Reusing Code in C++

1. Reusing Code in C++ • Has-a relationship • Classes with member objects(containment) • The valarray template class • Private & protected inheritance • Multiple inheritance • Virtual base class • Template class • Template specializations

2. Classes with object members • A student class • A name (string class) • A set of quiz scores • A fixed-size array? dynamic memory allocation? • An array-of-double

3. The valarray class

4. The Student class design Containment (composition, layering) is a has-a relationship Program studentc.h, studentc.cpp, use_stuc.cpp

5. Private inheritance • With private inheritance, public and protected members of the base class become private members of the derived class • Program studenti.h, studenti.cpp, use_studi.cpp

6. Containment or private inheritance • Has-a relationship can be generated by containment or private inheritance • In general, it is easy to use containment to build up a “has-a” relationship • Private inheritance provides some privileges, such as accessing protected member, or redefining virtual function, which can not be implemented by containment

7. Containment or private inheritance We should use containment to model a has-a relationship Use private inheritance if the new class needs to access protected members in the original class or if it needs to redefine virtual functions

8. Varieties of inheritance

9. Redefining access with using Public members of a base class become protected or private when one uses protected or private derivation

10. Multiple inheritance • Program worker0.h, worker0.cpp, worktest.cpp

11. Ambiguous inheritance • Program worker.h, worker0.cpp, worktest.cpp // point out the ambiguities

12. Type conversion class SingingWaiter:public Singer, public Waiter { }; SingingWaiter ed; Worker *pw = &ed; // ambiguous Worker *pw1 = (Waiter *) &ed; // the Worker in Waiter Worker *pw2 = (Singer *) &ed; // the Worker in Singer

13. Inherit two base-class objects

14. Virtual base classes class Singer : virtual public Worker { }; class Waiter : public virtual Worker { }; class SingingWaiter: pubic Singer, public Waiter { …};

15. Using virtual base class • If a class has an indirect inherit virtual base class, a constructor for that class should explicitly invoke a constructor for the virtual base class unless all that is needed is the default constructor for the virtual base class • SingingWaiter(const Worker &wk, int p=0, int v=Singer::other) : Waiter(wk, p), Singer(wk, v) { } // flawed • SingingWaiter(const Worker &wk, int p=0, int v=Singer::other) : Worker(wk), Waiter(wk, p), Singer(wk, v) { } // ok

16. Indicate which method • Multiple inheritance can result in ambiguous calls SingingWaiter newhire(“Elise Hawks”, 2005, 6, soprano); newhire.Show(); // ambiguous newhire.Singer::Show(); // use Singer version void SingingWaiter::Show() // best way to do it { Singer::Show();}

17. Problems shooting void Worker::Show() const { cout << “Name:” << fullname << “\n”; cout << “Employee ID:” << id << “\n”;} void Waiter::Show() const { Worker::Show(); cout <<“Panache rating:” << panache << “\n”; } void HeadWaiter::Show() const { Waiter::Show(); cout << “presence rating:” << presence << “\n”; } void SingingWaiter::Show() { Singer::Show(); } // fail due to ignoring Waiter void SingingWaiter::Show() { Singer::Show(); Waiter::Show(); } // call Worker::show() twice

18. Using modular approach void Worker::Data() const { cout << “Name:” << fullname << “\n”; cout << “Empolyee ID:” << id << “\n”; } void Waiter::Data() const { cout << “Panache rating:” << panache << “\n”; void Singer::Data() const { cout << “Vocal range:” << pv[voice] << “\n”;} void SingingWaiter::Data() const { Singer::Data(); Waiter::Data(); } void SingingWaiter::Show() const { cout << “Category: singing waiter\n”; Worker::Data(); Data();} • Program workermi.h, workermi.cpp, workmi.cpp

19. Multiple inheritance synopsis A derived class with an indirect virtual base class should have its constructors invoke the indirect base-class constructors directly, which is illegal for indirect non-virtual base classes Name ambiguity is resolved via the dominance rule

20. Class templates • Inheritance and containment aren’t always the solution when you want to reuse code • Drawbacks of different datatype • Edit header file each time when one changes the type • One can use the technique to generate just one kind of the class per program • C++ class template provide a better way to generate generic class declaration • C++’s Standard Template Library (STL)

21. The original Stack class

22. Using a class template Stack Stack<int> kernels; // create a stack of int Stack<string> colonels; // a stack of string objects Template<class T> void simple(T t) { cout << t << “\n”; } … simple(2); // generate void simple(int) simple(“two”); // generate void simple(char *) • Program stacktp.h, stacktem.cpp

23. Stack pointer – incorrect version Stack<string> s; // original stack Stack<char *> st; // create a stack for pointers-to-char string po; • Three versions to replace “string po;” • char *po; cin >> po; // error • char po[40]; // template <class Type> // bool Stack<Type>::pop(Type &item) // { …; item = items[--top]; … } // item is Lvalue, cannot be array name 3. char *po = new char[40]; // push to the same address, and pop the same address value // pop outcome is always the last push in data

24. Correct version • Program stcktp1.h, stkoptr1.cpp

25. Array template • Template is usually used in container class ArrayTP<ArrayTP<int, 5>, 10> twodee; int twodee [10][5]; • Program arraytp.h, twod.cpp • Generate 2 separate class declarations • Just one class declaration through constructor

26. Template versatility

27. Using one than one type parameter • Program pairs.cpp

28. Template specializations • Template specializations • Implicit instantiation • Explicit instantiation • Explicit specialization • Partial specialization

29. Implicit instantiation The compiler doesn’t generate an implicit instantiation of the class until it needs an object

30. Explicit instantiation Generate ArrayTP<string, 100> class The compiler generates the class definition, including method definitions, even though no object of the class has yet been created or mentioned

31. Explicit specialization • An explicit specialization is a definition for a particular type of types that is to be used instead of the general template • A specialized class template definition has the form • To provide a SortedArray template specialized for the const char * type, using the current notation

32. Partial specializations C++ allows for partial specializations, which partially restrict the generality of a template

33. Partialspecializations Providing a special version for pointers Making a variety of restrictions The compiler would make the following choices

34. Member templates • Member templates • Program tempmemb.cpp • Templates as parameters • Program tempparm.cpp

35. Template classes and friends • Template class can have friends • 3 types of template friends • Non-template friends • Bound template friends • The type of the friend is determined by the type of the class when a class is instantiated • Unbound template friends • All specializations of the friend are friends to each specialization of the class

36. Non-template friend functions to template classes counts() function is a friend to HasFriend<int> or HasFriend<string>

37. Bound template friends The compiler would replace the template parameter T with int, giving in this form

38. Bound template friends The report() function is not itself a template function, it just has a parameter that is a template Need to define explicit specializations for the friends one plans to use

39. Template class with non-template friends Program frnd2tmp.cpp

40. Bound template friend functions to template classes Bound template friend involves three steps Step 1, declare each template function Step 2, declare the templates again

41. Bound template friend functions to template classes Step 3, declare an object of a particular specialization The compiler substitutes int for TT and generates the following class

42. Template friend to a template class Program tmp2tmp.cpp

43. Unbound template friend functions to template classes • show2(hfd, hfi2) gets matched to this specialization • Program manyfrnd.cpp