CSS 343 Data Structures, Algorithms, and Discrete Math II. Polymorphism. Yusuf Pisan

Transcription

1 CSS 343 Data Structures, Algorithms, and Discrete Math II Polymorphism Yusuf Pisan

2 Polymorphism Hierarchy of classes that are related by inheritance static linkage / early binding Decide on function to call at compile time late binding Decide on function to call at run time virtual Prevent early binding from compiler Rectangle rec(10, 7); rec.area(); Shape *s1; s1 = &rec; s1->area(); Shape *s2 = new Rectangle(10, 7); s2->area(); class Shape { protected: int width, height; Shape(int a = 0, int b = 0) { width = a; height = b; virtual int area() { cout << "Parent class area :" << endl; return 0; ; class Rectangle : public Shape { Rectangle(int a = 0, int b = 0) : Shape(a, b) { virtual int area() {... 2

3 C++ Name Mangling _ZN9wikipedia7article8print_toERSo How compiler changes the name bool print_to (std::ostream&); _Z N 9wikipedia 7article 8print_to E R So function overloading Start with _Z Namespace 9 character wikipedia 7 character article 8 character print_to End of name Reference parameter ostream nickname functions with different signatures have different names internally namespace wikipedia { class article { std::string format (void); /* = _ZN9wikipedia7article6formatEv */ bool print_to (std::ostream&); /* = _ZN9wikipedia7article8print_toERSo */ class wikilink { wikilink (std::string const& name); /* = _ZN9wikipedia7article8wikilinkC1ERKSs */ ; ; g++ -std=c++14 main.cpp -S -o- grep Shape (-S create assembly code) 3

4 Polymorphism - virtual copy constructor is delete Default copy constructor is not generated No copy of this object can be made somefunction is a virtual function All subclasses have to implement it GetNumberOfShapes is static Can be called using Shape::GetNumberOfShapes() rec.getnumberofshapes() class Shape {... Shape(const Shape& sp) = delete;... virtual int somefunction() = 0;... static int GetNumberOfShapes(); int main() { cout<<shape::getnumberofshapes(); 4

5 Puzzle: Display object class Shape { friend ostream &operator<<(ostream &os, const Shape &sp) { os << "Shape display"; return os But, we want to display Rectangle, Triangle, etc not the Shape class Shape is not allowed to know about its subclasses Only virtual functions dispatch based on the dynamic type of an object Only nonstatic member functions can be virtual. operator<< cannot be nonstatic, first argument must be ostream How can we do it? 5

6 Puzzle: Display object class Shape { friend ostream &operator<<(ostream &os, const Shape &sp) { os << "Shape display"; return os But, we want to display Rectangle, Triangle, etc not the Shape class Shape is not allowed to know about its subclasses Only virtual functions dispatch based on the dynamic type of an object Only nonstatic member functions can be virtual. operator<< cannot be nonstatic, first argument must be ostream Non-Virtual Interface (NVI) idiom Clients invoke only the public interface, which in turn invokes virtual functions 6

7 NVI Idiom class Shape { friend ostream &operator<<(ostream &os, const Shape &sp) { return sp.display(os); private: virtual ostream &display(ostream &os) const = 0;... ; class Rectangle : public Shape { private: virtual ostream &display(ostream &os) const { os << "Rectangle"; return os; ; 7

8 Fruit 1. You can eat different type of fruits 2. Banana and Apple are type of Fruit 3. Different fruits provide different energy levels: Banana 20, Apple Fruit is made at the FruitFactory 5. Design Fruit, FruitFactory, Apple and Banana classes 8

9 Fruit 1. You can eat different type of fruits 2. Banana and Apple are type of Fruit 3. Different fruits provide different energy levels: Banana 20, Apple Fruit is made at the FruitFactory 5. Design Fruit, FruitFactory, Apple and Banana classes void FruitTest() { vector<fruit *> eaten; string name; int totalenergy{0; cout << "Give name: "; while (cin >> name && name!= "") { Fruit *f = FruitFactory::create(name); if (f) totalenergy += f->eat(); cout << "Give name: "; cout << "Total Energy: " << totalenergy << endl; for (const auto& f: eaten) delete f; 9

10 Fruit & Poly Examples 10

11 Open/Closed Principle software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. Bertrand Meyer Fruit class is open for extension -- we can extend it without changing any code in the class Fruit class is closed for modification -- clients are using the interface, so we are not going to modify it 11

12 SOLID Principles for OO Programming Single responsibility a class should have only a single responsibility Open/closed "software entities should be open for extension, but closed for modification." Liskov substitution objects can always be replaced with instances of their subtypes Interface segregation many interfaces are better than one general-purpose interface Dependency inversion principle Abstractions should not depend on details. Details should depend on abstractions. 12

13 Single responsibility SOLID Principles for Fruit FruitFactory is responsible for making Fruit Open/closed Fruit can be extended Liskov substitution Can replace Apple object with Fruit object Interface segregation Fruit class is serving as an interface Dependency inversion principle Fruit does not know about Apple Apple relies on Fruit abstractions 13

14 Comment Validator We want EntryController to be more flexible Have multiple validator objects that are used for SPAM checking Not modify EntryController each time class EntryController { void AddComment(const string &str) { if (ValidateNotSpam(str)) { //Save the comment to the database private: bool ValidateNotSpam(const string &comment) { return true; ; 14

15 #include <algorithm> class CommentValidator { virtual bool Validate(const string &comment); ; class EntryController { private: vector<commentvalidator *> validators; EntryController() = default; void addvalidator(commentvalidator *cv) { validators.push_back(cv); // below could also be written as a for loop bool ValidateNotSpam(const string &comment) { return all_of(validators.begin(), validators.end(), [&comment](commentvalidator* cv) { return cv->validate(comment); ); ; 15

16 class Pet { How can we make it virtual void say() = 0; easier to add new pets? virtual ~Pet() = default; static Pet *create(string type); ; // Cat class, Cat isa Pet class Cat : public Pet { void say() override { cout << "Meeow" << endl; ; Pet *Pet::create(string type) { if (type == "Cat") return new Cat(); else return new Dog(); int main() { Pet *p = Pet::create("Cat"); p->say(); return 0; 16

17 Poly Pets Create a PetFactory that is responsible for making Pet objects Only PetFactory needs to be modified for adding Pets Problem: PetFactory has to be modified when new pets entered Solution: Each Pet has their own create function. PetFactory keeps a copy of a "prototype" Pet to make new ones. Keep list of objects in a map/hash table. Problem: Still have to register each Pet, or hardcode it Solution: Abstract PetFactory, concrete CatFactory, DogFactory, etc. Each Factory is now registered as being able to make that object Creating a CatFactory object, registers it Compile below with and without horse.cpp - no code change needed 17

18 Order of static variables // Create an instance of CatFactory which registers "Cat" static CatFactory global_catfactory; What happens if we try to create a CatFactory, but our map<string, PetFactory*> Pet::factories has not been defined yet? 18

19 class Pet { private: // moved static map<string, PetFactory*> factories; static void registertype(const string& type, PetFactory * factory) { getmap()[type] = factory; static map<string, PetFactory*>& getmap() { static map<string, PetFactory *> factories; return factories; static Pet * create(string type) { if (getmap()[type] == nullptr) { cout << "Don't know how to create " << type << endl; return nullptr; return getmap()[type]->create(); virtual void say() = 0; virtual ~Pet() { ; Make map<string, PetFactory *> factories inside a static function variable instead of a static class variable 19