Re: [boost] Re: Policy or Trait?

From: E. Gladyshev (eegg_at_[hidden])
Date: 2004-04-01 22:34:06

From: "Hurd, Matthew" <hurdm_at_[hidden]>

[...]
> It would be nice if something new like "aspect class" or some such could

Interesting name!  I second your opinion in many ways.

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;

Eugene