In the first case, there's no point in using placement new, since int
doesn't have a constructor.
In the second case, it's either pointless (if myClass
is trivial) or wrong, since there are already objects in the array.
You use placement new to initialise an object in a block of memory, which must be suitably aligned, and mustn't already contain a (non-trivial) object.
char memory[enough_bytes]; // WARNING: may not be properly aligned.
myClass * c = new (memory) myClass;
Once you've finished with it, you need to destroy the object by calling its destructor:
c->~myClass();
This separates the object's lifetime from that of its memory. You might also have to release the memory at some point, depending on how you allocated it; in this case, it's an automatic array, so it's automatically released when it goes out of scope.