Factory Method

[Bill Baxter]

Kyle Furlong wrote:
> Klaus Friedel wrote:
>> 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.
> 
> How many times are we going to have threads on this, by my count this is 
> the third time in the last few months this ambiguity has confused 
> someone. Just fix the damn thing already.


I was aware of the previous mentions of this problem in the docs. But 
Walter's usually good about making simple doc fixes as long as there's 
an actual bug filed for it.  And now there is, thanks to Klaus.

--bb