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CSE432S

CSE432S. Strategy and Template Method Patterns, Single User Protection. Strategy Pattern. “Define a Family of Algorithms, encapsulate each one, and make them interchangeable.”. Strategy – The General Idea.

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CSE432S

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  1. CSE432S Strategy and Template Method Patterns, Single User Protection CSE432S - Nick Beary

  2. Strategy Pattern “Define a Family of Algorithms, encapsulate each one, and make them interchangeable.” CSE432S - Nick Beary

  3. Strategy – The General Idea • 3 Participants: Strategy, Context, Client (book has Strategy, ConcreteStrategy, Context). • Client interacts with the same interface (Context), but has many options for algorithms (strategies). • Context is the intermediary, but Client must have knowledge of Strategy CSE432S - Nick Beary

  4. Strategy – Consequences • Families of related algorithms • An alternative to subclassing • Strategies eliminate conditional statements • A choice of implementations • Clients must be aware of different Strategies • Communication overhead between Strategy and Context • Increased number of objects. CSE432S - Nick Beary

  5. Strategy – Implementation Issues • Defining the Strategy and Context interfaces • Strategies as template parameters • Making Strategy objects optional CSE432S - Nick Beary

  6. Template Method Pattern “Define the skeleton of an algorithm in an operation, deferring some steps to subclasses.” CSE432S - Nick Beary

  7. Template Method – The General Idea • 2 participants: AbstractClass and ConcreteClass • Abstract Class defines parts of an algorithm that are invariant, leaves as virtual functions pieces of the algorithm that may or must be defined by Concrete Classes. • The Hollywood Principle – Clever metaphor, or needless pun? CSE432S - Nick Beary

  8. Template Method – Operations • Concrete Operations • Client class, Concrete Class • Abstract Class • Primitive Operations (abstract operations) • Hook Operations • Key Point: Distinguish operations which may be defined from those that must be defined. CSE432S - Nick Beary

  9. Template Method – Implementation Issues • Using C++ access control. • Minimizing primitive operations • Naming conventions CSE432S - Nick Beary

  10. Single User Protection Time for some sweet, sweet Template Method Pattern Action! CSE432S - Nick Beary

  11. Problems • Write Protection • How do we stop ourselves from making rash decisions about deletion and editing? • Read Protection • How do we keep things from our spouses and/or children? (Vlissides example, not mine) • Note: File System Level CSE432S - Nick Beary

  12. Write Protection • Should prevent any changes to a node (relatively) • Shouldn’t be able to be explicitly deleted, but protection might change at run-time, so can’t use const (C++). • Solution: Protect the destructor! CSE432S - Nick Beary

  13. Destructor Protection – Who Deletes? • The Node class • A class outside the Node class hierarchy • Creates unnecessary friend status. • A global function • Really no benefit over a static member function. CSE432S - Nick Beary

  14. Static or no? • Syntax like node->destroy(); and delete this; unsettles some. • Static member doesn’t support modification in subclasses. • Non-virtual member function, employing everybody’s best friend… CSE432S - Nick Beary

  15. TEMPLATE METHOD!!! • Write a destroy method that takes care of invariant deletion functions, leave details to primitive operations/hooks. • Primitives in this case include protection checking, warning actions, so forth. CSE432S - Nick Beary

  16. Read Protection • Add Templating to reading operations (i.e. streamOut). CSE432S - Nick Beary

  17. Summary • Strategy Pattern • 1 Context, interchangeable interfaces • Template Method Pattern • Define Algorithm Invariant, leave primitives to subclasses • Single-User Protection • Use Template Method Pattern to apply security protocols, protect you from yourself and prying, ignorant others. CSE432S - Nick Beary

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