IOStreams

1. CS 3370 IOStreams

2. Agenda • Inserters and Extractors • Stream State • Files Streams • String Streams • Formatting • Manipulators • Internationalization

3. Inserters • Insert an object into a stream • it does formattedoutput • Uses operator<< • the “left-shift” operator • the arrow suggests the direction of the data flow • Easy to define for your own classes

4. Inserter ExampleFormatted Output • A Date class inserter: ostream& operator<<(ostream& os, const Date& d) { char fillc = os.fill('0'); os << setw(2) << d.getMonth() << '-' << setw(2) << d.getDay() << '-' << setw(4) << d.getYear() << setfill(fillc); return os; }

5. Extractor ExampleFormatted Input istream& operator>>(istream& is, Date& d) { is >> d.month; char dash; is >> dash; if(dash != '-') { is.putback(dash); is.setstate(ios::failbit); // Input disabled return is; } is >> d.day; is >> dash; if(dash != '-') { is.putback(dash); is.setstate(ios::failbit); } is >> d.year; return is; }

6. Stream State • 4 states: • eof: set upon an attempt to read past end-of-file • sets fail automatically • eof is meaningless for output streams • fail: an operation failed (e.g., alpha chars when reading int) • bad: stream is broken (no memory for buffer, device failure) • good: none of the other 3 states occurred • When an error occurs (fail or bad), the stream is disabled • All subsequent stream operations are ignored • Can re-enable stream operations with clear( )

7. Testing the Stream State • Can test with associated member functions: • good( ), eof( ), fail( ), bad( ) • Can test for successful input like this: • if (strm) • same as if (!strm.fail( ) && !strm.bad() && ! strm.eof( )) • Can also use exceptions

8. Streams and Exceptions • Can have exceptions thrown instead of checking state • Call the exceptions( ) member function • Can pick which states you want to throw:myStream.exceptions(ios::badbit); • The exception type thrown is ios::failure • ios is a base class for streams • See strmexcept.cpp

9. Input IdiomMost operations return the stream • while (myStream >> x)// process x (this assumes no input failure) • while (getline(myStream, line)) // process line (ditto) • You can check conditions separately • if (myStream.eof( )) … • if (myStream.fail( )) …

10. Handy Input Functions • get • getline • ignore • putback • unget • peek

11. get • get( ) • returns the next character, or -1 • whitespace included • get(char& c) • puts the character read in c • returns the stream • get(char* s, int n, char delim = ‘\n’) • reads n characters or up to delim

12. getlineTwo versions • getline(char* s, int n, char delim = ‘\n’) • returns stream • reads and discards delim (different from get) • std namespace scope version • uses a string, not a char* • declared in <string> • getline(istream& is, string& s, char delim = ‘\n’) • returns stream

13. ignore • Discards characters • ignore(int n = 1, int delim = eof( )) • returns stream • For a large n, use: • std::numeric_limits<streamsize>::max()

14. unget, putback, peek • unget() • Moves the stream’s get pointer back one • The next input op re-reads the previous character • putback(char) • Puts an arbitrary character into the buffer • So the next input op reads that character • peek() • Returns the next character without moving the get pointer beyond it

15. File Streams • Classes ifstream, ofstream, fstream • declared in <fstream> • Constructors open, destructors close automatically • All normal stream operations apply • Additional member functions: • close( ), open( ) • Open modes • ios::in, ios::out, ios::app, ios::ate, ios::trunc, ios::binary • Can combine with a bitwise-or ( | )

16. Stream Positioning • Can move around in a stream • except when using the console, of course • Using functions seekp( ), seekg( ) • seekp( ) seeks in the output buffer (p = “put”) • seekg( ) seeks in the input buffer (g = “get”) • Simultaneous I/O streams share the same buffer • File streams keep the put/get pointers together • In string streams they’re independent

17. Files With Fixed-length Records • Often used by databases • can access records randomly • Fields must have only built-in data • numbers, C-style strings, static arrays • no pointers! • uses binary mode • Use the write and read member functions • See employeedb.cpp

18. String Streams • Classes istringstream, ostringstream, stringstream • declared in <sstream> • Writes to or reads from a string • or both • but remember the get/put pointers are independent • Useful for converting other data types to and from strings • Examples: C04/IString.cpp, C04/Ostring.cpp,C04/HTMLStripper2.cpp

19. Output Formatting • Can set stream attributes • width, fill character, alignment, numeric base, floating-point format, decimal precision, etc. • Use setf( ) and unsetf( ) • Example: C04/Format.cpp

20. Stream Buffers • The data area(s) held by the stream • One for input, one for output • Streams that support both, have both • Can access via the function rdbuf( ) • A “Way Station” for data en route • Usually don’t worry about it • One cool feature: • C04/SType.cpp,hexdec.cpp

21. Manipulators • A shorthand for setting/unsetting stream attributes • dec, hex, endl, flush • Achieved via a special overload convention • manipulators are functions • when inserted, the following function is called:ostream& ostream::operator<<(ostream& (*pf)(ostream&)) { return pf(*this);} • The function pf should do its work and return the stream

22. Creating a Manipulator • #include <iostream> • Define a function with the required signature (below) • Do your work and return the stream: ostream& nl(ostream& os) { return os << '\n'; } int main() { cout << "newlines" << nl << "between" << nl << "each" << nl << "word" << nl; } cout << nl becomes… cout.operator<<(nl), which executes nl(cout), which executes cout << ‘\n’

23. Manipulators with Arguments • setw(n), setfill(c), setprecision(n), etc. • Must include <iomanip> for these • Example: C04/Manips.cpp • Difficult to implement your own • not portable • Use Effectors instead • (see next slide)

24. Effectors • Create a class whose constructor formats a string according to its purpose • That class also provides an operator<< • Example: C04/Effector.cpp

25. Wide Streams • The streams we’ve been using traffic in bytes (char) • You can have streams that use wide characters (wchar_t) • displaying foreign characters requires platform support outside of C++ • C++ just stores code points internally

26. basic_istream • The template that governs the standard stream classes:template<class charT, class traits = char_traits<charT> >class basic_istream {...}; • typedef basic_istream<char> istream;typedef basic_istream<wchar_t> wistream;typedef basic_ifstream<char> ifstream;typedef basic_ifstream<wchar_t> wifstream;typedef basic_istringstream<char> istringstream;typedef basic_istringstream<wchar_t> wistringstream;

27. Locales • Cultural customization of I/O formatting • A stream has an associated locale • Can change it with imbue( ) • Example: Locale.cpp (Windows only) • Java’s locale support is much better