Placement New Syntax

B. looks closest to correct.  I would use:

SomeClass* ptr = new(pmem)SomeClass();

It is B , provided pmem is an allocated pointer (meaning memory already allocated to it).

See this link for more explanation: http://www.geekinterview.com/question_details/16559

E. new (pmem, SomeClass);

B) new(pmem) SomeClass;

It is
B. new(pmem) SomeClass;

pmem points to the memory which shall be used.

B

Where you use placement new, it is important to explicitly call the destructor for the object before re-using the memory.

Fred * fred = new(pmem) Fred();
...
fred->~Fred();
fred = 0;

George * george(pmem) George();