CS 376b Introduction to Computer Vision 01 / 23 / 2008 Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 376b - Spring 2008 Today’s Topics More C++ –similarities to Java, operators –arrays –objects in C++ vs. Java references –pointers, references –struct, class –pass by reference

C++ Like Java, –C++ has for, while and do-while loops, –C++ has if's and else's –C++ has switch statements with cases –C++ has same arithmetic operators –C++ has same relational operators Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ >> and << when used on ints do a shift. int x = 10; x = x << 2; // shift left by 2 bits multiplies by 4 cout << x << endl; // will be 40 >> is a shift right (divides by 2 * right operand)‏ padding high bits with 0's? Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ More types –array similar to Java –int nums[30]; // array of 30 ints, index starts at 0 –int ages[] = {21, 18, 35, 52, 65}; –can't put brackets before the name (but in Java can)‏ Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ More types –objects a simple declaration in Java creates a reference to a class –Card a; // a is a Card reference a simple declaration in C++ creates an object of that class by calling the default constructor –string a; // actually calls string's default constructor string s1(“Hello”); // calls a different constructor string s2 = string(“Bye”); // also calls a constructor s1 = s2; // actually makes a COPY Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ More types –objects string s1(“Hello”); // calls a different constructor string s2 = string(“Bye”); // also calls a constructor s1 = s2; // actually makes a COPY s2 = “So long”; // doesn't affect s1 Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ More types –objects/references –In Java we can have: Card a; // a reference to a Card (does NOT call Card's default constructor)‏ Card b = new Card(2, 2); a = b; // a and b now both refer to the same object b.setRank(5); // the ONE object has rank 5 and both a and b refer to it –This is the typical behaviour in Java (except for Strings where = actually makes a “copy” of the data not an alias.) Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ More types –objects/references –In C++ we can have: Card &a; // a reference to Card (does NOT call Card's default constructor)‏ Card b = new Card(2, 2); a = b; // a and b now both refer to the same object b.setRank(5); // the ONE object has rank 5 and both a and b refer to it –This is the typical behaviour in Java (except for Strings where = actually makes a “copy” of the data not an alias.) –see example programs Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ a struct is a record in C++ and in C e.g. struct PhoneNumber { int areaCode; // first 3 digits, e.g. 518 int exchange; // next 3 digits e.g. 580 int last4; // last 4 digits e.g } if you know structs from C, they are different in C++. In C++ they are essentially the same as classes except that by default a struct's members are public and by default a class's members are private. Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ classes in C++ –can have multiple inheritance –typically divided into header and implementation (usually in different files)‏ –class definitions end with a semicolon –divided into sections public: private: protected: –inline functions - code in the definition, for speed (one liners)‏ –header (interface) file vs. implementation file –prototypes of functions and classes must appear before use –extern (says variable is declared elsewhere and nothing more is allocated) –let's look at the Card.h and Card.cpp files Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ classes in C++ –destructors (class name preceded by ~, automatically called when object goes out of scope, usually put code in there for deallocating memory for data members if necessary)‏ –if you don't create them C++ automatically provides: default constructor (no parameters)‏ a copy constructor –provides shallow copy access –if you need deep copy, write your own a destructor –may need your own to release allocated memory –I just wanted to make you aware of these things now, we'll see more details later after understanding explicit memory alloc/dealloc. Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ pointers in C++ –a pointer is a variable that holds a memory address the size of a pointer is the size of a memory address for your system –the memory address stored in a pointer is typically but not always an address in the heap –a pointer can refer to different places during a program –a pointer can point to data structures that change in size or whose size is not known at compile-time –NULL == 0 is the value of a pointer that does not reference a memory location (doesn't point to a memory address)‏ –a pointer can be explicitly dereferenced to refer to the data at which the pointer points (use the * operator for dereference)‏ Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ pointers in C++ –a pointer to a class can refer to members of the class by using the pointer operator -> Card *c = new Card(3,3); // note use of new c->setRank(5); // same as (*c).setRank(5); // let's see this in code... –a pointer can be subscripted if it is pointing to an array –can do pointer arithmetic (again useful if pointing to an array), e.g. adding 1 to a pointer adds the size of the type to which the pointer points Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ pointers in C++ –from Budd's book: int i = 7; int j = 11; int *p = &i; // & is address of, point now points to I // p holds the address of i // i holds the value 7 *p = *p + j; // p is being dereferenced, i now has value 18 p = &j; // p now holds the address of j // let's put this in a program and print the values... Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ const is used to make something constant (like Java final)‏ e.g. const int x = 10; // like Java's final int x = 10; when used with pointers though be careful which is being declared constant (the pointer or the thing being pointed to)‏ from Budd's book p.33: –int i = 7; –const int *p = &i; // pointer to a constant –int * const q = &i; // constant pointer –const int * const r = &i; // a const pointer to a const –// let's put this in a program Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ void * –can be used to hold any type of pointer –think of Object references in Java –double d = 3.5; –double *dp = &d; –void *p = dp; // make p point to d (the thing dp points to)‏ –double dp2; –dp2 = *((double *)p); // cast p to be a double pointer and deref –// can't simply do: dp2 = *p; –// let's put this in code Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ function pointers –double fdiv(int i, int j)‏ –{ return i / (double) j; –} –double (*fptr) (int, int); // fptr is a pointer to a function whose –// arguments are two ints and has a return type of double –// can assign to fptr the address of a function that has those –// properties –fptr = &fdiv; Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ function pointers –useful when a function can be generically written to take a pointer to a function which it then calls internally –see qsort example in Budd p. 34 Michael Eckmann - Skidmore College - CS 376b - Spring 2008

C++ references used as parameters (for pass by reference)‏ –use & in parameter –no need to do anything else –void passByRef(int &i)‏ –{ i++; –} –int x = 7; –passByRef(x); –// x is now 8 –// write program with Card functions (pass by ref and val)‏ Michael Eckmann - Skidmore College - CS 376b - Spring 2008