Re: [boost] Re: placement 'new' returns pointers with undefinedbehavior

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-11 17:09:27

--- "David B. Held" <dheld_at_[hidden]> wrote:
[...]
> This example is in the standard?  Unbelievable.
> 
> > [Example:
> > struct B {
> >   virtual void f();
> >   void mutate();
> >   virtual ~B();
> > };
> > struct D1 : B { void f(); };
> > struct D2 : B { void f(); };
> > void B::mutate() {
> >   new (this) D2; // reuses storage -- ends the lifetime of *this
> > [...]
> 
> You mean to tell me that this exact example is in the
> standard?  Where?  First, where does ~B() get called?
> Second, how can you be sure in general that *this is the right
> size to hold a D2?
> 

Yes, it is in 3.8/5

  11) For example, before the construction of a global object of non-POD
  class type (_class.cdtor_).

    (_expr.dynamic.cast_).  [Example:
      struct B {
              virtual void f();
              void mutate();
              virtual ~B();
      };

      struct D1 : B { void f(); };
      struct D2 : B { void f(); };
      void B::mutate() {
              new (this) D2;          // reuses storage - ends the lifetime of *this
              f();                    // undefined behavior
              ... = this;             // OK, this points to valid memory
      }
      void g() {
              void* p = malloc(sizeof(D1) + sizeof(D2));
              B* pb = new (p) D1;
              pb->mutate();
              &pb;                    // OK: pb points to valid memory
              void* q = pb;           // OK: pb points to valid memory
              pb->f();                // undefined behavior, lifetime of *pb has ended
      }
     --end example]

The whole "reusing storage - object lifetime" thing is unbelievable. :)

Eugene

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