Status of a stream

Input

...

Class Complex

...

Class ostream

Output

Standard streams

Topic 16. STREAMS

Along with <stdio.h> library, containing input/ouput functions printf, scanf, etc., C++ language has also the library <iostream.h>. In this library there are classes and functions for information input and output. To use this library the program should contain the statement

#include <iostream.h>

Advantages of <iostream.h>

1. The syntax is more simple and more elegant that syntax of printf and scanf.

2. Formatted output can be easily redefined.

3. Th library <iostream.h> has a wide hierarchy of classes for buffered and non-buffered output.

In C++ language there are 4 predefinned objects for input/output:

· cin - standard input (usually keyboard), corresponding to stdin in C language;

· cout - standard output (usually screen), corresponding to stdout in C language;

· cerr - standard file for error messages (the screen by default), corresponding to stderr in C language;

· clog - full buffered cerr object.

For input and output following operators are used:

>> - overloaded input operator “get”

<< - overloaded output operator “put”.

These operators if necessary can be overloaded (redefined) for user defined classes.

EXAMPLE:

stream output in C++	output in C
cerr<<”Error”	printf(“Error”);
cerr<<”Error number”<<i<<”\n”;	printf(“Error number %d\n”,i);

In C++ language there is standard predefined class for output

{

ostream& operator<<(char*);

ostream& operator<<(int);

ostream& operator<<(long);

ostream& operator<<(double);

ostream& put(char);

};

The operator << has two operands: the first operand is of class ostream, the second is the value for output. Operator << (“put”) returns the reference to ostream object. It is necessary for so called cascade output:

cout<<”x=”<<x;

This line is interpreted as

(cout,operator<<(“x=”)).operator<<(x);

From declaration of ostream class it can be seen that operator << is overloaded for all predefined data types: char, int, long and double.

16.2.1.Overloading << for user defined classes

Consider previously implemented class for Complex numbers. It can be modified for standard output into cout with the help of overloaded operator <<.

#include <stdio.h>

#include <iostream.h>

{

double Re;

double Im;

public:

Complex(double r, double i) {Re=r; Im=i;};

Complex Add(Complex a,Complex b);

Complex& Assign(Complex a);

friend ostream& operator<<(ostream& s, Complex x);

friend istream& operator>>(istream& s, Complex& a);

};

ostream& operator<<(ostream& s, Complex x)

{

return s<<"("<<x.Re<<","<<x.Im<<")";

}

Now we can print complex numbers in the form (re,im), for example:

main()

{

Complex x(3,4);

cout<<”x=”<<x<<’\n”;

}

In fact we expanded class ostream with our own operator << for Complex objects.

Input is similar to output. There is class istream, which supplies operator >> (“get”) for standard data types of C language:

class istream

{

...

public:

istream& operator>>(char*);

istream& operator>>(char&);

istream& operator>>(int&);

istream& operator>>(long&);

istream& operator>>(lfloat&);

istream& operator>>(double&);

istream& get(lchar& c); // character

istream& get(char* p, int n); // character string

};

EXAMPLE:

C++ stream input	C input
int x;	int x;
float y;	float y;
cin>>x>>y;	scanf(“%d &f”,x,y);

User can add his own operator >> for new classes.

EXAMPLE: input operator for Complex class.

istream& operator>>(istream& s, Complex& a)

{

// a is expected in form (re,im)

double re,im;

char c;

int ok=0;

a.Re=0;a.Im=0;

s>>c;

if (c=='(') {

s>>re>>c;

if (c==',') {

s>>im>>c;

if (c!=')') {

cerr<<"Wrong complex number"<<"\n";

}else{

ok=1;

}

}

}

if (ok) {a.Re=re; a.Im=im;}

return s;

}

Operators << and >> for Complex class cann be tested with the help of a following program:

main()

{

Complex a(0,0);

Complex b(0,0);

Complex c(0,0);

cout<<"Please enter the first number in form (re,im):";

cin>>a;

cout<<"And now the second number in form (re,im):";

cin>>b;

c.Add(a,b);

cout<<"c="<<c<<"\n";

return 0;

}

Every stream is connected with error states. They are described as follows in clas ios (stands for input/output status).

Дата добавления: 2014-11-28