Since each of the containers needs to be able to copy its elements in order to function properly, the type of the elements must fulfill CopyConstructible and Assignable requirements.ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §23.1 Container requirements [lib.container.requirements] para. 4 For a given container, all elements must belong to the same type. For instance, one cannot store data in the form of both char and int within the same container instance.
