CSE 332: C++ IO

We’ve Looked at Basic Input and Output Already• How to move data into and out of a program

– Using argc and argv to pass command line args– Using cout to print data out to the terminal– Using cin to obtain data from the user at run-time– Using an ifstream to read data in from a file– Using an ofstream to write data out to a file

• How to move data between strings, basic types– Using an istringstream to extract int values– Using an ostringstream to assemble a string

• This module will expand on those ideas– Towards understanding better how steams work– Towards using them even more effectively

CSE 332: C++ IO

Example: Moving Data Into and Out of a Struct

// struct, operator declarations (in point2d.h)

struct Point2D { Point2D (int x, int y); bool operator< (const Point2D &) const;int x_;int y_;

};

// extraction operator(also in point2d.h)istream & operator>> (istream &, Point2D &);

// insertion operator(also in point2d.h)ostream & operator<< (ostream &, const Point2D &);

CSE 332: C++ IO

Defining Stream Operators for a Struct// operator definitions (in point2d.cpp)

// extraction operator(also in point2d.h)istream & operator>> (istream &i, Point2D &p) { i >> p.x_ >> p.y_; // extract both variables return i;}

// insertion operator(also in point2d.h)ostream & operator<< (ostream &o, const Point2D &p) { o << p.x_ << " " << p.y_; // space delimited return o;}

CSE 332: C++ IO

Now Can Use the Struct with Various Streams// uses standard io streams

#include <iostream>

using namespace std;

#include "point2d.h"

int main(int, char*[]){

while (true) {

Point2D p(0, 0);

cout << "Please enter 2

coordinates (or hit

Ctrl-C to quit): ";

cin >> p;

cout << endl

<< "You entered "

<< p << endl;

}

return 0;

}

// uses string streams

#include <iostream>

#include <string>

#include <sstream>

using namespace std;

#include "point2d.h"

int main(int, char*[]){

Point2D p(0, 0);

string s ("64 81");

istringstream i(s);

i >> p;

ostringstream o;

o << p;

cout << "The string is "

<< o.str() << endl;

return 0;

}

CSE 332: C++ IO

Flushing Streams (and Stream Manipulators)• An output stream may hold onto data for a while, internally

– E.g., writing chunks of text rather than a character at a time is efficient– When it writes data out (e.g., to a file, the terminal window, etc.) is

entirely up to the stream, unless you tell it to flush out its buffers

• If a program crashes, any un-flushed stream data is lost– Terminal output & files are as of last flush, not as of where it crashed– So, flushing streams reasonably often is an excellent debugging trick

• Can tie an input stream directly to an output stream– Output stream is then flushed by call to input stream extraction operator – E.g., my_istream.tie(&my_ostream);– cout is already tied to cin (useful for prompting the user, getting input)

• Also can flush streams directly using stream manipulators – E.g., cout << flush; or cout << endl; or cout << unitbuf;

• Other stream manipulators are useful for formatting streams– Field layout: setwidth, setprecision, etc.– Display notation: oct, hex, dec, boolalpha, noboolalpha, scientific, etc.

CSE 332: C++ IO

A Few More Useful Details• Cannot copy or assign stream objects

– Copy construction or assignment syntax using them results in a compile-time error

• Extraction operator consumes data from input stream– “Destructive read” that reads a different element each time– Use a variable if you want to read same value repeatedly

• Need to test streams’ condition states– E.g., calling the is_open method on a file stream– E.g., use the stream object in a while or if test– Insertion and extraction operators return a reference to a

stream object, so can test them too

• File stream destructor calls close automatically