Re: Constructing Derived in shell of Base <shudder>

Victor Bazarov <>
Tue, 13 Jul 2010 18:59:30 -0400
On 7/13/2010 4:35 PM, Kenneth 'Bessarion' Boyd wrote:

On Jul 13, 1:47 pm, Victor Bazarov<v.baza...@comcast.invalid> wrote:

Well, "messing with its storage" should in this case be enough because
the expression 'new (&b) Derived' is supposed to *construct* the Derived
object in the memory that 'b' occupies. The act of construction of that
object *should* include setting up the virtual function resolution
mechanism properly for the call to 'b.f()' *if* 'b' were a reference. ....


Here is the fixed program, which works "as the OP expected":

This...I'm not sure sure it works as expected, or merely works by
common empirical facts about implementations.


struct B { int j; virtual void f() { std::cout<< "B::f\n"; } };
struct D : B { virtual void f() { std::cout<< "D::f\n";} };

int main()
     B b;
     B& rb = b;
     rb.f(); // B::f() invoked

       /* undefined behavior when sizeof(B)<sizeof(D)

'When' is the key word. The OP started with the premise that the sized
*are* the same. So, no UB.

        * we are also gambling that the memory requirement indicated
        * by sizeof is actually contiguous, but I haven't even seen
        * for such exotic hardware so don't worry about that.

Uh... Objects are defined to occupy contiguous storage. What specs for
what exotic hardware?

        * Cause a compile error for this; C++0X would use
static_assert. */


       typedef B_D_same_size[sizeof(B)>=sizeof(D) ? 1 : -1];

     new (&b) D; // create D in shell of B
     rb.f(); // D::f() invoked

       // ~B() invoked at end of scope on object intended to be of type
D: do we get lucky?

The destructor of the D object created in 'B' by means of placement new,
has to be directly invoked (using the same syntax). And the second
invocation of B::~B has to be prevented since it was invoked already


How about the following?

int main()
     unsigned char buffer[sizeof(B)>=sizeof(D) ? sizeof(B) :
     B* b = new (buffer) B;
     b->f(); // B::f() invoked
     b = new (buffer) D;
     b->f(); // D::f() invoked

That's fine. Not the original question, but fine.


[The main context where I have considered using such gymnastics, is
reimplementing the STL. There's some a priori sense in being able to
make the node-type backing classes be derived from raw structs that
actually can use C memory management.]

Why were you considering reimplementing STL? Exercise?

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