Re: AbstractFactory Implementation in ModernC++ Design

Gil <>
Wed, 16 Feb 2011 14:31:39 -0800 (PST)
On Feb 16, 5:25 am, Jan <> wrote:

Hi Folks,

I was reading Andrei Alexandrescu's Modern C++ Design and I decided to
get my hands dirty with the code.

I tried implementing parts of his code and tried to put up a simple

I ran into some compile issues.

My main.cpp is here
My AbstractFactoryClass header file is here
My TypesCollection header file is here

I get the following error when I try to compile this code:

main.cpp: In function 'main':
main.cpp:59: error: expected primary-expression before '>' token
main.cpp:49: error: expected primary-expression before ')' token
main.cpp:50: error: expected primary-expression before '>' token
main.cpp:50: error: expected primary-expression before ')' token

I tried fixing this by changing Line 26 in AbstractFactoryClass header

from : return unit.Create(TypeMapping<T>());
to : return unit.template Create(TypeMapping<T>());

but that does not fix this error either and I got the same errors

I would be grateful if someone can help me with this error.

On the same note, I would like to add that I get the same error when I
try to use Alexandrescu's Loki library. So maybe my understanding
itself is wrong. Kindly help me learn.


the 3rd template parameter of your AbstractFactoryImpl already defines
the list of concrete products.
that is, it ultimately instatiates things like:

class ProductCreatorUsingNew< Derived1, boiler_plate_type >
  : public boiler_plate_type {
  //... type list boiler plate
    Derived1* Create(TypeMapping<Base1>)
      return new Derived1;

so when you do then

Base1* ptr1 = factory.Create<Base1>();

what you actually get is a Derived1 object address, as *intended*.

so you define the abstract products with the abstract factory type (1)
and you define the concrete products when you define factory
implementation type (2):

//I want to create objects derived from Base1, Base2 etc
(1) typedef AbstractFactory<
      TYPECOLLECTION_2(Base1, Base2)
    > AbstractBaseFactory;

//with their concrete type Derived1, Derived2 etc by using
//this creator ProductCreatorUsingNew
(2) typedef AbstractFactoryImpl<
      TYPECOLLECTION_2(Derived1, Derived2)
    > DerivedFactory1;

additionally, in your scenario, you cannot call

Base1* ptr1 = factory.Create<Derived1>();

because AbstractFactory::Create needs the base (abstract in your
terminology) type to find the right "Unit creator" and dispatch it to
the right Create overloaded method.

I don't have Alexandrescu's book but I looked at loki-lib sources on
sourceforge and I suspect this is a generic implementation of a an
abstract factory pattern.

if so you may want to check a more simple (and reasonable?) approach:

note: you needn't add template prefix on AbstractFactory::Create call,
that method is not a template method.


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