Do you have any well defined proble/ segment to delegate?
Tarık Özkanlı
On Wed, Mar 21, 2012 at 4:27 PM, Yves Jaradin <yves.jara...@uclouvain.be>wrote:
> Le 20/03/12 20:44, Denys Duchier a écrit :
>
>> Yves Jaradin<yves.jaradin@**uclouvain.be <yves.jara...@uclouvain.be>>
>> writes:
>>
>> not sure what you mean here. could this have to do with the placement
>>>> argument to the new operator?
>>>>
>>>> new (ptr) X(v1,...,vn)
>>>>
>>>>
>>> That is related to it. Basically if you have an A* and want a B*, you
>>> have to
>>> find out the beginning of the memory block in which A is, give that to
>>> the
>>> placement new and hope that the numeric value of the pointer you get
>>> back is
>>> equal to the one you started with.
>>>
>>
>> you can overload new where appropriate:
>>
>> void* operator new(size_t,void* p) { return p; }
>>
>
> That is guaranteed to be the behaviour of placement new on non-arrays, no
> need to overload it.
>
>
>> Finding the beginning of the memory for the
>>> A object is implementation specific and error-prone if one wants to
>>> consider
>>> all cases including when the dynamic type of the object pointed to is a
>>> subclass of A.
>>>
>>
>> since we are talking about polymorphic classes, dynamic_cast<void*>(p)
>> is garanteed to return a pointer to the "complete" object containing *p
>> (you could even used a virtual base class to provide a method to return
>> that address). you can then pass that pointer as the placement argument
>> for new.
>>
>
> I did re-read the standard to be sure of where the problems are.
>
> The situation would be as follow:
> A class P (probably callled Node or something like that) with virtual
> methods and child classes A and B (corresponding to the two concrete types
> we are switching from and to).
>
> We start with a P* p that points to an object of actual (most-derived)
> class A. The way to do the conversion would be:
> dynamic_cast<P*>(new (dynamic_cast<void*>(p)) B(init_params))
> But there is no guarantee that the offset of the P subobject in A is the
> same as that of the P subobject in B [class.derived]§5, so the pointer
> returned by the expression may be different from p. Having P as the
> reference type and doing the conversion with
> new (reinterpret_cast<void*>(p)) P(init_params) isn't better because the
> layout of a base class subobject may be different from the layout of a most
> derived object of the same type [class.derived]§8.
>
>
>> Cheers,
>>
>> --Denys
>>
>
> Cheers,
> Yves
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