Assignment 2 Visitor pattern.

In this assignment, we’ll look at the Visitor pattern. We’ll be developing a program to “draw” shapes. The program can also save shapes to a file & re-create them from the file using a simple factory.
1 Shapes
We’ll use an abstract Shape class with 3 concrete derived classes: Circle, Rectangle & Triangle. These classes in turn use a Point class. The Shape class contains pure virtual methods to “draw” & save a shape:
class Shape { // ABC public: virtual ~Shape() {} virtual void draw() const = 0; virtual void save(std::ostream& = std::cout) const = 0; // see section 2 for an additional method };
The 3 concrete derived classes implement the above pure virtual methods together with some additional methods. (See provided files). The following illustrate the output of the draw method:
[C: (1,-2), 3] [R: (2,-3), (4,-5)] [T: (5,-6), (7,-8), (2,1)]
Note that the output is written to standard output. The first line indicates a circle with center (1,-2) & radius 3. The second line indicates a rectangle with 2 corners at (2,-3) & (4,-5). The last line indicates a triangle with the 3 vertices (5,-6), (7,-8) & (2,1). When the above 3 shapes are saved to a file (using the save method), the content is as follows:
circle (1,-2) 3 rectangle (2,-3) (4,-5) triangle (5,-6) (7,-8) (2,1)
Make sure your output format for draw & for save match exactly the formats shown above.
2 The Visitor Pattern
The various shape classes demonstrate a classic OO design with concrete derived classes implementing or overridng virtual functions in an abstract base class. However, there is a disadvantage to this design — if ever we need to add a new operation on shapes, we’ll need to add new methods in all the different shape classes. We may, for example, want to add a “translate” operation that moves a shape by a certain amount. To do this, we’ll need to add translate methods to Shape & its derived classes. This will necessitate the recompilation of all the affected classes. The Visitor pattern solves this problem. To make it easier to add new operations, each of the shape classes has a method named accept that accepts a visitor (of class ShapeVisitor in our case). When a “shape visitor” visits a shape object, it performs operations on that object (e.g. translate it). Different types of “shape visitors” (deriving from ShapeVisitor) perform different operations on shapes. The ShapeVisitor class has different “visit” methods to visit different types of shapes:
1
Winter ’16 class ShapeVisitor { public: virtual ~ShapeVisitor() {} virtual void visitCircle(Circle *s) = 0; virtual void visitTriangle(Triangle *s) = 0; virtual void visitRectangle(Rectangle *s) = 0; // ... more methods if there are more types of shapes }; In the above, the “visit” methods are pure virtual. Concrete derived classes need to implement these methods to perform specific operations on circles, triangles & rectangles. Each of the shape classes has an accept method that accepts a ShapeVisitor. This method is basically declared as: virtual void accept(ShapeVisitor&); In this assignment, we’ll have 3 types of “shape visitors”: TranslationVisitor, XReflectionVisitor & YReflectionVisitor. They all inherit from ShapeVisitor. A TranslationVisitor translates a shape by a certain “amount” (the displacement, which technically is a vector but which we’ll represent as a Point) when it visits the shape. Each point in the shape is translated by the same amount. As an example, if the displacement is (1,2), the point (3,4) is translated to (4,6). This is basically addition of points. An XReflectionVisitor reflects a shape about the x-axis when it visits the shape. This means that each point in the shape is reflected about the x-axis. Under this type of reflection, the point (1,2), for example, is mapped to (1,-2) (i.e. the x-coordinate remains the same, the y-coordinate is negated). Analogously, a YReflectionVisitor reflects a shape about the y-axis when it visits the shape, i.e., each point in the shape is reflected about the y-axis. For example, the point (1,2) is mapped to (-1,2) (i.e. the y-coordinate remains the same, the x-coordinate is negated). 3 The Shape Factory To read shapes stored in a file, we use a ShapeFactory class: class ShapeFactory { public: explicit ShapeFactory(std::istream& in); Shape* create(); // ... }; The create method reads the type name of a shape from its associated input stream & then calls the appropriate constructor. Each of the classes Circle, Triangle & Rectangle has a constructor that takes an input stream as an argument. (See provided header files.) This constructor reads the appropriate data members from the input stream. create returns the null pointer on failure. 4 The Main Program The program must be invoked with the name of a data file as a command-line argument. This file may be non-existent. But if it does exist, it must be a file containing shape data (written by the save method). In that case, the program reads in the shapes, stores & draws them & then waits for user input after displaying a prompt () to standard error. If the specified file does not exist, the reading & drawing of shapes on startup is skipped. 2
Winter ’16 The user can repeatedly issue commands until the end-of-file key is entered. When end-of-file is encountered, the program saves all its current objects to the file specified on the command-line. If the file did not exist before, it is created. If the file did exist, its content is now completely overwritten. The program then exits. The following shows an interaction with the program. Each line that starts with a ‘’ is a command issued by the user. (The ‘’ is the prompt printed by the program to standard error.) The other lines are the output of the program (written to standard output). The output assumes that there are no shapes at the start: c circle (1,-2) 3 # create circle with specified centre & radius [C: (1,-2), 3] c rectangle (2,-3) (4,-5) [C: (1,-2), 3] [R: (2,-3), (4,-5)] c triangle (5,-6) (7,-8) (2,1) [C: (1,-2), 3] [R: (2,-3), (4,-5)] [T: (5,-6), (7,-8), (2,1)] x # reflect about x-axis [C: (1,2), 3] [R: (2,3), (4,5)] [T: (5,6), (7,8), (2,-1)] y # reflect about y-axis [C: (-1,2), 3] [R: (-2,3), (-4,5)] [T: (-5,6), (-7,8), (-2,-1)] t (10,-10) [C: (9,-8), 3] [R: (8,-7), (6,-5)] [T: (5,-4), (3,-2), (8,-11)] d # draw all shapes [C: (9,-8), 3] [R: (8,-7), (6,-5)] [T: (5,-4), (3,-2), (8,-11)] Note that the program processes user input line by line & for each line, it just reads in sufficient information to carry out the command & ignores the rest of the line. This is shown in some of the commands above. As can be seen, c is used to create & add a shape, x, y & t are used to transform all current shapes & d is for drawing all shapes. Note the “arguments” to the c & t commands. Invalid commands (including those that have invalid arguments) should be ignored — no error messages should be printed. Sample input/output files will be provided. Make sure your program output matches exactly those in the output files provided. 5 Submission This assignment is due at 11pm, Friday, April 1, 2016. Submit a zip file to “Share In” in the directory: \COMP\3512\a2\set<X\ where <X is your set. Your zip file should be named <name id.zip, where <name id is your name & student ID separated by an underscore (for example, SimpsonHomer a12345678.zip). Do not use spaces in your zip file name. Your zip file must unzip directly to your source files without creating any directories. We’ll basically compile your files using 3
Winter ’16 g++ -std=c++11 -W -Wall -pedantic *.cpp after unzipping. Do not submit rar files. They will not be accepted. If you need to submit more than one version, name the zip file of each later version with a version number after your name, e.g., SimpsonHomer a12345678 v2.zip. If more than one version is submitted, we’ll only mark the version with the highest version number. (If it is unclear to us which version to mark, you may fail to get credit for the assignment.) This assignment will be marked mainly based on testing the required features. In order to be able to test your program at all, it must at least be able to load shapes from a file & draw them. Your program must compile without errors or warnings using the command shown above. If that is not the case, you may receive a score of 0 for the assignment. Otherwise, the grade breakdown is approximately as follows: Code clarity 10% Reading/writing shapes from/to files† 20% Drawing shapes 15% Translation? 15% x-reflection? 15% y-reflection? 15% Error-handling 10% †requires ShapeFactory for reading ?requires user input, draw & visitor