Re: Constructing Derived in shell of Base <shudder>

Victor Bazarov <v.bazarov@comcast.invalid>
Tue, 13 Jul 2010 14:47:14 -0400
On 7/13/2010 2:23 PM, Francesco S. Carta wrote:

Stephen Howe <sjhoweATdialDOTpipexDOTcom>, on 13/07/2010 19:10:47, wrote:


For the code

struct Base { int j; virtual void f(); };
struct Derived : public Base { virtual void f(); };
void fooBar
Base b;
b.f(); // Base::f() invoked
new (&b) Derived; // placement new => as same size, construct Derived
in shell of Base
b.f(); // Which f() is invoked, Base or Derived?

1) Is it well-defined? if not, why not?
2) If it is well defined, which f() is invoked? I think it is the
Derived f() but a case could be made for Base.

Well, as it is, strictly speaking, it simply doesn't compile (things
like missing declarations of both f(), missing parentheses after fooBar,
missing main()).

Then, if you tried to make something compilable out of it you would have
discovered that only Base::f() gets called - after all, b is an instance
of Base (during both calls of f()) and your code doesn't transform it to
a Derived - you're simply messing with its storage and I think your code
leads to UB.

More experienced people hanging around will give you better insights.

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.
Since 'b' isn't a reference, but a plain object, the compiler creates
the call to B::f when you write 'b.f()'.

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

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

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