IOStream Library

Unit - 10 IOStream Library Input / Output in C++

Unit Introduction This unit covers different features in standard IOStream Library

Unit Objectives After covering this unit you will understand… • IOStream library • File operations • Buffering operations • Seeking operations • Output stream formatting • IOStream manipulators • Creating manipulators

IOStream Introduction • Input and Output are the basic programming functions • Without IOStream Library: • requires hundreds of lines of code for usage and manipulation • are error prone with dangling pointers, uninitialized pointers etc

IOStream Library • IOStream Library helps in: • ease of coding • error handling • safer and efficient results • Deals with the following I/O functions in a safe, efficient and easier manner: • Standard Input • Standard Output • Files • Memory Blocks

IOStreams Features • Provides an easier interface • Handles menial tasks like closing the file pointer, when the file is not in use etc • Error handling is automatic when I/O error occurs • Constructor handles all the initialisation • Destructor handles all the cleanup tasks like removing dangling pointers, memory cleanup etc

File IOStreams • File IOStreams are simple to use: • Declare an Object • Use the file I/O functions • Destroy the object (the destructor is called automatically when the object goes out of scope)

Example: File IOStreams #include <fstream.h> #include <iostream.h> void main() { const int sz = 100; // Buffer size; char buf[sz]; { ifstream in(”myfile.cpp"); // Read ofstream out(”myfile.out"); // Write int i = 1; // Line counter while(in.get(buf, sz)) { // Line input in.get(); // get() does not read ‘\n’ cout << buf << endl; // use endl for ‘\n’ // File output just like standard I/O: out << i++ << ": " << buf << endl;

Example: File IOStreams (contd.) } } // Destructors close in & out ifstream in(”myfile.out"); // More convenient line input: while(in.getline(buf, sz)) { char* cp = buf; while(*cp != ':') { cp++; } cp += 2; // Past ": " cout << cp << endl; } }

File Open Modes • You can control the way a file is opened by changing a default argument • These flags can be combined using a bitwise OR • The different modes are: • ios::in • ios::out • ios::app • ios::nocreate

File Open Modes (contd.) • ios::noreplace • ios::trunc • ios::binary

Buffering in IOStreams • You can buffer all the bytes in a file using buffering • It provides an easy and efficient solution to reading files or other I/O features

Example: Buffering IOStreams #include <fstream.h> #include <iostream.h> void main() { ifstream in(“myfile.cpp”); cout << in.rdbuf(); // outputs entire file // another way to output the file while (in.get(*cout.rdbuf())) // redirects it to cout { in.ignore(); // ignore the terminator ‘\n’ } }

Seeking in IOStream • You can move the stream pointer to different positions and read or write bytes • There are three seek functions: • ios::beg From beginning of stream • ios::cur Current position in stream • ios::end From end of stream

Example: Seeking in IOStream #include <iostream> #include <fstream> void main() { ifstream in("Iofile.cpp"); ofstream out("Iofile.out"); out << in.rdbuf(); // Copy file in.close(); out.close(); // Open for reading and writing: ifstream in2("Iofile.out", ios::in | ios::out); ostream out2(in2.rdbuf()); cout << in2.rdbuf(); // Print whole file out2 << "Where does this end up?"; out2.seekp(0, ios::beg); // put pointer

Example: Seeking in IOStream (contd.) out2 << "And what about this?"; in2.seekg(0, ios::beg); // get pointer cout << in2.rdbuf(); }

Output Stream Formatting • You can format the output stream in STL, just like you can using printf in C++ • The different formatting functions are: • Internal formatting data • ios::skipws • ios::showbase • ios::showpoint • ios::uppercase • ios::showpos • ios::unitbuf

Output Stream Formatting (contd.) • ios::stdio • Format fields • ios::basefield • ios::dec • ios::hex • ios::oct • ios::floatfield • ios::scientific • ios::fixed • automatic • ios::adjustfield • ios::left • ios::right

Output Stream Formatting (contd.) • ios::internal • Width, fill and precision • int ios::width( ) • int ios::width(int n) • int ios::fill( ) • int ios::fill(int n) • int ios::precision( ) • int ios::precision(int n)

Iostream Manipulators • Output stream formatting can be a tedious task • To make things easier to read and write, a set of manipulators are provided • The manipulators are: • showbase / noshowbase • showpos / noshowpos • uppercase / nouppercase • showpoint / noshowpoint

IOStream Manipulators (contd.) • skipws / noskipws • left / right / internal • scientific / fixed • Some manipulators can have arguments, which require <iomanip.h> header • The argument supplied manipulators are: • setiosflags (fmtflags n) • resetiosflags(fmtflags n)

IOStream Manipulators (contd.) • setbase(base n) • setfill(char n) • setprecision(int n) • setw(int n)

Example: IOStream Manipulators #include <fstream> #include <iomanip> void main() { ofstream trc("trace.out"); int i = 47; float f = 2300114.414159; char* s = "Is there any more?"; trc << setiosflags(ios::unitbuf | ios::showbase | ios::uppercase | ios::showpos); trc << i << endl; // Default to dec trc << hex << i << endl; trc << resetiosflags(ios::uppercase) << oct << i; trc.setf(ios::left, ios::adjustfield); trc << resetiosflags(ios::showbase) << dec << setfill('0'); trc << "fill char: " << trc.fill() << endl; trc << setw(10) << i << endl;

Example: IOStream Manipulators (contd.) trc << resetiosflags(ios::showpos) << setiosflags(ios::showpoint) << "prec = " << trc.precision() << endl; trc.setf(ios::scientific, ios::floatfield); trc << f << endl; trc.setf(ios::fixed, ios::floatfield); trc << f << endl; trc << setprecision(20); trc << "prec = " << trc.precision() << endl; trc << resetiosflags( ios::showpoint | ios::unitbuf); }

Creating Manipulators • You can create your own manipulators • The manipulator may or may not have arguments • The declaration for endl is ostream& endl(ostream&); cout << “howdy” << endl; • the endl produces the address of that function • Applicator calls the function, passing it the ostream object as an argument

Example: Creating Manipulators #include <iostream> ostream& nl(ostream& os) { return os << '\n'; } void main() { cout << "newlines" << nl << "between" << nl << "each" << nl << "word" << nl; } /* The expression os << '\n'; calls a function that returns os, which is what is returned from nl */

Unit Summary In this unit you have covered … • Overview of the IOStream library • Different features of IOStream • IOStream operations & manipulators • Creating custom manipulators

IOStream Library