Re: [Boost-users] [Boost.Parameter] How to Use Named Parameters in a Class Hierarchy?

From: Wu Yinghui, Freddie (yhwu_at_[hidden])
Date: 2006-10-25 22:02:35

> Wu Yinghui, Freddie wrote:
>> Dear all,
>>
>> I'm implementing a hierarchy of classes to handle different types of
>> objects. The characteristics of these objects are mostly similar (so I
>> can put them into a container that knows only about the pure abstract
>> base interface type), but some properties are specific to each object
>> types.
>>
>> I plan to use Boost.Parameter for its capability of providing named
>> parameter to these object types. However, I have one problem in this
>> approach: Since Boost.Parameter requires that the function processing
>> the ArgumentPack to be a template, I cannot make it virtual (a member
>> function template cannot be virtual) in my pure abstract base
>> interface. Furthermore, due to the lack of "virtual" member function,
>> I cannot use
>> the base class pointer to provide these named parameters to some
>> subclasses.
> 
> Depends on what you actually want to do. Perhaps you could use something
> like the template pattern (pseudo-code):
> 
> -----
> struct Parameters
>  : boost::parameter::parameters< ... >
> {};
> 
> struct Base
> {
>    BOOST_PARAMETER_MEMFUN(void, Work, 1, 4, Parameters)
>    {
>        DoWork( p[arg1], p[arg2|0], p[arg3|1] ... );
>    }
> 
> private:
>    virtual void DoWork(actual, args, goes, here) = 0;
> };
> 
> class Derived : public Base
> {
>    virtual void DoWork(actual, args, goes, here)
>    {
>        ... use args ...
>    }
> };
> 
> class Derived2 : public Base
> {
>    virtual void DoWork(actual, args, goes, here)
>    {
>        ... use args ...
>    }
> };
> 
> -----
> 
> I've used something similar myself, hope that gives an idea on how to solve
> your problem.
> 
> // Johan

Thanks for your quick reply, Johan.

I actually thought of a similar solution. I guess I didn't explain my
problem clear enough... Unfortunately, this solution doesn't seem to
suit my need.

The situation is like this: In the pseudo-code you provided, DoWork()
function is declared in the Base class, and is overridden by Derived1
and Derived2, with a fixed number of arguments. My problems is that, my
base class cannot predict the exact number of parameters that a derived
class may have. (A derived class may have 10 parameters, while another
may have none.) So it looks a little bit out-of-shape in terms of the OO
programming style I'm familiar with if I'm going to provide overloads of
DoWork() for all the possible parameters used by all derived classes
(not to mention the undesirable dependency introduced).

Thanks again for your comments! Any further thoughts? :)

Cheers,

Freddie

-- 
Wu Yinghui, Freddie
Research & Development
Senior Software Engineer
Volume Interactions Pte Ltd
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