CS 246: Software Abstraction and Specification. Lecture 8. Object Composition

Transcription

1 CS 246: Software Abstraction and Specification Lecture 8 Object Composition Agenda: Object Composition Implementing Composite Objects "Law" of Demeter Reading: Eckel, Vol. 1 Ch. 14 Inheritance and Composition References Lieberher et al. Formulations and Benefits of the Law of Demeter", ACM SIGPLAN Notices, March 1989 Herb Sutter and Andrei Alexandrescu, C++ Coding Standards U Waterloo CS246se (Spring 2011) p.1/20

2 Review: Aggregation vs. Composition Transcript GPA 1..* Term Record term termgpa academic standing * enrollment 1 Program name U Waterloo CS246se (Spring 2011) p.2/20

3 Optional Unshared Component Address street no. street name unit no. city province postal code country local 0..1 Student Record name studentid Design decisions: data representation of component? initial value of component? is StudentRecord object responsible for creating/destroying associated Address object? is StudentRecord copy constructor a deep or shallow copy? U Waterloo CS246se (Spring 2011) p.3/20

4 class Address; Optional Component class StudentRecord { public: StudentRecord(string name, int id, Address* addr = 0); //... private: StudentRecord(StudentRecord&); string name_; int id_; Address *local_; //... }; // pointer member StudentRecord::StudentRecord(string n, int i, Address *a) : name_ (n), id_ (i), local_(a) { } StudentRecord::~StudentRecord() { delete local_; } U Waterloo CS246se (Spring 2011) p.4/20

5 Exactly One Unshared Component Address street no. street name unit no. city province postal code country 1 permanent Student Record name studentid Design decisions: data representation of component? initial value of component? is StudentRecord object responsible for creating/destroying components? U Waterloo CS246se (Spring 2011) p.5/20

6 Initializers for Object Data Members #include "Address.h" class Transcript; class StudentRecord { public: StudentRecord(string name, int id, Address *l, Address &p); //... private: string name_; int id_; Address *local_; // pointer data member //... }; Address perm_; // object data member StudentRecord::StudentRecord (..., Address *l, Address &p) : //..., local_(l), perm_ (p), // calls member's copy constructor //... U Waterloo CS246se (Spring 2011) p.6/20

7 One-to-One Association Student Record 1 1 Transcript name GPA studentid Design decisions: data representation of associated object(s)? initial value of associated object(s)? is StudentRecord object responsible for creating/destroying components? U Waterloo CS246se (Spring 2011) p.7/20

8 One-to-One Association class StudentRecord; class Transcript { public: Transcript(StudentRecord&); private: StudentRecord &sr_; float GPA_; }; Transcript::Transcript(StudentRecord &s) : sr_(s), GPA_(0.0) {} StudentRecord::StudentRecord (..., Address *l, Address &p) :... local_(l), perm_ (p), // calls member's (copy) constructor transcript_(*(new Transcript(*this))) // constructs trans { } U Waterloo CS246se (Spring 2011) p.8/20

9 StudentRecord s; Object Construction Object is built inside out: components before composite - in the order they are declared in the class declaration s: StudentRecord cout s.name_:string name_(n) id_(i) perm_(a) s.id_ : int s:local_:address // components of Address transcript_(* new Transcript))... s:transcript_:transcript // components of Transcript << "Inside constructor body" U Waterloo CS246se (Spring 2011) p.9/20

10 Collection of Components Work Term employer term evaluation coop history 0..6 Transcript 1..* GPA Term Record term termgpa academic standing Design Decisions: data representation of components? container of objects or of pointers to objects? initial value of associated object(s)? is Transcript object responsible for creating/destroying components? U Waterloo CS246se (Spring 2011) p.10/20

11 Collection of Components class StudentRecord; class TermRecord; class WorkTerm; class Transcript { public: Transcript(StudentRecord&); void addtermrecord(string term); void addworkterm (int wtnum, string, string, string); private: StudentRecord &sr_; float GPA_; vector<termrecord*> academic_; // vector of TermRecord* WorkTerm *work_[6]; // array of WorkTerm* }; Transcript::Transcript(StudentRecord &sr) : sr_(sr), GPA_(0.0), academic_(0), // call to vector constructor (size = 0) work_() // initialize array elements to NULL { } U Waterloo CS246se (Spring 2011) p.11/20

12 Composite Creates Components void Transcript::addTermRecord(string term) { TermRecord *tr = new TermRecord(term, 0.0, ""); academic_.push_back(tr); } void Transcript::addWorkTerm (int wtnum, string term, string employer, string eval) { work_[wtnum] = new WorkTerm(term, employer, eval); } U Waterloo CS246se (Spring 2011) p.12/20

13 Shared Components Term Record term termgpa academic standing * enrollment 1 Program name Design decisions: data representation of component? initial value of component? U Waterloo CS246se (Spring 2011) p.13/20

14 Shared Components class TermRecord { string term_; float termgpa_; string academicstanding_; Program &enrollment_; // reference object member public: TermRecord (string term, float GPA, string as, Program &p); //... }; TermRecord::TermRecord(string t, float g, string as, Program &p) : term_(t), termgpa_(g), academicstanding_(as), enrollment_(p) // reference must be set in initializer { } U Waterloo CS246se (Spring 2011) p.14/20

15 Composition and Data Encapsulation Address street no. street name unit no. city province postal code country permanent 1 local 0..1 Student Record name ID 1 Work Term employer term evaluation coop history Transcript GPA 1..* Term Record term termgpa academic standing * enrollment 1 Program name Information Hiding: Modular design should hide design and implementation details, including information about components. 1..* Course Enrollment grade credits earned * 1 Course number description credits U Waterloo CS246se (Spring 2011) p.15/20

16 Composition and Data Encapsulation GPA Transcript addtermrecord (term) computetermgpa (term) printtermrecord (term) 1..* TermRecord term termgpa academic standing - TermRecord (term) - computegpa() : float - print() vs. GPA Transcript addtermrecord(termrecord) TermRecord& trecord (term) 1..* TermRecord term termgpa academic standing + TermRecord (term) + computegpa() : float + print() Client Code Transcript *t;... t->addtermrecord("spring2011"); t->computetermgpa("spring2011"); t->printtermrecord("sring2011"); Client Code Transcript *t; TermRecord *tr;... tr = t->gettermrecord("spring2011"); tr->computegpa(); tr->print(); U Waterloo CS246se (Spring 2011) p.16/20

17 Composition and Data Encapsulation GPA Transcript addtermrecord (term) computetermgpa (term) printtermrecord (term) 1..* TermRecord term termgpa academic standing - TermRecord (term) - computegpa() : float - print() vs. GPA Transcript addtermrecord(termrecord) TermRecord& trecord (term) 1..* TermRecord term termgpa academic standing + TermRecord (term) + computegpa() : float + print() Client Code Composite object offers Transcript *t;... t->addtermrecord("spring2011"); methods for accessing and t->computetermgpa("spring2011"); manipulating component t->printtermrecord("sring2011"); information. Client Code Transcript *t; TermRecord *tr;... tr = t->gettermrecord("spring2011"); tr->computegpa(); tr->print(); U Waterloo CS246se (Spring 2011) p.17/20

18 Another Example client code Store 1 composite object * Customer + pay (total) * PayPal Account 1..* + pay (total) CreditCard + charge (total) void Store::settlebill(float total) {... Customer->getPayPal()->getCreditCard()->charge(total);... } U Waterloo CS246se (Spring 2011) p.18/20

19 Component Encapsulation client code Store 1 composite objects * Customer +pay (total) * PayPal Account - pay (total) 1..* CreditCard - charge (total) void Store::settlebill(float total) {... Customer->pay(total);... } U Waterloo CS246se (Spring 2011) p.19/20

20 "Law" of Demeter G B E b: B c: C x: int m1 (D) m2 ( ) A D C F H I Limit of A's direct dependencies. "Law" tests encapsulation: an object "talks only to its neighbours" Method A::m1 can only call methods of A itself Aʼs data members m1ʼs parameters any object created by m1 not methods of other classes (unless already allowed by above conditions) (e.g., not methods of an object simply because object is returned by a method call) U Waterloo CS246se (Spring 2011) p.20/20