Re: [Boost-users] Need a method to compare types of data objects at runtime

From: kauai (kauai_diver_at_[hidden])
Date: 2005-04-24 22:42:04

I ran into a similar problem.  If you read any recent C++
book the authors will tell you to limit your use of
dynamic_casts (or any casts for that matter) in favor of
virutal functions.  

I know you want your base class simple, but you can add
set_pressure() as virtual in your base and have it do
nothing, then just override in the class where it does
something.  You can also override in all sub classes and
simply throw() in classes where it doesnt apply to be
called.

--- Stephen Torri <storri_at_[hidden]> wrote:
> If I have a abstract class with two classes that
> implement it how can I
> make a check that tests for the type of the
> implementation classes?
> 
> For example:
> 
>         class Base {
>         public:
>         	virtual void run (){}
>         	virtual void stop (){}
>         };
>         
>         class Gas_Pedal : public Base {
>         public:
>         	virtual void run (){}
>         	void set_rate (unsigned int);
>         private:
>         	unsigned int m_rate;
>         };
>         
>         class Brake_Pedal : public Base {
>         public:
>         	virtual void stop (){}
>         	void set_pressure (unsigned int);
>         private:
>         	unsigned int m_pressure;
>         };
> 
> When I have a client that receives a Base object I want
> to be able to
> check at run-time. So that if I am expecting a Gas_Pedal
> object and I
> receive something else I can perform some recovery
> action. The software
> I am designing contains an untold number of arrangements
> of items. Each
> item expect a distinctive input type. If it receives
> something that it
> does not know how to handle or need I want to throw an
> error.
> 
> In order to do this I would like to do something like:
>         
>         void process (Base* sobj)
>         {
>         	if (! (isExpectedType(sobj))
>         	{
>         		throw NotForMeException ();
>         	}
>         
>         	Brake_Pedal* bp =
> dynamic_cast<Brake_Pedal>(sobj);
>         	bp.set_pressure (50);
>         }
> 
> So perhaps my design is flawed that I am running into
> this situation. I
> am attempting to keep the interface for the Base class
> simple. The
> underlying types could have different data and access
> methods because
> they differ so widely in what they will do. This example
> was given to
> keep things simple.
> 
> Stephen
> 
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
> 

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