C++ I/O and Other Topics

C++ I/O and Other Topics

Using C++ Stream I/O • Default input stream is called cin • Default output stream is called cout • Use the extraction operator >> with cin • Use the insertion operator << with cout • We will see later that cin and cout are actually objects

Using cin • integers and floats • cin >> i1 >> f1; // skips over whitespace • characters • cin >> ch1 >> ch2; // skips over whitespace • To not skip whitespace, use cin.get: • ch = cin.get(); // reads any character

Using cin (con’t) • strings (char*) • cin >> str; // reads until first whitespace • Better to use cin.getline: • cin.getline(str, buffer_size, end_char); • where buffer_size = size of str, including ‘\0’ • end_char = character to stop at • Reading stops when end_char is encountered, the end-of-file is encountered, or when buffer_size-1 has been exceeded. ‘\n’ is the default for end_char. Automatically appends ‘\n’ to the string.

Using cout • integers and floats • cout << i1 << f1; // displayed in default format • characters • cout << ch; // single character displayed • strings (char*) • cout << str; // displays up to ‘\0’ character

General File I/O Steps • Declare a file name variable • Associate the file name variable with the disk file name • Open the file • Use the file • Close the file

C++ File I/O • Declare a file name variable • #include <fstream.h> • ifstream input_filename_var; // input file • ofstream output_filename_var; // output file

C++ File I/O (con’t) • Associate the file name variable with the disk file name and open it • input_filename_var.open(“pathname/filename”, ios::in); • output_filename_var.open(“pathname/filename”, ios::out); • where ios::in and ios::out are optional • Files may be opened in other modes such as ios::app (append) and ios::binary (binary input or output)

C++ File I/O (con’t) File name declaration and opening/association may be combined: ifstream input_filename_var(pathname/filename, ios::in); ofstream input_filename_var(pathname/filename, ios::out); where ios::in and ios::out are optional

C++ File I/O (con’t) • Use the file • Use an input file as you would use the cin input stream • ifile1 >> x >> y; // x and y are integers • ifile2 >> ch; // ch is a char • ch = ifile3.get(); // ch is a char • ifile4.getline(buffer, buffer_size) // buffer is char*

C++ File I/O (con’t) • Use an output file as you would use the cout output stream • ofile1 << x << y; // x and y are integers • ofile2 << ch; // ch is a char • ofile3 << “Hi there!” << endl; // literal string • ofile4 << str; // str is a char*

C++ File I/O (con’t) • Close the file • input_filename_var.close(); • output_filename_var.close(); • All files are closed automatically upon termination of program execution, but it is a good habit to close them explicitly. Also, close them as soon as they are no longer needed by the program.

Checking That Files Have Opened Successfully • Always check that all files (input or output) have been successfully opened • If any file cannot be opened, send a message to the user and terminate program execution • ifstream myFile(“inputData”); • if (!myFile) { • cerr << “Input file could not be opened” << endl; • exit(1); // must #include <stdlib.h> to use exit function • }

Checking for End-of-File ifstream inFile(“somefile.txt”); . . . if (inFile) . . . OR while (inFile) . . . Checks to see if the last operation on inFile was successful. If yes, then the condition is true; otherwise, it is false. So, if the last operation was a read, then you are checking to see if the end-of-file has been reached.

Sample Program Using File I/O #include <fstream.h> #include <iostream.h> #include <stdlib.h> void main() { ifstream inFile(“numbers.dat”); int x; if (!inFile) { cerr << “Cannot open input file. Exiting.” << endl; exit(1); }

Sample Program (con’t) inFile >> x; while (inFile) { // OR while (inFile >> x) and inFile >> x; // omit the priming read cout << x << endl; } inFile.close(); }

Passing by Reference In C: swap(&x, &y); // call passes addresses void swap(int* x, int* y) { // ptrs receive addresses int temp; temp = *x; // dereference pointer *x = *y; // dereference pointers *y = temp; // dereference pointer }

Passing by Reference (con’t) In C++: swap(x, y); // just pass variable void swap(int& x, int&y) { // x and y are references int temp; temp = x; // no dereferencing x = y; // no dereferencing y = temp; // no dereferencing }

Constants #define PI 3.14 Preprocessor replaces all occurrences of PI with the value 3.14. #define actually defines a macro. const int PI = 3.14; Is not replaced by the processor. Value cannot be changed -- runtime error.

Other C++ Topics • Can us endl in place of ‘\n’ • Variables can be declared anywhere in C++ code (but pick a logical place) • Output can be formatted by using stream manipulators (e.g., setw, setprecision) (Ch. 11.6) • Namespaces - now part of ANSI C++ (Ch. 21)

Other Things to be Aware of (both C and C++) • in-line functions • avoids a function call by “advising” the compiler to generate a copy of the function in place of each call • faster execution, but larger object file • inline float area(int h, int w) { return h * w} • int main() . . .

Other Things to be Aware of (both C and C++) (con’t) • static storage class • scope • block • function • file • class (C++ only)

C++ I/O and Other Topics