TITLE: template-based clone function

(Source: comp.lang.c++.moderated, 13 Mar 2001

TROM:

> class One
> {
>    public:
>         virtual void Function(void)
>         {
>                 cout<<"Function One"<<endl;
>         }
>
>         virtual One *Create(void)
>         {
>                 return new One;
>         }
> };//class One
>
> class Two : public One
> {
>    public:
>         virtual void Function(void)
>         {
>                 cout<<"Function Two"<<endl;
>         }
> };//class Two
>
[...]
>
> I'd like to use a 'Create' function in the base class (class One) that
> I could use with all the derived classes (like class Two). Is possible
> without I must to repeat the same code in all the classes??
>
> The solution I don't like is:
>
> class Two : public One
> {
>    public:
>         virtual void Function(void)
>         {
>                 cout<<"Function Two"<<endl;
>         }
>
>         virtual One *Create(void)
>         {
>                 return new Two;
>         }
> };//class Two


MENZL: "Gerhard Menzl" <gerhard.menzl@sea.ericsson.se>

So what you want is a polymorphical function that default-constructs
another instance of the derived class your base class pointer is
pointing to, but you don't want to retype "return new <derived class
name>" in every derived class, right? An unusual design - are you sure
you don't want to clone the orginal? -  but it can easily be achieved
anyway using Jim Coplien's Curiously recurring template pattern:

   class Base
   {
   public:
      virtual Base* Create () = 0;   // keep non-leaf classes abstract
   };

   template <typename T> class Creator : public Base
   {
   public:
      virtual Base* Create () { return new T; }
   };

   class Derived : public Creator<Derived>
   {
      //...
   };

This effectively creates an intermediary class between Base and every
derived class that performs the creation work. That is, you still need
separate creation code for every derived class, but you only have to
type it once, and the compiler will generate all those repetitive
new-expressions for you. Is that what you were looking for?


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