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

foo* p1 = new (buffer) foo(1);
foo* p2 = new (buffer) foo(2);
p1->~foo();
p2->~foo();

You are destructing the same object twice, and that alone is undefined behavior. Your implementation may decide to order a pizza when you do that, and it would still be on the right side of the spec.

Then there is the fact that your buffer may not be properly aligned to emplace an object of type foo, which is again non standard C++ (according to C++03, I think C++11 relaxes this).

Update: Regarding the question specified in the title,

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

Yes, is it well-defined to placement-new multiple times at the same address, provided that it points to raw memory.