Design Patterns Explained

252 Part V • Handling Variations with Design Patterns
Decorators in testing I have used the Decorator to wrap precondition and postcondition
tests on an object to be tested with nice results. During test, the first
object in the chain can do an extensive test of preconditions prior to
calling its trailing object. Immediately after the trailing object call,
the same object calls an extensive test of postconditions. If I have
different tests I want to run at different times, I can keep each test in
a different Decorator and then chain them together according to the
battery of tests I want to run.
Summary
In this chapter The Decorator pattern is a way to add additional function(s) to an
existing function dynamically. In practice, it requires building a
chain of objects that give the desired behaviors. The first object in
this chain is called by a Client that had nothing to do with the
building of it. By keeping the creation of the chain independent
from its use, the Client object is not affected by new requirements
to add functionality.
Supplement: C++ Code Examples
Example 15-2 C++ Code Fragments
class SalesTicket : public Component
{ public:
void prtlicket();
}
SalesTicket::prtTicket() {
// sales ticket printing code here
}
class Decorator : public Component
{ public:
virtual void prtTicket();
Decorator( Component *myC);
private:
Component *myComp; }
(continued)