Re: Empty objects as members
On 11/1/2010 6:07 PM, Marcel M?ller wrote:
Victor Bazarov wrote:
But why must a pointer to a non polymorphic object be unique? They do
not support dynamic casts anyway. And why does this not apply to one
base class?
I probably don't understand the question, but why would
polimorphic-ness matter?...
Classes with at least one virtual function are guaranteed to support
RTTI. For this to work the relation between objects and pointers must be
bijective.
Data members in the same object and elements of the same
array wouldn't be distinguishable if pointers weren't unique.
In my opinion there is no need to have unique pointer for zero length
objects, since non polymorphic objects always must have a static type,
known at compile time.
Most probably I missed something, but I don't know what.
Most probably *I* didn't give it enough thought. What you're saying, I
think, is that if an object doesn't have a state and is not polymorphic
(IOW, doesn't have member data that can have [different] values or can
have a different dynamic type), there is no need to allocate storage for
it, right? To extend this reasoning, the member functions for such a
class don't have to have a unique 'this' pointer, since there is no
*need* to distinguish between objects (they are all the same), yes? The
objects of that type don't have identity, but the question to answer is,
"why would then need it?" The compiler then can optimize and reuse some
pointer for all 'this' pointers when calling their member functions,
yes? Now, imagine that the identity is not stored (state) but implied
in the behavior, like
int NoIdentityClass::foo(NoIdentityClass const& other) const
{
if (this == &other)
return 0;
else
return 42;
}
how would the compiler know not to optimize? I am not sure what the use
of such algorithm would be (so it could be just theoretical), but it's
quite real. I can write a program to have a limited number of some
stateless objects that would require to be unique, so I would need to
disallow the compiler to optimize the size to 0 for *that* particular
class. I am guessing that requiring the compiler to always allocate at
least one byte for those is both *inexpensive* and solves the identity
problem.
Anyway, interesting. Thanks for making me think about it.
V
--
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