cs 3370. inserters and extractors stream state files streams string streams formatting ...

CS 3370

Inserters and ExtractorsStream StateFiles StreamsString StreamsFormattingManipulators Internationalization

Insert an object into a stream it does formatted output

Uses operator<< the “left-shift” operator the arrow suggests the direction of the

data flowEasy to define for your own classes

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;}

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;}

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( )

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

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

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( )) …

getgetline ignoreputbackungetpeek

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

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

Discards characters ignore(int n = 1, int delim =

eof( )) returns stream

For a large n, use: std::numeric_limits<streamsize>::max()

unget() Moves the stream’s get pointer back one The next input op re-reads the previous

characterputback(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

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 ( | )

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

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

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

Can set stream attributes width, fill character, alignment, numeric

base, floating-point format, decimal precision, etc.

Use setf( ) and unsetf( )Example: C04/Format.cpp

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 routeUsually don’t worry about itOne cool feature:

C04/SType.cpp, hexdec.cpp

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

#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’

setw(n), setfill(c), setprecision(n), etc.

Must include <iomanip> for theseExample: C04/Manips.cppDifficult to implement your own

not portableUse Effectors instead

(see next slide)

Create a class whose constructor formats a string according to its purpose

That class also provides an operator<<

Example: C04/Effector.cpp

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

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;

Cultural customization of I/O formatting

A stream has an associated localeCan change it with imbue( )Example: Locale.cpp (Windows only) Java’s locale support is much better