PRODUCT  :  Borland C++                           NUMBER  :  1014
  VERSION  :  3.0
       OS  :  ALL
     DATE  :  October 19, 1993                         PAGE  :  1/2

    TITLE  :  Casting a base pointer to a derived pointer.





  As pointed out in 'The C++ Programming Language' ( Bjarne
  Stroustrup - Second Edition ), "...A pointer to a Class B
  may be explicitly converted to a pointer to a Class D
  that has B has a direct or indirect base class if an
  unambiguous conversion from D to B exists and if B is not
  a virtual base class.   Such a case from a base to a
  derived class assumes that the object of the base class
  is a sub-object of an object of the derived class;  the
  resulting pointer points to the enclosing object of the
  derived class..."   It must be noted that the compiler
  does *not* and will *not* ensure that the subject pointer
  refers to an object suitably aligned in storage:  this is
  the programmer's responsibility!   In other words, the
  cast is only valid if the pointer was originally of the
  derived type.

  The following program shows a cast from a pointer to Object
  to a pointer to the derived class String.   The conversion
  operator ( const char * ) in String class is used to confirm
  the success of the cast.


  #include <iostream.h>
  #include <array.h>

  Array A(2);

  int main( void )
  {
      String *str1 = new String( "one" );
      String *str2 = new String( "two" );
      String *Sptr;
      A.add(*str1);
      A.add(*str2);
      Sptr = (String *)(&(A[1]));   // Cast (Object*) to String*
      cout << Sptr->operator const char *() << endl;
      return( 0 );
  }


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  subject to the terms of the No-Nonsense License Statement that













  PRODUCT  :  Borland C++                           NUMBER  :  1014
  VERSION  :  3.0
       OS  :  ALL
     DATE  :  October 19, 1993                         PAGE  :  2/2

    TITLE  :  Casting a base pointer to a derived pointer.




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