Factory Method
Klaus Friedel
d at friedelprivat.de
Wed Sep 19 13:48:06 PDT 2007
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 ?
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