HP Integrity NonStop J-Series - Manual (Page 206)

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©Copyright 1996 Rogue Wave Software
A Few Friendly Warnings
Persistence is a useful quality, but requires care in some areas. Here are a few things to look out
for when you use persistence on objects.
Always Save an Object by Value before Saving the Identical
Object by Pointer
In the case of both isomorphic and polymorphic persistence of objects, you should never stream
out an object by pointer before streaming out the identical object by value. Whenever you design
a class that contains a value and a pointer to that value, the saveGuts and restoreGuts member
functions for that class should always save or restore the value then the pointer.
Here is an example that creates a class that uses isomorphic persistence, then instantiates objects
of the class and attempts to persist those objects isomorphically. This example will fail. See the
explanation that follows the code.
class Elmer {
public:
/* _ */
Elroy elroy_;
Elroy* elroyPtr_; // elroyPtr_ will point to elroy_.
};
RWDEFINE_PERSISTABLE(Elmer)
void rwSaveGuts(RWFile& file, const Elmer& elmer) {
// Create a value for elroyPtr_ and stream it:
file << *elroyPtr_;
// If elroyPtr_ == &elroy_, store a reference
// to elroy_ that points to the created value for
// elroyPtr_:
file << elroy_;
}
void rwRestoreGuts(RWFile& file, Elmer& elmer){