Tools.h++ Manual

104011 Tandem Computers Incorporated 17-13

would be disastrous as we would have to dereference

passengers_

strm << *passengers_;

Instead, we pass it to

RWvostream& operator<<(RWvostream&, const

RWCollectable*);

which automatically detects the nil pointer and stores a record of it.

Multiply-referenced objects

Note – a passenger name can exist in the set pointed to by

customers_

and in

the set pointed to by

passengers_

. That is, both collections contain the same

string. When the

Bus

is restored, we want to make sure that this relationship

is maintained.

We don’t have to do anything! Consider the call:

Bus aBus;

RWFile aFile(“busdata.dat”);

aBus.addPassenger(“John”);

aFile << aBus;

Note – the name “John” will be on both the customer list and the passenger

list. Note that both lists will be saved in this single call: ﬁrst the customer list,

then the passenger list. The persistence machinery will save the ﬁrst reference

to “John”, but for the second reference it will merely store a reference to the

ﬁrst copy. During the restore (see below), both references will resolve to the

same object, replicating the original morphology of the collection.