Using new (this) to reuse constructors

C++0x will introduce syntax to allow constructors to call other constructors.

Until then, new(this) works in some cases, but not all. In particular, once in the constructor, your base class(es) are already fully constructed. Reconstructing via new(this) re-calls the base constructors without calling the base destructors, so expect problems if the base classes weren't expecting this kind of hackery - and they probably weren't.

An example for clarity:

class Base
{
public:
   char *ptr;
   MyFile file;
   std::vector vect;
   Base()
   {
       ptr = new char[1000];
       file.open("some_file");
   }
   ~Base()
   {
       delete [] ptr;
       file.close();
   }
};

class Derived : Base
{
    Derived(Foo foo)
    {
    }
    Derived(Bar bar)
    {
       printf(ptr...);  // ptr in base is already valid
       new (this) Derived(bar.foo); // ptr re-allocated, original not deleted
       //Base.file opened twice, not closed
       // vect is who-knows-what
       // etc
    }
}

or as they say 'hilarity ensues'

The members and base classes will be initialised before entering the constructor body, then initialised again when you call the second constructor. In general this will lead to memory leaks and possibly undefined behaviour.

So the answer is "no, this is not sound code".