14 class design (1)

Chapter 14 Graph class design Bjarne Stroustrup www.stroustrup.com/Programming

Abstract We have discussed classes in previous lectures Here, we discuss design of classes Library design considerations Class hierarchies (object-oriented programming) Data hiding Stroustrup/Programming

Ideals Our ideal of program design is to represent the concepts of the application domain directly in code. If you understand the application domain, you understand the code, and vice versa . For example: Window – a window as presented by the operating system Line – a line as you see it on the screen Point – a coordinate point Color – as you see it on the screen Shape – what’s common for all shapes in our Graph/GUI view of the world The last example, Shape , is different from the rest in that it is a generalization. You can’t make an object that’s “just a Shape” Stroustrup/Programming

Logically identical operations have the same name For every class, draw_lines() does the drawing move(dx,dy) does the moving s.add(x) adds some x ( e.g. , a point) to a shape s . For every property x of a Shape, x() gives its current value and set_x() gives it a new value e.g., Color c = s.color(); s.set_color(Color::blue); Stroustrup/Programming

Logically different operations have different names Lines ln; Point p1(100,200); Point p2(200,300); ln.add(p1,p2); // add points to ln (make copies) win.attach(ln); // attach ln to window Why not win.add(ln) ? add() copies information; attach() just creates a reference we can change a displayed object after attaching it, but not after adding it (100,200) p1: (200,300) p2: attach() add() Stroustrup/Programming (100,200) (200,300) &ln ln: win:

Expose uniformly Data should be private Data hiding – so it will not be changed inadvertently Use private data, and pairs of public access functions to get and set the data c.set_radius(12); // set radius to 12 c.set_radius(c.radius()*2); // double the radius (fine) c.set_radius(-9); // set_radius() could check for negative, // but doesn’t yet double r = c.radius(); // returns value of radius c.radius = -9; // error: radius is a function (good!) c.r = -9; // error: radius is private (good!) Our functions can be private or public Public for interface Private for functions used only internally to a class Stroustrup/Programming

What does “private” buy us? We can change our implementation after release We don’t expose FLTK types used in representation to our users We could replace FLTK with another library without affecting user code We could provide checking in access functions But we haven’t done so systematically (later?) Functional interfaces can be nicer to read and use E.g., s.add(x) rather than s.points.push_back(x) We enforce immutability of shape Only color and style change; not the relative position of points const member functions The value of this “encapsulation” varies with application domains Is often most valuable Is the ideal i.e., hide representation unless you have a good reason not to Stroustrup/Programming

“ Regular” interfaces Line ln(Point(100,200),Point(300,400)); Mark m(Point(100,200), 'x'); // display a single point as an 'x' Circle c(Point(200,200),250); // Alternative (not supported): Line ln2(x1, y1, x2, y2); // from (x1,y1) to (x2,y2) // How about? (not supported): Square s1(Point(100,200),200,300); // width==200 height==300 Square s2(Point(100,200),Point(200,300)); // width==100 height==100 Square s3(100,200,200,300); // is 200,300 a point or a width plus a height? Stroustrup/Programming

A library A collection of classes and functions meant to be used together As building blocks for applications To build more such “building blocks” A good library models some aspect of a domain It doesn’t try to do everything Our library aims at simplicity and small size for graphing data and for very simple GUI We can’t define each library class and function in isolation A good library exhibits a uniform style (“regularity”) Stroustrup/Programming

Class Shape All our shapes are “based on” the Shape class E.g., a Polygon is a kind of Shape Rectangle Image Stroustrup/Programming Shape Polygon Text Open_polyline Ellipse Circle Marked_polyline Closed_polyline Line Mark Lines Marks Axis Function

Class Shape – is abstract You can’t make a “plain” Shape protected: Shape(); // protected to make class Shape abstract For example Shape ss; // error: cannot construct Shape Protected means “can only be used from a derived class” Instead, we use Shape as a base class struct Circle : Shape { // “a Circle is a Shape” // … }; Stroustrup/Programming

Class Shape Shape ties our graphics objects to “the screen” Window “knows about” Shape s All our graphics objects are kinds of Shape s Shape is the class that deals with color and style It has Color and Line_style members Shape can hold Point s Shape has a basic notion of how to draw lines It just connects its Point s Stroustrup/Programming

Class Shape Shape deals with color and style It keeps its data private and provides access functions void set_color(Color col); Color color() const; void set_style(Line_style sty); Line_style style() const; // … private: // … Color line_color; Line_style ls; Stroustrup/Programming

Class Shape Shape stores Point s It keeps its data private and provides access functions Point point(int i) const; // read-only access to points int number_of_points() const; // … protected: void add(Point p); // add p to points // … private: vector<Point> points; // not used by all shapes Stroustrup/Programming

Class Shape Shape itself can access points directly: void Shape::draw_lines() const // draw connecting lines { if (color().visible() && 1<points.size()) for (int i=1; i<points.size(); ++i) fl_line(points[i-1].x,points[i-1].y,points[i].x,points[i].y); } Others (incl. derived classes) use point() and number_of_points() why? void Lines::draw_lines() const // draw a line for each pair of points { for (int i=1; i<number_of_points(); i+=2) fl_line(point(i-1).x, point(i-1).y, point(i).x, point(i).y); } Stroustrup/Programming

Class Shape (basic idea of drawing) void Shape::draw() const // The real heart of class Shape (and of our graphics interface system) // called by Window (only) { // … save old color and style … // … set color and style for this shape… // … draw what is specific for this particular shape … // … Note: this varies dramatically depending on the type of shape … // … e.g. Text, Circle, Closed_polyline // … reset the color and style to their old values … } Stroustrup/Programming

Class Shape (implementation of drawing) void Shape::draw() const // The real heart of class Shape (and of our graphics interface system) // called by Window (only) { Fl_Color oldc = fl_color(); // save old color // there is no good portable way of retrieving the current style (sigh!) fl_color(line_color.as_int()); // set color and style fl_line_style(ls.style(),ls.width()); draw_lines(); // call the appropriate draw_lines() // a “virtual call” // here is what is specific for a “derived class” is done fl_color(oldc); // reset color to previous fl_line_style(0); // (re)set style to default } Note! Stroustrup/Programming

Class shape In class Shape virtual void draw_lines() const; // draw the appropriate lines In class Circle void draw_lines() const { /* draw the Circle */ } In class Text void draw_lines() const { /* draw the Text */ } Circle , Text , and other classes “ Derive from” Shape May “override” draw_lines() Stroustrup/Programming

class Shape { // deals with color and style, and holds a sequence of lines public: void draw() const; // deal with color and call draw_lines() virtual void move(int dx, int dy); // move the shape +=dx and +=dy void set_color(Color col); // color access int color() const; // … style and fill_color access functions … Point point(int i) const; // (read-only) access to points int number_of_points() const; protected: Shape(); // protected to make class Shape abstract void add(Point p); // add p to points virtual void draw_lines() const; // simply draw the appropriate lines private: vector<Point> points; // not used by all shapes Color lcolor; // line color Line_style ls; // line style Color fcolor; // fill color // … prevent copying … }; Stroustrup/Programming

Display model completed attach() Stroustrup/Programming Circle draw_lines() Text draw_lines() Window Display Engine draw() draw() draw() our code make objects wait_for_button() Shape Shape draw_lines() draw_lines()

Language mechanisms Most popular definition of object-oriented programming: OOP == inheritance + polymorphism + encapsulation Base and derived classes // inheritance struct Circle : Shape { … }; Also called “inheritance” Virtual functions // polymorphism virtual void draw_lines() const; Also called “run-time polymorphism” or “dynamic dispatch” Private and protected // encapsulation protected: Shape(); private: vector<Point> points; Stroustrup/Programming

A simple class hierarchy We chose to use a simple (and mostly shallow) class hierarchy Based on Shape Rectangle Image Stroustrup/Programming Shape Polygon Text Open_polyline Ellipse Circle Marked_polyline Closed_polyline Line Mark Lines Marks Axis Function

Object layout The data members of a derived class are simply added at the end of its base class (a Circle is a Shape with a radius) Stroustrup/Programming points line_color ls Shape: points line_color ls ---------------------- r Circle:

Benefits of inheritance Interface inheritance A function expecting a shape (a Shape& ) can accept any object of a class derived from Shape. Simplifies use sometimes dramatically We can add classes derived from Shape to a program without rewriting user code Adding without touching old code is one of the “holy grails” of programming Implementation inheritance Simplifies implementation of derived classes Common functionality can be provided in one place Changes can be done in one place and have universal effect Another “holy grail” Stroustrup/Programming

Access model A member (data, function, or type member) or a base can be Private, protected, or public Stroustrup/Programming

Pure virtual functions Often, a function in an interface can’t be implemented E.g. the data needed is “hidden” in the derived class We must ensure that a derived class implements that function Make it a “pure virtual function” ( =0 ) This is how we define truly abstract interfaces (“pure interfaces”) struct Engine { // interface to electric motors // no data // (usually) no constructor virtual double increase(int i) =0; // must be defined in a derived class // … virtual ~Engine(); // (usually) a virtual destructor }; Engine eee; // error: Collection is an abstract class Stroustrup/Programming

Pure virtual functions A pure interface can then be used as a base class Constructors and destructors will be describe d in detail in chapters 17-19 Class M123 : public Engine { // engine model M123 // representation public: M123(); // construtor: initialization, acquire resources double increase(int i) { /* … */ } // overrides Engine ::increase // … ~M123(); // destructor: cleanup, release resources }; M123 window3_control; // OK Stroustrup/Programming

Technicality: Copying If you don’t know how to copy an object, prevent copying Abstract classes typically should not be copied class Shape { // … private: Shape(const Shape&); // don’t copy construct Shape& operator=(const Shape&); // don’t copy assign }; void f(Shape& a) { Shape s2 = a; // error: no Shape copy constructor (it’s private) a = s2; // error: no Shape copy assignment (it’s private) } Stroustrup/Programming

Technicality: Overriding To override a virtual function, you need A virtual function Exactly the same name Exactly the same type struct B { void f1(); // not virtual void f2(char); void f3(char) const; void f4(int); }; Stroustrup/Programming struct D : B { void f1(); // doesn’t override void f2(int); // doesn’t override void f3(char); // doesn’t override void f4(int); // overrides };

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14 class design (1)

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