Placement new allows you to construct an object in memory that’s already allocated.
You may want to do this for optimization when you need to construct multiple instances of an object, and it is faster not to re-allocate memory each time you need a new instance. Instead, it might be more efficient to perform a single allocation for a chunk of memory that can hold multiple objects, even though you don’t want to use all of it at once.
DevX gives a good example:
Standard C++ also supports placement
new operator, which constructs an
object on a pre-allocated buffer. This
is useful when building a memory pool,
a garbage collector or simply when
performance and exception safety are
paramount (there’s no danger of
allocation failure since the memory
has already been allocated, and
constructing an object on a
pre-allocated buffer takes less time):
char *buf = new char[sizeof(string)]; // pre-allocated buffer string *p = new (buf) string("hi"); // placement new string *q = new string("hi"); // ordinary heap allocation
You may also want to be sure there can be no allocation failure at a certain part of critical code (for instance, in code executed by a pacemaker). In that case you would want to allocate memory earlier, then use placement new within the critical section.
Deallocation in placement new
You should not deallocate every object that is using the memory buffer. Instead you should delete only the original buffer. You would have to then call the destructors of your classes manually. For a good suggestion on this, please see Stroustrup’s FAQ on: Is there a “placement delete”?