C++ IO All I/O is in essence, done one character at a time - PDF document

C++ IO C++ IO  All I/O is in essence, done one character at a time  Concept: I/O operations act on For : COP 3330. streams (sequences) of ASCII Object oriented Programming (Using C++) characters ht t p: / / www. com pgeom . com / ~pi yush/ t each/ 3330 Piyush Kumar Interactive I/O C++ IO #include <iostream>  cout standard output stream sequence of characters printed to the monitor output data input data  cin standard input stream executing Keyboard Screen sequence of characters input program from the keyboard cin cout  both cout and cin are data objects and are defined as classes ( type istream ) ( type ostream ) class class Example Example Namespaces: They provide a way to avoid name collision. Ostream object named cout. Be careful about using this. #include <iostream> #include <iostream> using namespace std; using namespace std; Standard IO library for C++. Defines two int main(void){ int main(void){ fundamental types, istream and ostream. cout << “Hello World” ; cout << “Hello World” ; cout << endl; cout << endl; return 0; return 0; Equivalent to: } } operator<< (cout, “Hello World”); Its calling a friend function of ostream with input data. Stream: A flow of characters (1 or 2 bytes long). Can flow in and out of Files, strings, etc. Uses function declaration (approx): ostream& operator<<( ostream&, const char * ) 1

Example Special Output Characters invokes a manipulator function called endl. endl looks something like this: ostream& endl( ostream& os) { os << '\n'; \n new line #include <iostream> os.flush(); \t tab return os; using namespace std; } \b backspace int main(void){ \r carriage return cout << endl; return 0; Equivalent Compiler statement: \ ʹ single quote std::cout.operator<<( } std::endl(std::cout) \ ʺ double quote ); \\ backslash Scope Operator for namespaces. Stream IO headers A Stream  iostream -- contains basic information  A flow of characters. required for all stream I/O operations  Buffers: IO to streams goes thru a  iomanip – used for performing formatted I/O buffer. C++ allows you change the with stream manipulators default behavior of associated buffers.  fstream – used for performing file I/O  State: Each stream is associated with operations a state indicating various things like if  stringtream -- used for performing in- an error has occurred or not… memory I/O operations (i.e., into or from strings in memory) C++ IO Class hierarchy C++ IO Class hierarchy ios ios istream ostream streambuf istream ostream streambuf cin cout ifstream istringstream ostringstream ofstream ifstream istringstream ostringstream ofstream stringbuf stringbuf filebuf filebuf 2

C++ IO Hierarchy Other Predefined Streams  The ios hierarchy defines the  cerr - the standard destination for interface of the IO system. error messages (often the terminal window). Output through this stream  The streambuf hierarchy defines the is unit-buffered, which means that implementation of the IO system, characters are flushed after each mostly provides the facilities of block of characters is sent. buffering and byte-level I/O  clog - like cerr, but its output is buffered. Formatting with predefined Stream IO streams. Inside ios  << (left-shift operator)  Remember: Due to inheritance, anything you learn about formatting IO  Overloaded as stream insertion operator with predefined streams (cin, cout, clog, cerr) also applies to file IO and  >> (right-shift operator) string IO.  Overloaded as stream extraction operator  Anything available or defined in the  Both operators used with cin , cout , ios class is available everywhere in cerr , clog , and with user-defined the IO subsystem. stream objects Example << operator  cin >> cin >> Variabl able;  << is overloaded to work on built-in  cout << cout << Varia riable; le; types of C++.  clog << clog << Variab riable; le;  Can also be used to output user-  Buffere Buffered defined types.  cerr << cerr << Varia riable; le;  Other interesting examples:  Unbuffered, Unbuffered, pr prints V ints Variable able  cout << ‘\n’; // newline. imme immediately diately.  cout << “1+2=” << (1+2) << endl;  Note: Note: Variable ty able types pes a are e availa ailable to ble to  cout << endl; // newline.  cout << flush; // flush the buffer. the compiler. the compile 3

<< operator Stream insertion: One char.  Associates from left to right, and returns a reference to  put member function its left-operand object (i.e. cout ). This enables  Outputs one character to specified stream cascading. cout.put(‘C');  Outputs “char *” type as a string.  Returns a reference to the object that called  If you want to print the address, typecast it to (void *). it, so may be cascaded  Example:  char name[] = “cop3330”; cout.put( ‘C' ).put( '\n' );  cout << name << static_cast<void *>( name ) << endl;  May be called with an ASCII-valued  static_cast<void *>( name ) equivalent to ((void *) expression name) in C except that it happens at compile time. cout.put( 65 ); • Outputs A Input Stream Input Stream : Looping  >> (stream-extraction) while (cin >> fname)  Used to perform stream input  Normally ignores whitespaces (spaces, tabs, newlines)  Returns zero ( false ) when EOF is encountered, “>>” returns 0 ( false ) when EOF otherwise returns reference to the object from which it was invoked (i.e. cin ) encountered and loop terminates. • This enables cascaded input cin >> x >> y;  >> controls the state bits of the stream  failbit set if wrong type of data input  badbit set if the operation fails Example Program Output #include <iostream> using std::cout; $ ./a.exe using std::cin; Enter the heights: (enter end of file to end): 72 using std::endl; 89 54 int main(void) { 33 int height = 0, maxheight = 0; 68 66 cout << "Enter the heights: (enter end of file to end): "; Tallest person's height = 89 while(cin >> height) if( height > maxheight) maxheight = height; cout << "Tallest person's height = " << maxheight << endl; return 0; } 4

istream member function: get istream member function: get get (array_name, max_size) ; char fname[256]  char ch = cin.get(); cin.get ( fname, 256 );  Inputs a character from stream (even white spaces) and returns it.  Read in up to 255 characters and inserts a null at the end of the string “fname". If  cin.get( c ); a delimiter is found, the read terminates.  Inputs a character from stream and The array acts like a buffer. The stores it in c delimiter is not stored in the array, but is left in the stream. istream member function: istream member functions: getline (array_name, max_size) ignore() char fname[256]  cin.ignore ( ) ; cin.getline ( fname, 256 );  Discards one character from the input stream.  cin.ignore (10) ;  Same as get, except that getline  Discards 10 characters. discards the delimiter from the stream.  cin.ignore(256,’\n’);  Discards 256 characters or newline, whichever comes first. istream member functions: FILE IO Example. peek(), putback() Copy File “first.txt” into “second.txt”.  char ch = cin.peek ( ) ; #include <iostream> #include <fstream>  Peeks into the stream’s next character. using namespace std;  cin.putback (‘A’) ; int main(void) {  Puts ‘A’ back in the stream. ifstream source("first.txt"); ofstream destin("second.txt"); char ch; while (source.get(ch)) destin<<ch; return 0; } 5

Slightly modified More IO functions #include <iostream> #include <fstream>  read() using namespace std;  cin.read(fname, 255); • Reads 255 characters from the input int main(void) { stream. Does not append ‘\0’. ifstream source("first.txt");  cout.write(fname,255); ofstream destin("second.txt"); • Writes 255 characters. char ch;  gcount: returns the total number of while (source.peek() != EOF){ source.get(ch); characters read in the last input destin.put(ch); operation. } return 0; } C++ IO Class Hierarchy Another example Revisited #include <iostream> #include <sstream> ios What if I replace this #include <string> with istringstream? istream ostream using namespace std; cin cout int main() { ifstream ofstream int i; string line; while(getline(cin,line)){ stringstream sfstream(line); istringstream ostringstream iostream while (sfstream >> i){ cout << i << endl; } } return 0; } stringstream fstream stringstream operations #include <iomanip> Stream  stringstream strm; Manipulators  stringstream strm(mystring);  Initializes strm with a copy of mystring.  strm.str();  Returns the content of strm in string format.  strm.str(s);  Copies the string s into strm. Returns void. 6