How does placement new know which layout to create?

Question

#include 
#include 
struct A { int a; };
struct B : virtual A { int b; };
struct C : virtual A  { int c; };
struct D : B,C { int d; };
in         


        

Answer 1

There is no option to create what you call a "separate" layout, other than creating a derived type, and fishing B out of it.

"Layout of B as portion of its derived class" is not the same as "Layout of B". Placement new and regular new use the layout based on the type itself, with is the default, stand-alone layout.

Where is the guarantee that the layout created by new fits into a buffer of size sizeof(B)?

sizeof(B) returns the size of B itself, not B-as-part-of-some-other-class. That is all the space needed to store a stand-alone B, regardless of the way that you allocate memory for it.