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2014
Design Patterns
Strategy Pattern
© 2014 by Prof. Christian Madritsch
Literature
• E. Freeman, Head First Design Patterns, O‘Reilly, 2005
• E. Gamma, R. Helm, R. Johnson, J. Vlissides, Design Patterns.
Elements of Reusable Object‐Oriented Software, Addison‐
Wesley Longman, 1994
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Design Patterns ‐ General
• In software engineering, a design pattern is a general
reusable solution to a commonly occurring problem in
software design.
• A design pattern is not a finished design that can be
transformed directly into code.
• It is a description or template for how to solve a
problem that can be used in many different situations.
• Object‐oriented design patterns typically show
relationships and interactions between classes or
objects, without specifying the final application classes
or objects that are involved.
• Independent of Programming Language
Design Patterns ‐ General
• Design patterns can speed up the
development process by providing tested,
proven development paradigms.
• Split into the following sections:
– Creational Patterns
– Behavioral Patterns
– Structural Patterns
– Concurrency Patterns
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Design Patterns Overview
• Creational Patterns • Behavioral Patterns
– Factory – Command
– Abstract Factory – Observer
– Singleton – Strategy
• Structural Patterns – Template Method
– Adapter
– Decorator
– Facade
SimUDuck app 1.0
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SimUDuck app 2.0
but also...
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How about an Interface?
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Implementation vs. Interface
• Programming to an implementation:
Dog *h = new Dog();
h‐>bark();
• Programming to an interface/supertype:
Animal *animal = new Dog();
tier‐>makeSound();
• Assign the concrete implementation
object at runtime:
Animal = getAnimal();
animal‐>makeSound();
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class Duck {
protected: FlyBehavior* flyBehavior;
protected: QuackBehavior* quackBehavior;
protected: Duck() : flyBehavior(0), quackBehavior(0) {
}
public: virtual void setFlyBehavior(FlyBehavior* fb) {
flyBehavior = fb;
}
public: virtual void setQuackBehavior(QuackBehavior* qb) {
quackBehavior = qb;
}
public: virtual void performFly() {
flyBehavior->fly();
}
public: virtual void performQuack() {
quackBehavior->quack();
}
public: virtual void swim() {
cout << "All ducks float, even decoys!" << endl;
}
public: virtual void display() = 0;
};
class MallardDuck : public Duck {
public: MallardDuck() {
quackBehavior = new Quack();
flyBehavior = new FlyWithWings();
}
public: virtual void display() {
cout << "I'm a real Mallard duck" << endl;
}
};
class DecoyDuck : public Duck {
public: DecoyDuck() {
quackBehavior = new MuteQuack();
flyBehavior = new FlyNoWay();
}
public: virtual void display() {
cout << "I'm a duck Decoy" << endl;
}
};
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class FlyBehavior {
public: virtual void fly () = 0;
};
class FlyWithWings : public FlyBehavior {
public: virtual void fly () {
cout << "I'm flying!!" << endl;
}
};
class FlyNoWay : public FlyBehavior {
public: virtual void fly () {
cout << "I can't fly" << endl;
}
};
int main(int argc, char* argv[])
{
MallardDuck mallard;
mallard.display();
mallard.performFly();
mallard.performQuack();
mallard.swim();
RedHead redHead;
redHead.display();
redHead.performFly();
redHead.performQuack();
redHead.swim();
RubberDuck rubberDuckie;
rubberDuckie.display();
rubberDuckie.performFly();
rubberDuckie.performQuack();
rubberDuckie.swim();
return 0;
}
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