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From: Hurd, Matthew (hurdm_at_[hidden])
Date: 2004-04-02 00:32:42

> On Behalf Of E. Gladyshev
> Sent: Friday, 2 April 2004 1:34 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] Re: Policy or Trait?
> From: "Hurd, Matthew" <hurdm_at_[hidden]>
> [...]
> > It would be nice if something new like "aspect class" or some such
> Interesting name! I second your opinion in many ways.

Would be nice to stop the confusion / confounding by describing somewhat
rigorously a classification / nomenclature for "aspect classes".

Constant aspects (bad name?, plain? simpleton?)
  - static constants and types only (bad name)

Static aspect
  - constant aspect extended to include static methods

Base aspect
  - intended to be inherited from by a client aka common
    MCD policy class - policy aspect perhaps?

Chain aspect
  - not sure how to describe this

Recurring base aspect
  - a base aspect that requires the first parameter to be the
    client class, this is the CRTP.

Generic "blah" aspect
  - "blah" parameterised by additional template parameters beyond the
     e.g. generic recurring base aspect is the CRTP with additional

A generic base aspect class would describe much of the MCD style policy

A static aspect could be used in a base aspect like manner or without
the inheritance.

"Aspect" might be inappropriate as I'm not 100% on my AOP nomenclature,
but it feels appropriate.

A trait or policy could be classified in such terms which might avoid
some confusion.

Needs some work, but it might start something...

> I don't think that differentiating between traits and policy
> makes too much sense or really necessary.
> As for now to me, they both are parts of the same concept.
> The idea is that a class can define an API
> specification. The user is responsible
> for implementing the API and supplying the implementation
> to the class as a template parameter.
> Traditionally an API is not a set of just behaviors
> or just data structures. It is a combination of both
> and it doesn't have to be stateless.
> So what's so unique about API's from the OOP standpoint
> especially. Well, an API as such cannot be
> INSTANTIATED. It just EXISTs as a set of rules, etc.
> It is like namespaces in C++.
> You cannot instantiate a namespace!
> You can instantiate data types defined by the API though.
> So to me, both traits and policies are sort of
> interchangeable namespaces.
> Perhaps my view is too simplistic.
> I sometimes do something like this
> template< typename MatrixApi >
> struct equation_system_solver {...};
> struct api
> {
> private: //** note private here
> api();
> };
> struct my_vector_api : api {...};
> template< typename VectrorApi >
> struct my_matrix_api : api {...};
> equation_system_solver< my_matrix_api<my_vector_api> > s;


Especially handy I'd imagine if you use the CRTP with the api to give
yourself some static polymorphism.

Another pattern trait / policy style pattern I've used recently is
having integer offsets and types associated with names in a type that
represents a packet type, these "aspect classes" are combined with a
packet type in a client to handle different financial exchanges in my

This would be a "constant aspect" in my nomenclature above.

It is a trait of a concept rather being a trait of a specific type or

Gets interesting when one of those types in the constant aspect is a
static aspect that describes operations on the type to be found at the
offset indicated.

The packet aspect is really nothing much more than a bunch integers and
associated types and it feels like a trait (it is a trait of a concept);
it is used like a policy.



Matt Hurd

Susquehanna Pacific P/L

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