The process of loading an archive may result in the creation of new objects. That
same process may throw an exception at some point. In order to prevent memory leaks
and invalid pointers, these situations must be considered. Unfortunately, there is
no simple universal solution to this problem. The manner of addressing this must
depend on the design of the data structures to be serialized. Below, we discuss
varying scenarios in increasing order of difficulty. This discussion presumes that
the class member functions are exception safe before considering serialization.
That is, the destructor could be called at anytime without referencing
an invalid pointer, or creating a memory leak.
class contains no pointers
No problem here.
class contains only owned pointers
From here on, we have to make a distinction between pointers used
to manage heap storage (owned pointers) and pointers used to refer
to related objects (referenced pointers). Programs containing owned
pointers must contain code for deleting these objects and returning the
deallocated storage to the heap. Programs containing referenced pointers
must be designed to ensure that no such referenced pointers are de-referenced
after the object pointed to has been destroyed and its storage returned
to the heap. If a pointer is stored in only one place, it must be an owned
The load function traps any exceptions that occur between the time an object
is created and its pointer is stored. Should an exception occur while
reading an archive, the created object is deleted and the de-serialized
pointer is set to NULL. This ensures that there are no memory leaks.
The fact that there are no other copies of this pointer ensures that
no pointers are left invalid. The object's destructor should
be able to delete any other existing objects in the normal manner
illustrates this case.
class contains one or more referenced pointers
This situation can be further subdivided into two cases
owned pointers are always serialized before referenced pointers
Object tracking will ensure that no new objects will be created
by the loading of a referenced pointer.
If an exception occurs, referenced pointers will not need to be deleted
so there will be no memory leaks. The destructor of this class won't attempt to
delete these pointers so there will be no problem with dangling references.
Owned pointers are handled exactly as described above.
class contains referenced pointers which might be created by load
If a referenced pointer is loaded before its corresponding owned
pointer, the object will be allocated on the heap. In certain cases
it cannot be known which pointers were created by their owners and which
were created by the load function. To address this:
is a program that illustrates this case.
- Trap exceptions with a
- Within the catch part, invoke the archive function
delete_created_pointers() to delete any pointers
created by the class load. Without other action, objects created in
this way would end up as memory leaks as they are not considered owned
pointers and hence aren't destroyed.
- The object's destructor won't try
to delete referenced pointers so any dangling references will
cause no harm.
Situations not covered above are pointers for which the classifications of
referenced and owned are not applicable. This might occur where
pointers are created by one class but consumed and deleted by another. These
may be addressed with an ad hoc analysis similar to the above. As the
situation becomes more complex this becomes more difficult and error prone.
Eventually, it will be have to addressed by building heap management into the
pointer itself - that is into
The library includes serialization of
previously mentioned, this required a tiny alteration in one of the
boost::shared_ptr implementation files in order to permit
access by the serialization system.
© Copyright Robert Ramey 2002-2004.
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)