TDDB84 Design Patterns Lecture 10. Prototype, Visitor

Transcription

1 Lecture 10 Prototype, Visitor Peter Bunus Dept of Computer and Information Science Linköping University, Sweden Prototype Peter Bunus 2 1

2 Prototype Non Software Example Peter Bunus 3 Motivational Example Peter Bunus 4 2

3 The Prototype Pattern Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype Peter Bunus 5 Motivational Example - Dragons Peter Bunus 6 3

4 The Dragon Classes class DragonPrototype{ protected: string name; DragonPrototype(){; DragonPrototype(const DragonPrototype& copy); ~DragonPrototype(){; virtual DragonPrototype* clone() const; void setname(string name); string getname() const; ; class MultipleHeadedDragon : public DragonPrototype{ protected: int numberofheads; MultipleHeadedDragon(){; MultipleHeadedDragon(const MultipleHeadedDragon& copy); ~MultipleHeadedDragon(); virtual DragonPrototype* clone() const; void setnumberofheads(int noheads); int getnumberofheads(); ; Peter Bunus 7 The Dragon Classes DragonPrototype::DragonPrototype(const DragonPrototype& copy){ name = copy.name; void DragonPrototype::setName(string p_name){ name = p_name; DragonPrototype* DragonPrototype::clone() const{ return new DragonPrototype(*this); MultipleHeadedDragon::MultipleHeadedDragon(const MultipleHeadedDragon &copy): DragonPrototype(copy){ numberofheads = copy.numberofheads; DragonPrototype* MultipleHeadedDragon::clone() const{ return new MultipleHeadedDragon(*this); void MultipleHeadedDragon::setNumberOfHeads(int noheads){ numberofheads = noheads; ; Peter Bunus 8 4

5 The Dragon Client cout << "Testing the dragon prototypes" << endl; MultipleHeadedDragon *dragonmh = new MultipleHeadedDragon(); dragonmh->setname("ziggy"); DragonPrototype *anotherdragon = dragonmh->clone(); anotherdragon->setname("zippy"); dragonmh->setnumberofheads(7); Peter Bunus 9 The Tree Stooges class Stooge { virtual void slap_stick() = 0; ; class Larry: public Stooge { void slap_stick(){ cout << "Larry: poke eyes\n"; ; class Moe: public Stooge{ void slap_stick(){ cout << "Moe: slap head\n"; ; class Curly: public Stooge{ void slap_stick(){ cout << "Curly: suffer abuse\n"; ; Peter Bunus 10 5

6 The Tree Stooges int main(void) { vector roles; int choice; while (true) { cout << "Larry(1) Moe(2) Curly(3) Go(0): "; cin >> choice; if (choice == 0) break; Larry(1) Moe(2) Curly(3) Go(0): 2 else if (choice == 1) Larry(1) Moe(2) Curly(3) Go(0): 1 roles.push_back(new Larry); else if (choice == 2) Larry(1) Moe(2) Curly(3) Go(0): 3 roles.push_back(new Moe); Larry(1) Moe(2) Curly(3) Go(0): 0 else Moe: slap head Larry: poke eyes Curly: suffer abuse roles.push_back(new Curly); for (int i = 0; i < roles.size(); i++) roles[i]->slap_stick(); for (int i = 0; i < roles.size(); i++) delete roles[i]; Peter Bunus 11 The Tree Stooges Prototype Variant class Stooge { virtual Stooge* clone() = 0; virtual void slap_stick() = 0; ; class Larry: public Stooge { class Moe: public Stooge{ Stooge* clone() {return new Larry; Stooge* clone() { return new Moe; void slap_stick(){ void slap_stick(){ cout << "Larry: poke eyes\n"; cout << "Moe: slap head\n"; ; ; class Curly: public Stooge{ Stooge* clone() {return new Curly; void slap_stick(){ cout << "Curly: suffer abuse\n"; ; Peter Bunus 12 6

7 The Tree Stooges Prototype Variant class Factory { static Stooge* make_stooge( int choice ); private: static Stooge* s_prototypes[4]; ; Stooge* Factory::s_prototypes[] = { 0, new Larry, new Moe, new Curly ; Stooge* Factory::make_stooge( int choice ) { return s_prototypes[choice]->clone(); Peter Bunus 13 The Tree Stooges Prototype Variant int main( void ) { vector roles; int choice; while (true) { cout << "Larry(1) Moe(2) Curly(3) Go(0): "; cin >> choice; if (choice == 0) break; roles.push_back( Factory::make_stooge( choice ) ); for (int i=0; i < roles.size(); ++i) roles[i]->slap_stick(); for (int i=0; i < roles.size(); ++i) delete roles[i]; Larry(1) Moe(2) Curly(3) Go(0): 2 Larry(1) Moe(2) Curly(3) Go(0): 1 Larry(1) Moe(2) Curly(3) Go(0): 3 Larry(1) Moe(2) Curly(3) Go(0): 0 Moe: slap head Larry: poke eyes Curly: suffer abuse Peter Bunus 14 7

8 Prototype Pattern Interaction Diagram Peter Bunus 15 Applicability The prototype pattern is used when a system should be independent of how its products are created, composed, and represented; and when the classes to instantiate are specified at run-time; for example, the dynamic loading to avoid building a class hierarchy of factories that parallels the class hierarchy of products; or when instances of a class can have one of only a few different combinations of state Peter Bunus 16 8

9 Consequences Isolating concrete product classes from the client. Dynamically adding and removing products at run-time. Specifying new objects by varying values. Specifying new objects by varying structure. Reducing the need for sub-classing. Configuring an application with classes dynamically. Main liability: Clone() needed. Peter Bunus 17 Visitor Peter Bunus 18 9

10 Visitor Non Software Example Peter Bunus 19 Visitor Pattern Structure Peter Bunus 20 10

11 Motivational Example Asssume that we have a binary tree with an integer in each node and each leaf We want: To print all the numbers Compute their sum using separate methods Peter Bunus 21 Java Tree Implementation abstract class Tree { protected int n; abstract void print(); abstract int sum(); class Node extends Tree { private Tree left, right; void print() { left.print(), System.out.print(n); right.print(); int sum() { return left.sum() + n + right.sum(); class Leaf extends Tree { void print() { System.out.print(n); int sum() { return n; Peter Bunus 22 11

12 Tree Visitor interface TreeVisitor { void visit(node node); void visit(leaf leaf); Each tree class must have an accept method abstract class Tree { protected int n; abstract void accept(treevisitor visitor); class Leaf extends Tree { void accept(treevisitor visitor) { visitor.visit(this); class Node extends Tree { private Tree left, right; void accept(treevisitor visitor) { left.accept(visitor), visitor.visit(this); right.accept(visitor); Peter Bunus 23 Tree Print Visitor interface TreeVisitor { void visit(node node); void visit(leaf leaf); class PrintVisitor extends TreeVisitor { public void visit(node node) { System.out.print(node.getN()); public void visit(leaf leaf) { // Usually the visit methods differ. System.out.print(leaf.getN()); The Client TreeVisitor visitor = new PrintVisitor(); Tree tree =... tree.accept(visitor); Peter Bunus 24 12

13 The Sum Visitor interface TreeVisitor { void visit(node node); void visit(leaf leaf); class SumVisitor implements TreeVisitor { int sum = 0; public void visit(node node) { sum += node.getn(); public void visit(leaf leaf) { sum += leaf.getn(); The Client TreeVisitor v = new SumVisitor(); Tree tree =... tree.accept(visitor); Peter Bunus 25 Visitor Benefits/Drawbacks Allows you to add operations to a Composite structure without changing the structure itself Adding new operations is relativelly easy The code for operations performed by the Visitor is centralized The Composite classes encapsulation is broken when the Visitor is used Because the traversal function is involved, changes to Composite are more difficult. Peter Bunus 26 13

14 Flyweight Peter Bunus 27 Flyweight Non Software Example Peter Bunus 28 14

15 The FlyWeight Design Pattern The Flyweight pattern describes how to share objects to allow their use at fine granularities without prohibitive cost. A flyweight" object has state-dependent (extrinsic) part (stored or computed by client objects) state-independent (intrinsic) part (stored in the Flyweight) Peter Bunus 29 Flyweight Example Flyweights are stored in a Factory's repository. The Ant, Locust, and Cockroach classes can be "light-weight" because their instance-specific state has been de-encapsulated, or externalized, and must be supplied by the client. Peter Bunus 30 15

16 Seven Layers of Architecture Enterprise-Architecture Global-Architecture System-Architecture OO Architecture Application-Architecture Subsystem Macro-Architecture Frameworks Micro-Architecture Design-Patterns Objects OO Programming Peter Bunus 31 Antipatterns Sources Peter Bunus 32 16

17 Congratulations: You have now completed TDDB84 Peter Bunus 33 17