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

From: Wu Yinghui, Freddie (yhwu_at_[hidden])
Date: 2006-10-26 10:08:38

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> Wu Yinghui, Freddie wrote:
> 
> [snip]
> 
>>
>> 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).
> 
> In order to know which parameters to send through the interface, you'll
> have to know the exact subclass? Sorry, that doesn't make sense.
> 
> What's the real problem you are trying to solve - could Boost.Variant +
> static_visitor (or some other variant on the Visitor pattern) help?
> 
> / Johan

My situation is like this:

I have a class hierarchy briefly shown as below (assuming virtual member
function template is allowed, just to demonstrate the idea):
===== CODE =====
class Base
{
   //...

   // This is the function that makes use of Boost.Parameter
   template <typename ArgumentPack>
   virtual void Configure(ArgumentPack const &arguments) = 0;
};

// This class resides in Derived1.dll
class Derived1 : public Base
{
   template <typename ArgumentPack>
   void Configure(ArgumentPack const &arguments)
   {
      //...
   }
};

// This class resides in Derived2.dll
class Derived2 : public Base
{
   template <typename ArgumentPack>
   void Configure(ArgumentPack const &arguments)
   {
      //...
   }
};
===== CODE =====

To use the class hierarchy, my program loads the DLL requested by user
using LoadLibrary() (We're using Windows, but it should be similar to
Linux, I suppose), and makes use of the Derived1/Derived2 classes via a
pointer to the Base class.

Since I have no information about the derived classes when building my
main executable (only Base class is known then), I cannot make direct
use of Derived1/Derived2 types in my main program. -- Yet, I need a way
to provide some arbitrary set of arguments to the derived classes.

Hope my explanation clears your doubts. Do you have any suggestion or
alternative design in mind?

Cheers,

Freddie

- --
Wu Yinghui, Freddie
Research & Development
Senior Software Engineer

Volume Interactions Pte Ltd
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