Re: [boost] Re: pick_of() proposal

From: Jon Kalb (jonkalb_at_[hidden])
Date: 2003-02-12 01:57:28

On 2003-02-11 8:47 PM, "Philippe A. Bouchard" <philippeb_at_[hidden]>
wrote:

> Jon Kalb wrote:
> 
> [...]
> 
>> If a user attempts to use this on a non-polymorphic object, will they
>> get a compile time error or run-time error?
> 
> No compile-time error, but a run-time error may occur if multiple
> inheritance is used.  There is basically nothing we can do in this
> situation:
> 
> class A { char c; };
> 
> class B { char c; };
> 
> class C : public A, public B { char c; };
> 
> 
> C * pC = new C;    // Let's say pC == 0x8000
> 
> B * pB = pC;        // pB == 0x8001
> 
> pick_of(pB) will return 0x8001.  It is similar to a "delete pB" attempt.

This is as I suspected.

I wonder if it wouldn't be better to have a compile time error in this
situation rather than to create a trap for the unsuspecting. It does put an
additional burden on the user to ensure that the types are indeed
polymorphic, but I think they have that burden in any case. The currently
implementation allows them to be blissfully ignorant of this situation.

I appreciate that your library will document that it only works for
polymorphic types, but I fear that to be an insufficient prophylactic.