Is it well-defined/legal to placement-new multiple times at the same address?

Question

(Note: this question was motivated by trying to come up with preprocessor hackery to generate a no-op allocation to answer this other question:

Macro that accept new

Answer 1

No - this doesn't look right.

When you use placement new, the object will be constructed at the address you pass. In this example you're passing the same address (i.e. &buffer[0]) twice, so the second object is just obliterating the first object that's already been constructed at this location.

EDIT: I don't think I understand what you're trying to do.

If you have a general object type (that may have non-trivial ctor/dtor's that might allocate/deallocate resources) and you obliterate the first object by placement new'ing over the top of it without first explicitly calling it's destructor, this will at least be a memory leak.