How to call a constructor on an already allocated memory?

Question

How can I call a constructor on a memory region that is already allocated?

Answer 1

The placement new constructor mentioned by the accepted answer is an old way before the allocator class defined in header. Now you really should do(in C++11 style):

allocator alloc;
//Allocate memory for one or n objects
auto p = alloc.allocate(1); 
//Construct an object of Foo on allocated memory block p, by calling one of Foo's constructors
alloc.construct(p, args, ...); 

//OK, p now points to a Foo object ready for use...

//Call Foo's destructor but don't release memory of p
alloc.destroy(p); 
//Release memory
alloc.deallocate(p, 1); 

That's it.