HP Integrity NonStop J-Series - Manual (Page 208)

operator<<(RWFile&, const Elroy&) is called, but this time the insertion operator notices from
the isomorphic save table that *(elmer.elroyPtr_) has already been stored, so a reference to
*(elmer.elroyPtr_) is stored to file instead of the value.
Everything seems okay, but trouble is looming. Trouble arrives with the statement:
file >> elmer2;
This statement calls the extraction operator operator>>(RWFile&, const Elmer&). Since elmer2
hasn't been restored yet, the value of elmer2 will be extracted from file and added to the
isomorphic restore table. In order to extract the value of elmer2, the members elroy_ and
elroyPtr_ must be extracted. The members of elmer2 are extracted in rwRestoreGuts(RWFile&,
Elmer&).
The function rwRestoreGuts(RWFile&, Elmer&) restores elroyPtr_ first, then elroy_. When
rwRestoreGuts restores the value *elroyPtr_, it calls the extraction operator
operator>>(RWFile&, Elroy*&).
The extraction operator operator>>(RWFile&, Elroy*&) sees that elmer2.elroyPtr_ hasn't yet
been extracted from file. So the extraction operator extracts the value *(elmer2.elroyPtr_) from
file, allocates memory to create this value, and updates elmer2.elroyPtr_ to point to this value.
Then the extraction operator makes a note in the isomorphic restore table that this value has been
created. After making the note, operator>>(RWFile&, Elroy&) returns to
rwRestoreGuts(RWFile&, Elmer&).
Back in rwRestoreGuts(RWFile&, Elmer&), it's time to restore elmer2.elroy_. Remember that
elmer.elroyPtr_ has the same address as elmer.elroy_. The rwRestoreGuts function calls the
extraction operator operator>>(RWFile&, Elroy&), which notices from the isomorphic save
table that *(elmer2.elroyPtr_) has already been extracted from file. Because a value has already
been stored in the restore table, the extraction operator extracts a reference to *(elmer.elroyPtr_)
from file instead of extracting the value. But the extraction operator notices that the address of
the value that elmer2.elroyPtr_ put into the restore table is different from the address of
elmer2.elroy_. So operator>>(RWFile&, Elroy&) throws an RWTOOL_REF exception, and the
restoration is aborted.
The solution to the problem in this particular case is easy: reverse the order of saving and
restoring Elmer's members! Here is the problem:
// WRONG!
void rwSaveGuts(RWFile& file, const Elmer& elmer) {
file << *elroyPtr_;
file << elroy_;
}
void rwRestoreGuts(RWFile& file, Elmer& elmer){
file >> elroyPtr_;
file >> elroy_;
}