Hands-On Design Patterns with C++

Pre-and post-conditions and actions

Another common use of the Template Method is dealing with pre-and post-conditions, or actions. In a class hierarchy, pre-and post-conditions generally verify that the design invariants of the abstraction provided by the interface are not violated by any specific implementation at any point during execution. Such verification naturally submits to the Template Method's design:

class Base {    public:    void VerifiedAction() {        assert(StateIsValid());        ActionImpl();        assert(StateIsValid());    }    virtual void ActionImpl() = 0;};class Derived : public Base {    public:    void ActionImpl() { ... }};

Of course, in software design, one man's invariants are another's customization points. Sometimes, it is the main code that remains ...