PersistentObject that supports reading and writing
of arbitrary class objects to a stream (p. 202) using the static functions
template <class T>
PersistentObject::write(ostream&, T*)
PersistentObject* PersistentObject::read(ostream&)
typeinfo
(RTTI) followed by the output of a user defined
operator<<(ostream&, const T&)
The result of PersistentObject::read(ostream&)
can be dynamically cast to T* because, for any class type
T, Persistent<T> is derived
from both PersistentObject and T.
In fact the ``real'' return type of
PersistentObject::read() is
Persistent<T>*, ensuring that e.g.
dynamic_cast<T*>(PersistentObject::read(istream&))
will return 0 or a pointer to the T-object
read from the istream.
For reading back objects, the system has to know which classes
correspond to the typeinfo strings. This information must be
supplied by calling Persistent<T>::register_class()
for each class T for which objects may be saved/read. The mapping
is stored in a global PersistentObject::registry_
which is defined in persistent.C.
The files persistent_write.C
and persistent_read.C illustrate
the use of PersistentObject by writing and reading
back a heterogeneous container of
Person (person.h) and
Student (student.h)
objects.