[boost] Re: suggestion for boost::any

From: Itay (itay_maman_at_[hidden])
Date: 2002-05-03 14:54:43

You can modify your test() function such that it will work with boost::any
as-is:

void test(any thing)
{
    A** ppa = any_cast<A*>(&thing);
    if (ppa)
      (*ppa)->do_something();
}

(I didn't test this code, so no guarantee is given, but I believe it should
work as expected)

-Itay.

"Darren Kessner" <dkessner_at_[hidden]> wrote in message
news:20020503180442.99383.qmail_at_web11503.mail.yahoo.com...
> Hello,
>
> I've just recently discovered Boost, so please excuse
> me if this topic has been discussed already.  I have a
> suggestion to make usage of boost::any for pointers
> more natural.
>
> My natural inclination is to use boost::any to hold
> different types of pointers.  Then, I'd like to check
> the type of the pointer at run-time with behavior
> similar to dynamic_cast:
>
>   void test(any thing)
>   {
>     A* pa = any_cast<A*>(thing);
>     if (pa)
>       pa->doSomething();
>   }
>
> Unfortunately, any_cast throws bad_any_cast if thing
> is not an A*.  In this way, the behavior of any_cast
> is similar to that of dynamic_cast, since thing is
> being passed by reference.  But the behavior is
> dissimilar in that I (may) know that thing is really
> holding a pointer, so any_cast should return 0 instead
> of throwing.
>
> My suggestion is to add an any_ptr class that holds a
> pointer to something:
>
>   class any_ptr
>   {
>     public:
>
>     template<typename T>
>     any_ptr(T* p)
>     {
>       thing_ = p;
>     }
>
>     any thing_; // public for illustration
>   };
>
> Then any_cast can be overloaded to behave like
> dynamic_cast when passed an any_ptr:
>
>   template<typename ValueTypePtr>
>   ValueTypePtr any_cast(const any_ptr& operand)
>   {
>     ValueTypePtr p = 0;
>
>     try {
>       p = any_cast<ValueTypePtr>(operand.thing_);
>     } catch (bad_any_cast){
>       p = 0;
>     }
>
>     return p;
>   }
>
> This works ok for me in simple tests, but I haven't
> looked at what issues there may be in detail.
>
>
> Darren
>
>
>
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