Re: Constructing Derived in shell of Base <shudder>

From:
"Kenneth 'Bessarion' Boyd" <zaimoni@zaimoni.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 13 Jul 2010 13:35:30 -0700 (PDT)
Message-ID:
<1a854086-0f69-4256-868b-9ca601a4c534@j4g2000yqh.googlegroups.com>
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 tha=

t

object *should* include setting up the virtual function resolution
mechanism properly for the call to 'b.f()' *if* 'b' were a reference. ...=

..

Right.

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.

#include <iostream>
#include <memory>

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
    b.~B();

      /* undefined behavior when sizeof(B)<sizeof(D)
       * we are also gambling that the memory requirement indicated
       * by sizeof is actually contiguous, but I haven't even seen
specs
       * for such exotic hardware so don't worry about that.

       * 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?

}


How about the following?

int main()
{
    unsigned char buffer[sizeof(B)>=sizeof(D) ? sizeof(B) :
sizeof(D)];
    B* b = new (buffer) B;
    b->f(); // B::f() invoked
    b->~B();
    b = new (buffer) D;
    b->f(); // D::f() invoked
    static_cast<D*>(b)->~D();
}

[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.]

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