at are for storing
heap-allocated or <a class="reference" href="reference.html#the-clonable-concept">cloned</a> objects (or in case of a map, the mapped object must be
a heap-allocated or cloned object). For each of the standard
containers there is a pointer container equivalent that takes ownership of
the objects in an exception safe manner.  In this respect the library is intended
to solve the so-called
<a class="reference" href="faq.html#what-is-the-polymorphic-class-problem">polymorphic class problem</a