operator-> ();
  T& operator*  ();
// implementation:
private:
  struct counter {
    T*  _p;
    int _n;
    counter (T* p = 0);
    ~counter ();
    int operator++ ();
    int operator-- ();
  };
  counter *_count;
#ifndef TO_CLEAN
public:
  int reference_counter() const { return _count != 0 ? _count->_n : -1; }
#endif // TO_CLEAN
};