The C++ Programming Language

Streams

Contents

Output Stream Input Stream Formatting Manipulators Files & Streams

Stream: Output (1)

cout << "a*b+c=" << a*b+c << '\n'; cout << "a^b|c=" << (a^b|c) << '\n'; cout << "a<<b=" << (a<<b) << '\n';

Notes

operator << is not new one, just overridden operator output stream

printf("a<<=%d\n", a<<b); printf("a^b|c=%d\n", a^b|c); printf("a*b+c=%f\n", a*b+c);

Stream: Output (2) Built-in Types

class ostream : public virtual ios { public: // ... ostream& operator<<(const char*); // string ostream& operator<<(char); // ... ostream& operator<<(long); ostream& operator<<(double); ostream& operator<<(const void*); // pointe

r };

main() { int i =0; int* p = new int(1); cout << "local " << &i << ", free store " << p << '\n'; }

Notes

cerr is pre-initiated as an object of ostream

cerr << "x = " << x; => (cerr.operator<<("x = ")).opera

tor<<(x);

Possible result: local 0x7ffead0, free store 0x500c

Stream: Output (3) User-defined Types

class complex{ double re, im; public: complex(double r = 0, double i = 0) { re=r, im=i; } friend double real(complex& a) {return a.re; } friend double imag(complex& a) {return a.im; } friend complex operator+(complex, complex); friend complex operator-(complex, complex); friend complex operator*(complex, complex); friend complex operator/(complex, complex); //...}; ostream& operator<<(ostream&s, complex z) { return s << "(" << real(z) << "," << image(z) << ")"; }

Notes

main() { complex x(1, 2); cout << "x = " << x << '\n'; }

produced x = (1,2)

Stream: Input (1) Examples

int a; char b[10];

cin >> a >> b;

Stream: Input (2) Built-in Types

class istream : public virtual ios { // ... public: istream& operator>>(char*); // string istream& operator>>(char&); // character istream& operator>>(short&); istream& operator>>(int&); istream& operator>>(long&); istream& operator>>(float&); istream& operator>>(double&); istream& get(char& c); istream& get(char* p, int n, char ='\n'); istream& putback(char& c); // ... };

Notes

istream::get(char &)} reads a single character into its argument main() { char c; while ( cin.get(c) ) cout << c; }

istream::get(char* p, int n, char ch ='\n') read at most n characters into where 'p' points to until the character read is same to ch.

void f() { char buf[100]; cin.get(buf, 100, '\n'); }

Stream: Input (3) Stream States

Every stream has a state associated with it class ios { // ... public: enum io_state { goodbit =0, eofbit =1, failbit =2, badbit =4, }; int eof() const; // end of file seen int fail() const; // next operation will fail int bad() const; // stream corrupted int good() const; // next operation might succeed};

Notes

good(): next input operation might succeed eof(): end of file The difference between the states fail() and

bad() is subtle and only really interesting to implementers of input operations

Stream: Input (3) User-defined Types

istream& operator>>(istream&s, complex&a) { double re =0, im = 0; char c = 0;

s >> c; if (c == '(') { s >> re >> c; if (c == ',') s >> im >> c; if (c != ')') s.clear(ios::badbit); // set state }

else { s.putback(c); s >> re; } if (s) a = complex(re,im); return s; }

Notes

An input operation can be defined for a user-defined type exactly as an output operation was, but for an input operation it is essential that the second argument is of reference type

Stream: Formatting (1) Typing of Streams

The tie() function is used to set up and break connections between an istream and an ostream (cf. t cin.tie(cout))

When an ostream is tied to an istream the ostream is flushed whenever an input operation is attempted on the istream

is.tie(0) unties the stream is from the stream it was tied to, if any.

A call without an argument , tie(), returns the current value

Notes

cin.tie(cout); cout << "Password: "; cout << "Password: "; cout.flush(); cin >> s; cin >> s;

Stream: Formatting (2) Output Fields

The width() function specifies the minimum number of characters to be used for the next numeric or string output operation cout.width(4) cout << '(' << 12 << ')';

The padding or filler character can be specified by the fill() function

cout.width(4); cout.fill('#'); cout << '(' << "ab" << ')'; // output: (##ab)

Default: as many characters as needed

A call of width() affects only the immediate following output operations

cout.width(4); cout.fill('#'); cout << '(' << 12 << ')', (" << '(' 12 << ")\n"; // output is (##12),(12)

Notes

Format State

class ios { public: // flags for controlling format: enum { skipw =01, // skip whitespace on input

// field adjustment: left =02, // padding after value right =04, // padding before value internal=010, // padding between sign and value

// integer base: octal =020, // octal dec =040, // decimal hex =0100, // hexadecimal showbase=0200, // show integer base showpoint=0400, // print tailing zeros uppercase=01000, // 'E', 'X' rather than 'e', 'x' showpos =02000, // explicit '+' for positive ints

//floating point notation scientific=04000, // .dddddd Edd fixed =010000, // dddd.dd

// flush output: unitbuf =020000, // after each output operation stdio =040000, // after each character };

// ... };

Format State (cont'd) An ios has a format state that is controlled by

the flags() and setf() functions The flags() function returns old option set.

const int my_ioflag = ios::left|ios::oct|ios::showpoint|ios::fixed;

old_flag = cout.flags(my_ioflag);

cout.setf(ios::showpos); // explicit '+' for positive int

NotesNotes

Integer Output

C++ provides a version of setf() that takes a second “pseudo argument'' indicating which kind of option we want to set in addition to the new value

cout.setf(ios::oct,ios::basefield); // octal cout << 1234; // output: 2322 cout.setf(ios::hex,ios::basefield); //hexadecimal cout << 1234; // output: 4d2 cout.setf(ios::showbase); cout << 1234; // 0x4d2

Field Adjustment

cout.width(4); cout << '(' << -12 << ")\n"; // output: ( -12) cout.width(4); cout.setf(ios::left, ios::adjustfield); cout << '(' << -12 << ")\n"; // output: (-12 )

cout.width(4); cout.setf(ios::internal, ios::adjustfield); cout << '(' << -12 << ")\n"; // output: (- 12)

NotesNotes

Internal adjustment places fill characters between the sign and the value

Floating-Point Output cout << 1234.56789 << '\n'; // 1234.57

cout.setf(ios::scientific,ios::floatfield); cout << 1234.56789 << '\n'; // 1.234568e+03

cout.setf(ios::fixed,ios::floatfield); cout << 1234.56789 << '\n'; // 1234.567890

cout.precision(8); cout << 1234.56789 << '\n'; // 1234.5679

cout.precision(4); cout << 1234.56789 << '\n'; // 1235

NotesNotes

Note that values are rounded rather than just truncated

Manipulators (1) Manipulators with No Arguments

cout << x; cout.flush(); cout << y;

class Flushtype { }; ostream& operator<<(ostream& os, Flushtype s) { return flush(os); } Flushtype FLUSH; cout << x << FLUSH << y << FLUSH;

ostream& flush(ostream&); typedef ostream& (*Omanip) (ostream&); ostream& operator<<(ostream& os, Omanip f) { return f(os); } cout << x << flush << y << flush;

class ostream: public virtual ios { // ... public: ostream& operator<<(ostream&, ostream& (*)(ostream&)); // ... }; istream& ws(istream& is) {return is.eatwhite(); } cin >> ws >> x;

Manipulators (2) Manipulators with Arguments

class Omanip_int { int i; ostream& (*f)(ostream&,int); public: Omanip_int(ostream& (*ff)(ostream&,int), int ii) : f(ff), i(ii) { } friend ostream& operator<<(ostream& os, Omanip_int& m) { return f(os,i); } };

ostream& _set_precision(ostream&,int); Omanip_int setprecision(int i) { return Omanip_int(&_set_precision,i); }

cout << setprecision(4) << angle;

Manipulators (3) Standard I/O Manipulators

ios& oct(ios&); //used octal notationios& dec(ios&); //used decimal notationios& hex(ios&); //used hexadecimal notation

ostream& endl(ostream&); //add '\n' and flushostream& ends(ostream&); //add '\0' and flushostream& flush(ostream&); // flush stream

istream& ws(istream&); // eat whitespace

SMANIP<int> setbase(int b);SMANIP<int> setfill(int f);

SMANIP<int> setprecision(int p);SMANIP<int> setw(int w);

SMANIP<long> resetiosflags(long b);SMANIP<long> setiosflags(long b);

cout << 1234 << ' ' << hex << 1234 << ' ' << oct << 1234 << endl; // 1234 4d2 2322

cout << setw(4) << setfill('#') << '(' << 12 << ")\n";

// (##12)

Files & Streams

File Stream: fstream,ifstream, ofstream

ofstream mystream("test.txt",ios::out|ios::nocreat); fstream dictionary("my.dic",ios::in|ios::out);

mystream.close();

String Stream char* p = "ABCDEFGHIJKLMN"; char* q = new char[max_size]; char c;

istrstream ist(p,strlen(p)); ostrstream ost(q,max_size);

while (ist>>c) ost << c;