Re: Constructing Derived in shell of Base <shudder>
On 2010-07-15 16:17:44 -0400, Kai-Uwe Bux said:
cpp4ever wrote:
[...]
may I suggest the following
//---------------------
struct B { int j; virtual void f() { std::cout << "B::f\n"; } virtual
~B() {} };
struct D : B { virtual void f() { std::cout << "D::f\n";} };
int main()
{
char obj[sizeof(D)];
B *p = new (obj) B;
p->f(); // B::f() involked
p->~B();
B *p = new (obj) D;
Probably, the above was meant to be:
p = new (obj) D;
Nice idea: assigning a new value to p circumvents [3.8/7] as no old pointer
to corps is in continued use.
Well, sure. That's the essence of what every memory manager does.
p->f(); // D::f() involked
p->~B(); // D Destructor will be called
}
//---------------------
This still requires care, as before creating an object with new, any
previously created object should be destroyed. Worse still the object
will not automatically be destroyed when it goes out of scope. Therefore
IMHO this sort of functionality should be encapsulated within a class
that ensures things are handled safely.
Just a question: does the standard guarantee that obj is correctly alligned
for objects of type D and B?
No. It's a char array, with alignment suitable for a char array.
In C++0x you'll get aligned storage with the aligned_storage template:
aligned_storage<sizeof(D), alignof(D)>::type obj;
or with attributes:
unsigned char obj [[ align(alignof(D)) ]] [sizeof(D)];
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)