xenial (7) smart_pointer.7rheolef.gz

NAME

       smart_pointer, smart_pointer_clone - reference counted safe pointer with true copy semantic

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
       };