The C++ Programming Language Single and Multiple Inheritance in C++ Douglas C. Schmidt www.cs.wustl.edu/schmidt/ [email protected] Washington University, St. Louis 1 Background Object-oriented programming is often de- ned as the combination of Abstract Data Typ es ADTs with Inheritance and Dy- namic Binding Each concept addresses a di erent asp ect of system decomp osition: 1. ADTs decomp ose systems into two-dimensional grids of mo dules { Each mo dule has public and private inter- faces 2. Inheritance decomp oses systems into three-dimensional hierarchies of mo dules { Inheritance relationships form a \lattice" 3. Dynamic binding enhances inheritance { e.g., defer implementation decisions until late in the design phase or even until run-time! 2 Data Abstraction vs. Inheritance DATA ABSTRACTION (2 DIMENTIONAL) (2 DIMENTIONAL) INHERITANCE (3 DIMENTIONAL) 3 Motivation for Inheritance Inheritance allows you to write co de to handle certain cases and allows other de- velop ers to write co de that handles more sp ecialized cases, while your co de contin- ues to work Inheritance partitions a system architec- ture into semi-disjoint comp onents that are related hierarchically Therefore, we may be able to mo dify and/or reuse sections of the inheritance hierarchy without disturbing existing co de, e.g., { Change sibling subtree interfaces i.e., a consequence of inheritance { Change implementation of ancestors i.e., a consequence of data abstraction 4 Inheritance Overview A typ e called a sub class or derived typ e can inherit the characteristics of another typ es called a sup erclass or base typ e { The term sub class is equivalent to derived typ e A derived typ e acts just like the base typ e, except for an explicit list of: 1. Sp ecializations { Change implementations without changing the base class interface Most useful when combined with dynamic binding 2. Generalizations/Extensions { Add new op erations or data to derived classes 5 Visualizing Inheritance Base Derived 2 Derived 1 Derived 5 Derived Derived 4 3 Derived 6 6 Typ es of Inheritance Inheritance comes in two forms, dep end- ing on numb er of parents a sub class has 1. Single Inheritance SI { Only one parent per derived class { Form an inheritance \tree" { SI requires a small amount of run-time over- head when used with dynamic binding { e.g., Smalltalk, Simula, Object Pascal 2. Multiple Inheritance MI { More than one parent per derived class { Forms an inheritance \Directed Acyclic Graph" DAG { Compared with SI, MI adds additional run- time overhead also involving dynamic bind- ing { e.g., C++, Ei el, Flavors a LISP dialect 7 Inheritance Trees vs. Inheritance DAGs Base Derived Derived 1 2 INHERITANCE Derived Derived TREE 4 3 Base Base 1 Derived 2 1 INHERITANCE Derived Derived 3 DAG 4 8 Inheritance Bene ts 1. Increase reuse and software quality Programmers reuse the base classes instead of writing new classes { Integrates black-b ox and white-b ox reuse by allowing extensibility and mo di cation with- out changing existing co de Using well-tested base classes helps reduce bugs in applications that use them Reduce object co de size 2. Enhance extensibility and comprehensibil- ity Helps supp ort more exible and extensible ar- chitectures along with dynamic binding { i.e., supp orts the op en/closed principle Often useful for mo deling and classifying hierarchically- related domains 9 Inheritance Liabilities 1. May create deep and/or wide hierarchies that are hard to understand and navigate without class browser to ols 2. May decrease p erformance slightly i.e., when combined with multiple inheritance and dynamic binding 3. Without dynamic binding, inheritance has only limited utility Likewise, dynamic binding is almost totally use- less without inheritance 4. Brittle hierarchies, which may imp ose de- p endencies up on ancestor names 10 Inheritance in C++ Deriving a class involves an extension to the C++ class declaration syntax The class head is mo di ed to allow a deriva- tion list consisting of base classes e.g., class Foo f /* :::g; class Bar : public Foo f /* :::g; class Foo : public Foo, public Bar f /* :::g; 11 Key Prop erties of C++ Inheritance The base/derived class relationship is ex- plicitly recognized in C++ by prede ned standard conversions { i.e., a p ointer to a derived class may always be assigned to a p ointer to a base class that was inherited publically But not vice versa::: When combined with dynamic binding, this sp ecial relationship between inherited class typ es promotes a typ e-secure, p olymor- phic style of programming { i.e., the programmer need not know the actual typ e of a class at compile-time { Note, C++ is not truly p olymorphic i.e., op erations are not applicable to objects that don't contain de nitions of these op- erations at some p oint in their inheritance hierarchy 12 Simple Screen Class The following co de is used as the base class: class Screen f public: Screen int = 8, int = 40, char = ' '; ~Screen void; ; g short height void const f return this->height short width void const f return this->width ; g void height short h f this->height = h; g void width short w f this->width = w; g Screen &forward void; Screen &up void; Screen &down void; Screen &home void; Screen &b ottom void; Screen &display void; Screen &copy const Screen &; // ::: private: short height , width ; char *screen , *cur pos ; g; 13 Sub classing from Screen class Screen can be a public base class of class Window e.g., class Window : public Screen f public: Window const Point &, int rows = 24, int columns = 80, char default char = ' '; void set foreground color Color &; void set background color Color &; void resize int height, int width; // ::: private: Point center ; ; Color foreground Color background ; // ::: g; 14 Multiple Levels of Derivation A derived class can itself form the basis for further derivation, e.g., class Menu : public Window f public: lab el const char *l; void set Menu const Point &, int rows = 24, int columns = 80, char default char = ' '; // ::: private: char *lab el ; // ::: g; class Menu inherits data and metho ds from b oth Window and Screen { i.e., sizeof Menu >= sizeof Window >= sizeof Screen 15 The Screen Inheritance Hierarchy Point Screen Color Window Menu Screen/Window/Menu hierarchy 16 Variations on a Screen: : : ps1 : ps2 : Screen Screen w : Window Menu A p ointer to a derived class can be as- signed to a p ointer to any of its public base classes without requiring an explicit cast: Menu m; Window &w = m; Screen *ps1 = &w; Screen *ps2 = &m; 17 Using the Screen Hierarchy e.g., class Screen f public: virtual void dump ostream &; = 0 g class Window : public Screen f public: virtual void dump ostream &; g; class Menu : public Window f public: virtual void dump ostream &; g; // stand-alone function void dump image Screen *s, ostream &o f // Some pro cessing omitted s->dump o; // *s->vptr[1] s, o; g Screen s; Window w; Menu m; Bit Vector bv; // OK: Window is a kind of Screen image &w, cout; dump // OK: Menu is a kind of Screen dump image &m, cout; // OK: argument typ es match exactly dump image &s, cout; // Error: Bit Vector is not a kind of Screen! dump image &bv, cout; 18 Using Inheritance for Sp ecialization A derived class sp ecializes a base class by adding new, more sp eci c state variables and metho ds { Metho d use the same interface, even though they are implemented di erently i.e., \overridden" { Note, there is an imp ortant distinction between overriding, hiding, and overloading::: A variant of this is used in the template metho d pattern { i.e., b ehavior of the base class relies on func- tionality supplied by the derived class { This is directly supp orted in C++ via abstract base classes and pure virtual functions 19 Sp ecialization Example Inheritance may be used to obtain the fea- tures of one data typ e in another closely related data typ e For example, class Date represents an ar- bitrary Date: class Date f public: Date int m, int d, int y; virtual void print ostream &s const; // ::: private: int month , day , year ; g; Class Birthday derives from Date, adding a name eld representing the p erson's birth- day, e.g., class Birthday : public Date f public: Birthday const char *n, int m, int d, int y : Date m, d, y, p erson strdup n fg ~Birthday void f free p erson ; g virtual void print ostream &s const; // ::: private: ; const char *p erson g; 20 Implementation and Use-case Birthday::print could print the p erson's name as well as the date, e.g., void Birthday::print ostream &s const f << " was born on "; s << this->p erson Date::print s; s << "\n"; g e.g., const Date july 4th 7, 4, 1993; Birthday my birthday "Douglas C. Schmidt", 7, 18, 1962; july 4th.print cerr; // july 4th, 1993 my birthday.print cout; // Douglas C. Schmidt was born on july 18th, 1962 birthday; Date *dp = &my dp->print cerr; // ??? what gets printed ??? // *dp->vptr[1]dp, cerr; 21 Alternatives to Sp ecialization Note that we could also use object com- p osition instead of inheritance for this ex- ample, e.g., class Birthday f public Birthday char *n, int m, int d, int y: m, d, y, p erson n fg date // same as b efore private: Date date ; char *p erson ; g; However, in this case we would not be able to utilize the dynamic binding facilities for base classes and derived classes { e.g., Date *dp = &my birthday; // ERROR, Birthday is not a sub class of date! { While this do es not necessarily a ect reusabil- ity, it do es a ect extensibility::: 22 Using Inheritance for Extension/Generalization Derived