C/C++ Coursework Module. Ian Reid, Trinity 2008

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1 C/C++ Coursework Module Ian Reid, Trinity 2008 The course takes you through a series of exercises designed to familiarise you with design and implementation issues in C++ programming. Objectives: Design and implementation of C++ programs. Functions, classes, recursion, inheritance, graphics,... References C/C++ course/lecture notes (IDR) ian/teaching/c++ Lipmann and Lajoie (3rd edition) Stroustrup (3rd edition) Pohl Structure: Exercises learning by example (Software-lab), Mini-lectures (9am, and 12noon or 2pm) Discussions design in small groups Project 3D graphical computer game (maybe?) 1 2 Introduction Interpreted language Source code Algorithms Data structures.m Interpreter Interpreter: a program that reads source line by line and carries out the action specified by the line of code Programs Interpreted language: Procedural programming: design focuses on algorithms e.g. matlab, C, Pascal, Fortran Object oriented programming: design focuses on data structures e.g. C++, Java good run-time error handling and identification rapid prototyping slow e.g matlab, various scripting languages 3 4

2 C/C++ compilation Source code.c,.c,.cc Compilation Object files.o Linking A simple program in matlab %% function to compute sum from 1 to n function res = sum(n) if (n <= 0) res = 0; else res = 0; for i=1:n res = res + i; end end Executable Example: CC -c prog.c CC -c main.c CC prog.o main.o -o myprog n = 0; n = input( Enter number: ); result = sum(n); printf( The sum is %d\n, result); 5 6 A simple program in C #include <iostream.h> // function to compute sum from 1 to n int sum(int n) if (n <= 0) return 0; int tot=0; for (int i=1; i<=n; i++) tot += i; return tot; int main(int argc, char *argv[]) int n=0; cout << "Enter number: "; cin >> n; int result = sum(n); cout << "The sum is " << result << endl; exit(0); Notes: function, arguments, main, control flow (for, if), input/output, compiler directives and standard libraries Variables A variable is associated with a memory location for storage of program data which can be retrieved and possibly modified during program execution. C/C++ are strongly typed languages the type of variables must be declared before use Choose variable name to be meaningful, without being overly verbose, e.g. counter, InputFile, window height A variable declaration: e.g. int num = 0; or int num(0); 7 8

3 Data types Classes Atomic types predefined in C/C++: bool, char, short, int, long, float, double Examples: c 40 \n \0 [char, 1 byte] x [int, 4 bytes] e [float, 4 bytes] C++ provides mechanisms for building up compound data structures; in particular classes and arrays A class is a compound data structure which encapsulates related data into a single data structure. class Complex double re, im; ; Members are accessed using the. operator Complex z, w1, w2; cout << z.re << endl; w1.re = 2.0*w2.re; w1.im = 2.0*w2.im; 9 10 Arrays An array is a data structure containing a numbered (indexed) collection of items of a single data type: int a[10]; res = a[0] + a[1] + a[4]; Complex zarray[20]; cout << zarray[5].re << endl; First index is always 0 The dimension must be a constant, and is fixed for the lifetime of the variable const int size = 3; float x[size]; or float x[] = 0.0, 1.0, 2.0; Multi-dimensional arrays e.g. double d[10][5]; has elements d[0][0]... d[0][4].. d[9][0]... d[9][4] Stored in the order of right-most index varying most rapidly. Note that C/C++ provides no inbuilt run-time checking of array bounds, so beware at best the program will crash (with something like Segmentation fault ), or worse, nonsense will result and be very difficult to trace

4 Methods In C++ a class encapsulates related data and functions. A class member function is sometimes called a method in the object-oriented programming literature. class Complex double re, im; ; double Mag() return sqrt(re*re + im*im); double Phase() return atan2(im, re); Methods are also accessed using the. operator Complex z; cout << "Magnitude=" << z.mag() << endl; cout << "Phase=" << z.phase() << endl; Constructor Whenever a variable is declared, space is allocated for it and it is possibly initialised. In general this is done by a constructor In C++ a user defined class can supply its own constructor. The compiler can supply one by default, but it is useful (required) for automatic initialisation of data members, or if the class requires some dynamic memory to be allocated. The constructor is a function with the same name as the class Complex(double x=0.0, double y=0.0) re = x; im = y; Example Data hiding / Encapsulation In procedural programming we want the functions to behave as black boxes with well defined interfaces. Avoiding side-effects means that we can use the functions as building blocks to create programs. In object-oriented programming we take this further and a class becomes a black box data structure, which has a public interface to private data. The outside program can only access the class through its well defined interface, minimising side-effects. class Complex Complex(double x=0.0, double y=0.0) re=x; im=y; double Re() return re; double Im() return im; double Mag() return sqrt(re*re + im*im); double Phase() return atan2(im, re); double re, im; ; The outside program can only access the private members through the public methods: Complex z(2.0, 5.2); cout << z.re() << endl; Re() is an accessor method

5 Data hiding ctd By encapsulating the representation of Complex in this way, the internal representation could change providing the interface remains the same For example, we could change the internal representation to polar coordinates class Complex Complex(double x=0.0; doubley=0.0) r = sqrt(x*x + y*y); theta = atan2(y,x); double Re() return r*cos(theta); double Im() return r*sin(theta); double Mag() return r; double Phase() return theta; double r; // magnitude double theta; // phase ; Functions and methods You should know how to program a simple function to take some values and return a result. We now discuss a number of more advanced function concepts: Scope Function prototype Class scoping Function call implementation Pass-by-value vs pass-by-reference parameters Function overloading Operators and operator overloading The interface of this class to the program remains unchanged Function prototype Scope A variable declared outside all functions is global. It can be accessed by any function. A variable declared inside a function is local to the function. It cannot be seen outside the function in which it is declared. If there exists a global variable of the same name then the local one takes precedence. It is redeclared/reallocated every time the function is called, so it does not remember its value from the last call. Function parameters are effectively local variables. 19 The function prototype provides enough information to the compiler so that it can check that it is being called correctly..h file: float exp(float x);.cc file: float exp(float x) const float precision = 1.0e-6; float term=1.0, res=0.0; int i=0; while (fabs(term)>precision) res += term; i++; term = pow(x,i)/factorial(i); The implementation of the function exp is in the.cc file, or even in a library somewhere. The prototype encapsulates the function interface and hides the implementation details. 20

6 Class scoping Class definitions can become cluttered with a lot of code if all methods are defined within them. Function call implementation Alternatively the class definition can contain a function prototype and the function declared elsewhere. We need a way of indicating that a given function implementation belongs to a particular class. This is done using the class scoping operator :: Memory CODE SEGMENT DATA SEGMENT machine code global variables.h file: class Complex double Mag();.cc file: double Complex::Mag() return sqrt(re*re + im*im); 21 STACK local variable m local variable 1 return location return value parameter n parameter 1 activation record 22 Parameter passing: rules and conventions Pass by value/reference int main() int i=5, j=10; swap(i, j); cout << i << j << endl; Pass by value Pass by reference The defaults in C/C++ are: atomic types: pass-by-value (i.e. copy) classes: pass-by-value (i.e. copy) arrays: pass-by-reference (i.e. location) Use pass-by-reference for an atomic type or a class if: void swap(int a, int b) int temp = a; a = b; b = temp; return; void swap(int& a, int& b) int temp = a; a = b; b = temp; return; modifications to the arguments must be preserved a large object must be passed the time and space costs to allocate and copy a large object onto the stack are often too great for real applications temp return location b = value of j a = value of i temp return location location of j location of i If using pass-by-reference for the latter and the data will not change, then use the const keyword void func(int x) void func(int &x) void func(const int &x) 23 24

7 Function overloading C++ allows several functions to share the same name to allow for different argument types. Example: double x, y;. y = exp(x); Complex w, z; z = exp(w); The two can be seen to be distinct names if we consider the argument types as part of the name. The same is true for class member functions. Function overloading ctd We can define exp for the Complex class as follows: #include <math.h> Complex exp(complex z) r = exp(z.re()); Complex zout(r*cos(z.im()), r*sin(z.im()); return zout; Operators/Expressions Arithmetic + * / - % (mod) << >> (shift left, right) Relational ==!= < > <= >= Boolean &&! They can be combined with the usual rules of precedence The precedence rules can be modified using round brackets ( ) Operator overloading An operator in C++ is just a function treated specially i.e. a + b is equivalent to operator+(a, b) Operators can be overloaded in the same way as functions: Complex operator+(complex z1, Complex z2) Complex zout(z1.re() + z2.re(), z1.im() + z2.im()); return zout; 27 28

8 Operator overloading, ctd Standard C/C++ does no checking on array bounds at run-time and this can result in catastrophic program crashes (one reason for the dreaded Segmentation Fault error is trying to access an array beyond its size). We can overload the [] operator to create a special array class that does does bounds checking: class MyArray float operator[](int i) if (i<0 i>=10) std::cerr << "Access out of bounds\n"; exit(1); else return a[i]; float a[10]; ; Advanced topics Inheritance Destructors Friend functions Templates Standard Template Library Recursive functions Lots of others not covered in this course MyArray x; Inheritance C++ allows programmers to build hierarchies of classes. An inherited or derived class is one which has all the functionality of the its parent, but with some extra data or methods. Inheritance, ctd Example class Window data: width, height posx, posy methods: raise(), hide() select(), iconify() Example: class GBall : public Ball class TextWindow data: methods: cursor_x, cursor_y backspace(), delete() clear() class GraphicsWindow data: background_colour Methods: clear(), fill() void Draw(); A GBall is identical to a Ball (same data and methods) with the addition of a method Draw which knows how to render a Ball onto a graphical window. class InteractiveGraphicsWindow data: Methods: MouseClick(), MouseDrag() 31 32

9 Destructor When an object ceases to exist (activation record is popped from the stack), the destructor is called. Like the constructor, the destructor has a default which the compiler can supply (this is what we have done throughout). The destructor has the same name as the class preceded by a : Recursion Recursion is the programming analogue of induction: if then p(0) and p(n) p(n + 1) p(n) n Define a function in terms of: Complex() Destructors are useful if memory has been dynamically allocated to the object (i.e. allocated at runtime). A destructor should ensure that this memory is freed up to the program. itself boundary conditions For example: Factorial: n! = n(n 1)!, 0! = Example: flood fill Consider a function to change the colour of a constant colour region to a new colour: const int SIZE=256; Bitmap im[size][size]; void fill(int x, int y, int old_colour, int new_colour) if (x>=0 && x<size && y>=0 && y<size) if (im[y][x]==old_colour) im[y][x] = new_colour; fill(x-1,y,old_colour,new_colour); fill(x+1,y,old_colour,new_colour); fill(x,y-1,old_colour,new_colour); fill(x,y+1,old_colour,new_colour); return; Templates Templating is a mechanism in C++ to create classes in which one or more types are parameterised. Recall our special array class, and suppose we wanted to store ints instead of floats. class MyArray float operator[](int i) if (i<0 i>=10) std::cerr << "Access out of bounds\n"; return 0.0; else return a[i]; float a[10]; ; 35 36

10 Templates ctd template <class Type> class MyArray Type operator[](int i) if (i<0 i>=10) std::cerr << "Access out of bounds\n"; return 0.0; else return a[i]; Type a[10]; ; MyArray<int> x; MyArray<double> y; MyArray<Complex> z; Templates ctd or even template <class Type, int N> class MyArray Type operator[](int i) if (i<0 i>=n) std::cerr << "Access out of bounds\n"; return 0.0; else return a[i]; Type a[n]; ; MyArray<int,10> x; MyArray<double,20> y; MyArray<Complex,5> z; Standard Template Library (STL) A number of data structures (container types) and algorithms that operate on them are so common that standard ways of representing and manipulating them have been developed. These comprise the Standard Template Library. Supports: stack (FILO structure) list (efficient insertion and deletion of elements) vector (extendible array) etc STL example std::vector<type> is an extendible array it can increase in size as the program needs it to it can be accessed just like an ordinary array (e.g. v[2]) it can tell you its current size (v.size()) you can add an item to the end without needing to know how big it is (v.push back(x)) Example: #include <vector> int main() std::vector<int> v; for (i=0; i<20; i++) v.push_back(i); for (i=0; i<v.size(); i++) std::cout << v[i] << std::endl; 39 40

11 STL example, ctd To create a new STL vector of a size specified at run-time: int size; std::vector<complex> z; std::cin >> size; z.resize(size); z[5] = Complex(2.0,3.0); or even std::vector< std::vector<float> > twodim; int width, height; twodim.resize(height); for (int i=0; i<height; i++) twodim[i].resize(width); twodim[2][3] = 10.0; Notes on style Write code which makes the meaning of each operation and each data structure clear. Use meaningful variable names Use comments to supplement the meaning Indent code for each block/loop Encapsulate groups of statements sensibly in functions Encapsulate related data sensibly in data structures Design top down Code bottom up Project notes The course takes you through a series of exercises designed to familiarise you with design and implementation issues in C++ programming. This should take at most two and half to three days, possibly less. After this you then have a loose specification for a labyrinth 3D graphical computer game. You have the remainder of the course to set about implementing and refining this. Write down what you hope to achieve before you start off Design your data structures before you start coding Don t be afraid to discard old ideas in favour of new ones during the life-cycle of the development, but keep old versions Discuss ideas with the demonstrators 43