Factory Method

[Klaus Friedel]

Bill Baxter Wrote:

> Klaus Friedel wrote:
> > Nathan Reed Wrote:
> > 
> >> Klaus Friedel wrote:
> >>> Im sure I missed something. I tried to create a factory class producing objects on the heap like I would in C++ or Java:
> >>>
> >>> //**************************************
> >>> class Cat{
> >>>   int id;
> >>> }
> >>>
> >>> class CatFactory{
> >>>   Cat * createCat(){
> >>>     Cat aCat = new Cat();
> >>>     return &aCat;
> >>>   }
> >>> }
> >>>
> >>> void test(){
> >>>   CatFactory factory = new CatFactory();
> >>>   Cat *c1 = factory.create();
> >>>   Cat *c2 = factory.create();
> >>> }
> >>> //**************************************************************
> >>>
> >>> I discoverd this would not work because aCat will be allocated on the stack instead the heap.
> >>> Why ???? Looks anything but intuitive to me.
> >>> What would be the correct way to create a object on the GC controlled heap an return a reference to that object ?
> >> In D, classes are reference types.  This means when you declare a 
> >> variable like "Cat aCat", that is really a reference to a Cat, and is 
> >> allocated on the heap, not the stack.  So, the correct way to do this 
> >> would be:
> >>
> >> class CatFactor {
> >>    Cat createCat () {
> >>      return new Cat;
> >>    }
> >> }
> >>
> >> The function allocates a Cat and returns a reference to it.  Pointers 
> >> are not necessary.
> >>
> >> Thanks,
> >> Nathan Reed
> > 
> > Thanks for the fast reply. I was sure I missed something ;-)
> > What really confused me was the following statement under "Memory Management" http://www.digitalmars.com/d/memory.html#stackclass :
> > 
> >>>> Class instances are normally allocated on the garbage collected heap. However, if they:
> >>>>
> >>>>    * are allocated as local symbols in a function
> >>>>    * are allocated using new
> >>>>    * use new with no arguments
> >>>>    * have the scope storage class
> >>>>
> >>>> then they are allocated on the stack. This is more efficient than doing an allocate/free >>>cycle on the instance. But be careful that any reference to the object does not survive the return of the function.
> > 
> > I thought of the "*" as "or" but after rereading I now understand it ment "and".
> > Maybe somebody should edit this chapter to prevent others from making the same mistake ?
> > 
> 
> Yeh, that is ambiguous.
> Care to file a doc bug on it?
> 
> I think inserting one 'and' is all that's needed:
> """
> * are allocated as local symbols in a function
> * are allocated using new
> * use new with no arguments
> * and have the scope storage class
> """
> 
> Bugzilla is here:
> http://d.puremagic.com/issues/
> 
> --bb

New Bugzilla issue 1521 created.