Re: Constructing Derived in shell of Base <shudder>
On Jul 16, 7:26 pm, Joshua Maurice <joshuamaur...@gmail.com> wrote:
On Jul 16, 10:57 am, "Francesco S. Carta" <entul...@gmail.com> wrote:
[...]
I still believe it's not our concern. We are assured that with placement
new, only sizeof(type) contiguous "chars" are needed/used.
Not really. You are guaranteed that if you allocate
sizeof(type) contiguous c chars, the code will work.
Well, let me phrase the question like this. Is the following code
legal?
#include <new>
class foo
{
public:
virtual void bar() {}
};
int main()
{
foo * f = new foo;
foo * f2 = new (f) foo;
f2 -> bar();
}
I would never write code like this as a matter of style and
sanity, but I would still like to know as a matter of
understanding all of the standard and its nuances. Does that
phrase disallow this?
No. Other things might; I'd have to check. Suppose, for
example, and implementation dyanamically allocated a new copy of
the vtable for each instance of the variable. (I think this
would be legal.) I'm not sure about what happens when you reuse
the memory underlying an object with non-trivial destructors
without calling the destructor.
From James
Kanze's reply, that might not be the intent, nor how it's commonly
understood.
Either I misstated my understanding, or you misunderstood what I
said. The expression sizeof(Type) returns the number of bytes
necessary to "contain" an instance of the object. Whether all
of those bytes are part of the object or not is a separate
question. And I think (but I'm not sure) that the
implementation is allowed to allocate other memory in the
constructor, and free it in the destructor. (Consider the oft
sited alternative to a vptr---the compiler allocates an entry in
a map somewhere.)
[...]
Given an answer to the above question is 1 or 3, I have an additional
question: What motivated this allowance?
The motivation for any allowances here is to allow a maximum of
freedom to the implementation. In no case, however, is there
any intent of allowing certain common idioms to fail. (Or
mayby... Suppose you derive from an empty PODS. Can you still
memcpy to it?)
--
James Kanze