Re: Run-time creation of an object of a dynamically determined class

From:

cbarron3@ix.netcom.com (Carl Barron)

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 12 Nov 2007 02:42:32 CST

Message-ID:

<1i7ft3x.aau3p3o86sq6N%cbarron3@ix.netcom.com>

Kodi Arfer <withheld@domain.tld> wrote:

Suppose I've got a base-class pointer which is pointing to an object
that belongs to any of a number of derived classes. (Please excuse my
brace style from Saturn.)

   struct Barney
    {Barney() {};
     virtual void greeting() {cout << "Barney says hello.\n";};};

   struct Dave : public Barney
     {Dave() {};
      virtual void greeting() {cout << "Dave says hello.\n";};};

   struct Dan : public Barney
     {Dan() {};
      virtual void greeting() {cout << "Dan says hello.\n";};};

   void funkshun()
   {Barney *bp1 = &ADaveOrADan;
    Barney *bp2;
    ...}

Now, suppose I want to make bp2 point to a new, dynamically allocated
object; not just any object, but one of the same class as *bp1.

  The simplest solution is a virtual 'factory' function in the base class
that must be defined in each Derived class to the actual creation
its a one liner but needs definition, such as [not tested]:

   struct Barney
   {
      virtual void greeting();
      virtual Barney *create()=0; // pure virtual function
      virtual ~Barney(){} // virt. dtor so Derived dtor is used to
            // destruct Barney *'s.
   };

   template <class Derived>
   struct BarneyCreate:public Barney
   {
      Barney *create() { return new Derived;}
   };

   struct Dave:public BarneyCreate<Dave>
   {
      void greeting();
   };

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