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C++ Overview, Part II (continued)

C++ Overview, Part II (continued). Reference: Chapter 2, Sections 2.2-2.9. CMSC 202 1. Function Calls and Argument Passing.

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C++ Overview, Part II (continued)

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  1. C++ Overview, Part II(continued) Reference: Chapter 2, Sections 2.2-2.9 CMSC 202 1

  2. Function Calls and Argument Passing • Passing by value vs. passing by reference (address) • C example • int a = 5, b = 10; • swap(&a, &b); // function call • void swap(int* a, int* b) • { • int temp = *a; • *a = *b; • *b = temp; • } CMSC 202 2

  3. Function Calls and Argument Passing (continued) • C++ supports reference parameters • C++ example • int a = 5, b = 10; • swap(a, b); // function call -- no addresses! • void swap(int& a, int& b) • { • int temp = a; • a = b; • b = temp; • } • No dereferencing of formal parameters is necessary CMSC 202 3

  4. Inline Functions • A function can have the code for its body included with • its prototype • Example • inline int max(int a, int b) {return(a > b ? a : b);} • Each function call is expanded by the compiler • Increases code efficiency • No guarantee that the compiler will do the expansion • Use only for very small (single statement) functions CMSC 202 4

  5. Command-line Arguments • The main function can receive arguments when invoked • Implemented as follows • int main(int argc, char *argv[]) • where argc = number of command line arguments, • including command line name (default = 1) • argv[0] = pointer to command line name as string • argv[n] = pointer to command line argument as string • Example • lowercase input.dat output.dat // lowercase program in text CMSC 202 5

  6. Vector Class Example // File: Vector.h class Vector { public: Vector() {} // default constructor Vector(float a, float b); // constructor overloaded Vector operator-(Vector a); // operating overloading float inner(Vector a); // vector inner product int nonzero(); // host vector nonzero void display(); // displays host vector private: float x; // vector x-coordinate float y; // vector y-coordinate }; CMSC 202 6

  7. Vector Class Example (continued) // File: Vector.C #include “Vector.h” #include <iostream.h> Vector::Vector(float a, float b) { // constructor overloaded x = a; y = b; } Vector Vector::operator -(Vector a) { // operator overloading Vector tmp; tmp.x = x - a.x; tmp.y = y - a.y; return(tmp); } CMSC 202 7

  8. Vector Class Example (continued) float Vector::inner(Vector a) { return(x * a.x + y * a.y); } int Vector::nonzero() { return(x != 0.0 || y != 0.0) } void Vector::display() { cout << “(“ << x << “, “ << y << “)”; } CMSC 202 8

  9. Default Constructors • A constructor with no arguments is called a default • constructor. • Usually used to set the attributes equal to default values • Vector::Vector() { • a = b = 0.0; • } • If a class has no constructors, a default constructor that • does nothing is supplied automatically. • If a class defines any constructor, no default constructor • is automatically supplied. The appropriate user-supplied • constructor will be invoked upon object instantiation. CMSC 202 9

  10. Operator Overloading • Predefined operators can be extended to operate on different • types of operands. This is called operator overloading. • Some consider this a type of polymorphism • Example • Vector Vector::operator -(Vector a) { // overloading minus • Vector tmp; • tmp.x = x - a.x; tmp.y = y - a.y; • return(tmp); • } • The minus operator now works with Vector objects: • u = v.operator - (w); OR u = v - w; • where u, v, and w are all of type Vector CMSC 202 10

  11. C++ I/O Streams • fstream is a predefined class in <fstream.h> • fstream represents a stream I/O class • General format • fstream stream-name(disk-filename, mode); • Examples • fstream inData(“mydata”, ios::in); • fstream outData(“results”, ios::out); • inData >> x; // read a value from input file • outData << x; // send value to output file • inData.close(); // done by default on some systems • outData.close(); // done by default on some systems CMSC 202 11

  12. Error Handling • Catch all errors that you can! • cerr is a predefined output stream used for error messages • Example • fstream myin(file, ios::in); • if (myin.fail()) { // fail is a member function of fstream • cerr << “Can’t open “ << file << ‘\n’; • exit(1); // terminates execution • } • Usually want to terminate execution and give the user a • meaningful error message for debug purposes. Use exit • function from <stdlib.h>. CMSC 202 12

  13. Student Exercise • Copy the code for program lowercase.C (pp. 67-68) from • the diskette in your text. • Make the following correction before running: • cerr << “Usage: lower infile outfile\n”; //incorrect • cerr << “Usage: “ << argv[0] << “ infile outfile\n”; • Create an appropriate input file. • Run the program with no arguments, one argument • (error), two arguments, and three arguments (error). CMSC 202 13

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