1、Chapter 14 Graph class design,Bjarne S have discussed classes in previous lectures Here, we discuss design of classes Library design considerations Class hierarchies (object-oriented programming) Data hiding,2,Stroustrup/Programming,Ideals,Our ideal of program design is to represent the concepts of
2、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 whats
3、 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 cant make an object thats “just a Shape”,3,Stroustrup/Programming,Logically identical operations have the same name,For every class, draw_lines() does the
4、 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);,4,Stroustrup/Programming,Logically different operations have diffe
5、rent 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 windowWhy 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 i
6、t,5,(100,200),p1:,(200,300),p2:,attach(),add(),Stroustrup/Programming,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 12c.set_radius(c.radius()
7、*2); / double the radius (fine)c.set_radius(-9); / set_radius() could check for negative,/ but doesnt yet double r = c.radius(); / returns value of radiusc.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
8、interface Private for functions used only internally to a class,6,Stroustrup/Programming,What does “private” buy us?,We can change our implementation after release We dont expose FLTK types used in representation to our users We could replace FLTK with another library without affecting user code We
9、could provide checking in access functions But we havent 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
10、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,7,Stroustrup/Programming,“Regular” interfaces,Line ln(Point(100,200),Point(300,400); Mark m(Point(100,200), x); / display a s
11、ingle 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): Rectangle s1(Point(100,200),200,300); / width=200 height=300 Rectangle s2(Point(100,200),Point(200,300); / width=100 height=100Rectangle s3
12、(100,200,200,300);/ is 200,300 a point or a width plus a height?,8,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 doesnt try to d
13、o everything Our library aims at simplicity and small size for graphing data and for very simple GUI We cant define each library class and function in isolation A good library exhibits a uniform style (“regularity”),9,Stroustrup/Programming,Class Shape,All our shapes are “based on” the Shape class E
14、.g., a Polygon is a kind of Shape,10,Rectangle,Image,Stroustrup/Programming,Class Shape is abstract,You cant make a “plain” Shape protected:Shape(); / protected to make class Shape abstract For exampleShape ss; / error: cannot construct Shape Protected means “can only be used from this class or from
15、 a derived class” Instead, we use Shape as a base class struct Circle : Shape / “a Circle is a Shape”/ ;,11,Stroustrup/Programming,Class Shape,Shape ties our graphics objects to “the screen” Window “knows about” Shapes All our graphics objects are kinds of Shapes Shape is the class that deals with c
16、olor and style It has Color and Line_style members Shape can hold Points Shape has a basic notion of how to draw lines It just connects its Points,12,Stroustrup/Programming,Class Shape,Shape deals with color and style It keeps its data private and provides access functionsvoid set_color(Color col);C
17、olor color() const;void set_style(Line_style sty);Line_style style() const;/ private:/ Color line_color;Line_style ls;,13,Stroustrup/Programming,Class Shape,Shape stores Points It keeps its data private and provides access functionsPoint point(int i) const; / read-only access to points int number_of
18、_points() const; / protected:void add(Point p); / add p to points/ private: vector points; / not used by all shapes,14,Stroustrup/Programming,Class Shape,Shape itself can access points directly:void Shape:draw_lines() const / draw connecting lines if (color().visible() ,15,Stroustrup/Programming,Cla
19、ss 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 de
20、pending on the type of shape / e.g. Text, Circle, Closed_polyline/ reset the color and style to their old values ,16,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) F
21、l_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 stylefl_line_style(ls.style(),ls.width();draw_lines(); / call the appropriate draw_lines() / a “virtual call”/ here is what is specific for
22、a “derived class” is donefl_color(oldc); / reset color to previous fl_line_style(0); / (re)set style to default ,17,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 c
23、lass Text void draw_lines() const /* draw the Text */ Circle, Text, and other classes “Derive from” Shape May “override” draw_lines(),18,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()vir
24、tual void move(int dx, int dy); / move the shape +=dx and +=dyvoid set_color(Color col); / color accessint color() const;/ style and fill_color access functions Point point(int i) const; / (read-only) access to pointsint number_of_points() const; protected:Shape(); / protected to make class Shape ab
25、stractvoid add(Point p); / add p to points virtual void draw_lines() const; / simply draw the appropriate lines private:vector points; / not used by all shapesColor lcolor; / line colorLine_style ls; / line styleColor fcolor; / fill color/ prevent copying ;,19,Stroustrup/Programming,Display model co
26、mpleted,20,attach(),Stroustrup/Programming,Language mechanisms,Most popular definition of object-oriented programming: OOP = inheritance + polymorphism + encapsulationBase and derived classes / inheritance struct Circle : Shape ; Also called “inheritance”Virtual functions / polymorphism virtual void
27、 draw_lines() const; Also called “run-time polymorphism” or “dynamic dispatch”Private and protected / encapsulation protected: Shape(); private: vector points;,21,Stroustrup/Programming,A simple class hierarchy,We chose to use a simple (and mostly shallow) class hierarchy Based on Shape,22,Rectangle
28、,Image,Stroustrup/Programming,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),23,Stroustrup/Programming,Benefits of inheritance,Interface inheritance A function expecting a shape (a Shape&) can accept any object of a
29、 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 f
30、unctionality can be provided in one place Changes can be done in one place and have universal effect Another “holy grail”,24,Stroustrup/Programming,Access model,A member (data, function, or type member) or a base can be Private, protected, or public,25,Stroustrup/Programming,Pure virtual functions,O
31、ften, a function in an interface cant 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
32、to electric motors/ no data/ (usually) no constructorvirtual 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,26,Pure virtual functions,A pure interface ca
33、n then be used as a base class Constructors and destructors will be describe d in detail in chapters 17-19Class M123 : public Engine / engine model M123/ representation public:M123(); / construtor: initialization, acquire resourcesdouble increase(int i) /* */ / overrides Engine :increase/ M123(); /
34、destructor: cleanup, release resources ;M123 window3_control; / OK,Stroustrup/Programming,27,Technicality: Copying,If you dont know how to copy an object, prevent copying Abstract classes typically should not be copiedclass Shape / Shape(const Shape / error: no Shape “copy assignment” (its deleted)
35、,Stroustrup/Programming,28,Prevent copying C+98 style,If you dont know how to copy an object, prevent copying Abstract classes typically should not be copiedclass Shape / private:Shape(const Shape / error: no Shape “copy assignment” (its private) ,Stroustrup/Programming,29,Technicality: Overriding,T
36、o override a virtual function, you need A virtual function Exactly the same name Exactly the same typestruct B void f1(); / not virtualvirtual void f2(char);virtual void f3(char) const;virtual void f4(int); ;,Stroustrup/Programming,30,struct D : B void f1(); / doesnt overridevoid f2(int); / doesnt o
37、verridevoid f3(char); / doesnt overridevoid f4(int); / overrides ;,Technicality: Overriding,To override a virtual function, you need A virtual function Exactly the same name Exactly the same typestruct B void f1(); / not virtualvirtual void f2(char);virtual void f3(char) const;virtual void f4(int);
38、;,Stroustrup/Programming,31,struct D : B void f1() override; / errorvoid f2(int) override; / errorvoid f3(char) override; / errorvoid f4(int) override; / OK ;,Technicality: Overriding,To invoke a virtual function, you need A reference, or A pointerD d1; B / calls D:f4(2) on d1 since bptr points to a D,Stroustrup/Programming,32,Next lecture,Graphing functions and data,33,Stroustrup/Programming,
