Is it legal to construct data members of a struct separately?

Question

class A;
class B;
//we have void *p pointing to enough free memory initially
std::pair<A,B> *pp=static_cast<std::pair<A,B> *>(p);
new (&pp->first) A(/*...*/);
new (&pp->second) B(/*...*/);

After the code above get executed, is *pp guaranteed to be in a valid state? I know the answer is true for every compiler I have tested, but the question is whether this is legal according to the standard and hence. In addition, is there any other way to obtain such a pair if A or B is not movable in C++98/03? (thanks to @StoryTeller-UnslanderMonica , there is a piecewise constructor for std::pair since C++11)

Answer 1

“Accessing” the members of the non-existent pair object is undefined behavior per [basic.life]/5; pair is never a POD-class (having user-declared constructors), so a pointer to its storage out of lifetime may not be used for its members. It’s not clear whether forming a pointer to the member is already undefined, or if the new is.

Neither is there a way to construct a pair of non-copyable (not movable, of course) types in C++98—that’s why the piecewise constructor was added along with move semantics.