IOstream

• An introduction to the iostream library
• Terminology of Iostreams
• Examples of how to use various aspects of the library
• Sample program
• Class filebuf
• Classes fstream, ifstream, and ofstream
• Class ifstream
• Class ios
• Class istream
• Class istream_withassign
• Class istrstream
• Class ofstream
• Class ostream
• Class ostream_withassign
• Class ostrstream
• Class stdiobuf
• Class stdiostream
• Class streambuf
• Class strstream
• Class strstreambuf

C++, like C, has no built-in input or output statements. Instead, I/O facilities are provided by a library. The standard C++ I/O library is the iostream library.

As with much of object oriented programming, iostreams can be difficult to understand without reading through the material more than once. A terminology section defines many of the basic terms.

Predefined Iostreams

The four predefined iostreams are:

• cin, connected to standard input.
• cout, connected to standard output.
• cerr, connected to standard error.
• clog, connected to standard error.

Basic Structure of Iostream Interaction

The iostream library allows a program to use any number of input or output streams. Each stream has some source or sink, which might be one of the following:

• Standard input
• Standard output
• A file
• An array of characters

• The lower layer implements sequences, which are simply streams of characters. These sequences are implemented by the streambuf class, or by classes derived from it.

• The upper layer performs formatting operations on sequences. These formatting operations are implemented by the istream and ostream classes, which have as a member a pointer to an object of a type derived from class streambuf. An additional class, iostream, is for performing input and output on streams.

The ifstream, ofstream, and fstream classes, which are derived from istream, ostream, and iostream respectively, handle input and output with files.

The istrstream, ostrstream, and strstream classes, which are derived from istream, ostream, and iostream respectively, handle input and output to and from arrays of characters.

When you want to open an input or output stream, you create an object of one of these types. You associate the streambuf member of the stream with a device or file. You generally do this association through the stream constructor, so you don't deal with the streambuf directly. The iostream library predefines stream objects for the standard input, standard output, and error output, so you don't have to create your own objects for those streams.

You use operators or iostream member functions to insert data into a stream (output) or extract data from a stream (input), and to control the format of data that you insert or extract.

To insert and extract a new data type (that is, one of your classes), you generally overload the insertion and extraction operators.

Iostream Terminology

The iostream libarary descriptions use the following terms, which are similar to terms from general programming but with specialized meanings.

Term/Definition

buffer

A word with two meanings, one specific to the iostream package and one more generally applied to input and output.

When referring specifically to the iostream library, a buffer is an object of the type defined by the class streambuf.

A buffer, generally, is a block of memory used to make efficient transfer of characters for input or output. With buffered I/O, the actual transfer of characters is delayed until the buffer is full or forceably flushed.

This confusion allows such odd phrases as "an unbuffered buffer", which refers to a streambuf where there is no buffer in the first sense defined above. Because of this confusion, this chapter avoids use of the term buffer to refer to streambufs. However, the next chapter and other C++ documents use the term buffer to mean streambufs.

extraction

The process of taking input from an iostream.

fstream

An input or output stream for use with files.

insertion

The process of sending output into an iostream.

iostream

Generally, an input or output stream but can also refer to any of the classes in the iostream library.

Iostream library

The library implemented by header files iostream.h, fstream.h, strstream.h, iomanip.h, and stdiostream.h. Because iostream is an object-oriented library, you should extend it. So, some of what you can do with the iostream library is not implemented.

stream

An iostream, fstream, strstream, or user-defined stream.

streambuf

A buffer that contains a sequence of characters with a put or get pointer, or both. It refers generally to any object of class streambuf or a class derived from streambuf. Any stream object contains an object, or a pointer to an object, of a type derived from streambuf.

strstream

An iostream for use with character arrays.

To use iostream routines, include the header files for the part of the library you need. The header files are:

Header File/Description

iostream.h

Declares basic features of iostream library.

fstream.h

Declares iostreams and streambufs specialized to files. Includes iostream.h.

strstream.h

Declares iostreams and streambufs specialized to character arrays. Includes iostream.h.

iomanip.h

Declares manipulators (values you insert into or extract from iostreams to have different effects).

fstream.h (obsolete)

Declares iostreams and streambufs specialized to use stdio FILEs. Includes iostream.h. stream.h (obsolete) For compatibility with old-style streams from C++ version 1. 2. Includes iostream.h, fstream.h, iomanip.h, and stdiostream.h

Output Using Iostreams

Output using iostreams usually relies on the overloaded left shift operator (<<) which, in the context of iostream, is called the insertion operator. To send a value to standard output, insert the value in the predefined output stream cout and use the insertion operator. For example:

cout << someValue;

The insertion operator, overloaded for all built-in types, converts the value of someValue to its correct output representation. If, for example, someValue is a float value, the << operator converts the value to the correct sequence of digits and includes a decimal point. Where it inserts float values on the output stream, << is called the float inserter. In general, given a type X, << is called the X inserter. The format of output and how to control it is discussed in the "Format Control" section.

The iostream library does not know about user-defined types. If you define types for output, you must define an inserter (that is, overload the << operator) to handle them correctly.

The << operator may be applied repetitively. To insert two values on cout, use the statement:

cout << someValue << anotherValue;

Output shows no space between the two values. To include spaces in the output use the statement:

cout << someValue << " " << anotherValue;

The << operator has the precedence of the left shift operator (its built-in meaning). As with other operators, you can always use parentheses to guarantee the order of action. It is often a good idea to use parentheses to avoid problems of precedence. Of the following four statements, the first two are equivalent, but the last two are not.

cout << a+ b; // + has higher precedence than <<
cout << (a+ b);
cout << (a& y);// << has precedence higher than &
cout << a& y;// probably an error: (cout << a) & y

Defining Your Own Insertion Operator
The string class defined in the "Sample Program" section has this definition:

class string {
private: 
   char* data;
   size_t size; 

public: 
   (functions not relevant here) 

friend ostream& operator<<(ostream&, 
       const string&); 
friend istream& operator>>(istream&, string&); 
}; 

ostream& operator<< (ostream& ostr, 
         const string& output) 
{ return ostr << output. data;}

	if (! cout) error("output error"); 

if (cout << x) return ; // return if successful 

if (cout. good() ) return; // return if successful 

enum io_state {goodbit= 0, eofbit= 1, failbit= 2, 
                badbit= 4, hardfail= 0x80} ; 

cout << "Needs to get out immediately." << flush; 

cout. write((char*)& x, sizeof(x)); 

Input Using Iostreams

Input using iostream is similar to output. Use the extraction operator >> and string together extractions the way you can with insertions. For example to get two values from standard input:

cin >> a >> b ; 

Defining Your Own Extraction Operators

When you want input for a new type, you overload the extraction operator for it, just as you overload the insertion operator for output.

Class string (shown in sections "Output Using Iostreams" and "Sample Program") defines its extraction operator like this:

istream& operator>> (istream& istr, string& input) 
{ 
   const int maxline = 256; 
   char holder[maxline]; 
   istr. get(holder, maxline, '\n'); 
   input = holder; 
   return istr; 
} 

The char* Extractor

This predefined extractor is mentioned here because it can cause problems. You use it like this:

char x[50];
cin >> x; 

Reading Any Single Character

The char extractor skips whitespace, which can be a problem if your program needs to read the next character whether or not it is whitespace. To do this with either form of function get member:

char c;
cin. get(c); // leaves c unchanged if input fails

int b;
b = cin. get(); // sets b to EOF if input fails

int a;
do { 
    a = cin. get(); 
while(a == ' ' ); 

To read binary values (such as those written with the member function write), use the read member function. To input the raw binary form of x using the read member function, do the following, which is the inverse of the earlier example that used write.

cin. read((char*)& x, sizeof(x)); 

Use the peek member function to look at the next character in the stream without extracting it. For example:

if (cin. peek() != c) return 0; 

By default, iostream extractors skip leading whitespace. You can turn off the skip flag to prevent this from happening. The following code turns off whitespace skipping from cin, then turns it on again.

cin. unsetf(ios::skipws); // turn off . . . 

cin. setf(ios::skipws, ios::skipws); // turn on

istr >> ws; 

By convention, an extractor whose first argument has a nonzero error state should not extract anything from the input stream and should not clear any error bits. An extractor that fails should set at least one error bit.

As with output errors, you should check the error state periodically and take some action (such as aborting) when you find a nonzero state. The ! operator returns the error state of an iostream. For example, the following code produces an input error if you type alphabetic characters for input:

#include <stream.h> 
#include <stdlib.h>
void error (const char* message) { 
   cout << message << "\n" ; 
   exit(1); 
} 
main() { 
   cout << "Enter some characters: "; 
   int bad; 
   cin >> bad; 
   if (! cin) error(" aborted due to input error"); 
   cout << "If you see this, not an error." << 
"\n"; 
   return 0; 
} 

Using Iostreams with stdio

You can use stdio with C++ programs, but problems can occur when you mix iostreams and stdio in the same standard stream within a program. For example, if you write to both stdout and cout, independent buffering will occur; results might not be what you expect. The problem is worse if you input from both stdin and cin, since independent buffering can turn the input into trash.

To eliminate this problem with standard input, standard output, and standard error, use the following instruction before performing any input or output. It connects all the predefined iostreams with the corresponding predefined stdio FILEs.

ios::sync_with_stdio(); 

Creating Iostreams

To read or write a stream other than the predefined iostreams, you must create your own iostream. In general that means creating objects of types defined in the iostream library. The following sections discuss the various types available.

Dealing with Files Using Class fstream

Dealing with files is similar to dealing with standard input and standard output; classes ifstream, ofstream, and fstream are derived from classes istream, ostream and iostream, respectively. As derived classes, they inherit the insertion and extraction operations (along with the other member functions) and also have members and constructors for use with files.

You must include the file fstream.h to use any of the fstreams. Use an ifstream when you only want to perform input, an ofstream for output only, and an fstream for a stream on which you want to perform both input and output. Use the name of the file as the constructor argument.

For example, to copy the file thisFile to the file thatFile:

ifstream fromFile(" thisFile");
if (! fromFile) 
   error(" unable to open 'thisFile' for input"); 
ofstream toFile (" thatFile"); 
if (!toFile ) 
   error(" unable to open 'thatFile' for output"); 
char c ; 
while (toFile && fromFile. get(c)) toFile. put(c); 

	enum open_mode {in= 1, out= 2, ate= 4, app= 8, 
		trunc= 0x10, nocreate= 0x20, noreplace= 0x40}; 

fstream inoutFile("someName", ios::in | ios::out); 

ofstream toFile; 
toFile.open(argv[1], ios::out); 

	ifstream infile; 
	for (char** f = &argv[1]; *f; ++f)
	{   
	   infile.open(*f, ios::in);
	   ...; 
	   infile.close();
	} 

ofstream outfile; 
outfile. attach(1); 

	streampos original = aFile.tellp(); // save current position
	aFile.seekp(0, ios::end); 	    // reposition to end of file
	aFile << x; 			    // write a value to file
	aFile.seekp(original); 		    // return to original position

	aFile.seekp(-10, ios::end); 

	aFile.seekp(10, ios::cur); 

Assigning Iostreams

Earlier versions of C++ allowed assignment of one stream to another. This is no longer allowed.

The problem with copying a stream object is that when two versions of the state information exist (such as a pointer to the current write point within an output file) each might be changed independently. This can cause problems. Copying stream objects is usually not necessary.

Format Control

Format control is described in the next chapter.

Manipulators

A manipulator is a value you can insert into or extract from iostreams to have special effects.

Because manipulators are ordinary identifiers, and therefore use up possible names, iostream does not define them for every possible function. Manipulators are listed and described in the next chapter.

To use predefined manipulators, include header file iomanip.h in your program.

You can define your own manipulators. The two basic kinds of manipulators are:

• Plain manipulator
• Parameterized manipulator.

A plain manipulator is a function that

1. Takes a reference to a stream (takes an istream&, ostream&, or ios& argument).
2. Operates on it in some way.
3. Returns its argument.

 ostream& tab(ostream& os) { 
         return os << '\t' ; 
	 } 
 ... 
 cout << x << tab << y ; 

 const char tab = '\t'; 
 ... 
 cout << x << tab << y; 

ostream& skipon(istream &is) { 
       is. setf(ios::skipws, ios::skipws); 
       return is; 
} 

ostream& skipoff(istream& is) { 
       is. unsetf(ios::skipws); 
       return is; 
} 

 ... 
cin >> skipon >> x >> skipoff >> y; 

A parameterized manipulator takes one or more parameters. It:

1. Takes an istream&, ostream&, or ios& argument, one additional argument (the parameter).
2. Operates on the stream.
3. Returns its stream argument.

	// file setfill.cpp 
	#include <iostream.h>
	#include <iomanip.h>

	// the private manipulator 
	static ios& sfill(ios& i, int f) { 
		 i.fill((char) f); 
		 return i; 
	} 

	// the public applicator 
	smanip_int setfill(int f) { 
		return smanip_int(sfill, f); 
	} 

	static ostream& xfield(ostream& os, long v)
	{ 
		long save = os. setf(ios::hex, ios::basefield); 
		os << v; 
		is. setf(save, ios::basefield); 
		return s; 
	}
 
	omanip_long print_hex(long v)
	{ 
		return omanip_long(xfield, v); 
	} 

See strstream in the next chapter.

stdiobufs: Iostreams for stdio FILEs

See stdiobuf in the next chapter.

Streambufs

Iostreams are the formatting part of a two-part (input or output) system. The other part of the system is made up of streambufs, which deal in input or output of unformatted streams of characters.

You usually use streambufs through iostreams, so you don't have to worry about the details of streambufs. You can use streambufs directly if you choose to, for example, if you need to improve efficiency or to get around the error handling or formatting built in to iostreams.

How Streambufs Work

A streambuf consists of a stream or sequence of characters and one or two pointers into that sequence. Each pointer points between two characters. (Pointers cannot actually point between characters, but it is helpful to think of them that way.)

° One is a put pointer, which points just before the position of the next character delivered.

° The other is a get pointer, which points just before the next character that will be fetched.

A streambuf can have one or both of these pointers.

Position of Pointers
The positions of the pointers and the contents of the sequences can be manipulated in various ways. Whether or not both pointers move when manipulated depends on the kind of streambuf used. Generally, with queue-like streambufs, the get and put pointers move independently; with file-like streambufs the get and put pointers always move together. A strstream is an example of a queue-like stream; an fstream is an example of a file-like stream.

Using Streambufs

You never create an actual streambuf object, but only objects of classes derived from class streambuf. Examples are filebuf and strstreambuf, which are described in filebuf and ssbuf, in the next chapter. You can derive your own classes from streambuf, to provide an interface to a special device, or to provide other than basic buffering. sbuf. pub and sbuf. prot, in the next chapter, discuss how to do this.

Apart from creating your own special kind of streambuf, you might want to access the streambuf associated with an iostream to get at the public member functions. In addition, each iostream has a defined inserter and extractor that take a streambuf pointer. When a streambuf is inserted or extracted, the entire stream is copied.

Here is another way to copy a file, with the error checking omitted.

 
ifstream fromFile("thisFile");
ofstream toFile ("thatFile"); 
toFile << (streambuf*) fromFile. rdbuf(); 

fromFile >> (streambuf*) toFile. rdbuf(); 

Sample Program

Here is a sample C++ program. It has a '' toy'' implementation of a string class, and a small program to test it. The class is a '' toy'' because it is not full-featured, yet it shows a number of features a real class must have.

Header file

The header file str.h for the string class string is:

/****************** str.h **********************/ 
/* header file str.h for toy C++ strings package 
*/ 
#include <string.h> /* for C string functions */ 
class ostream; /* to declare output of strings 

class string { 
public: 
   string(); 
   string(char *); 
   void append(char *); 
   const char* str() const; 
   string operator+ (const string&) const; 
   const string& operator=(const string&); 
   const string& operator=(const char*); 

private: 
   char *data; 
   size_t size; 
}; 
inline string::string() {size = 0; data = NULL;} 
inline const char* string::str() const { 
   return data;} 
ostream& operator<<(ostream&, const string&); 
istream& operator>>(istream&, string&); 

Implementation file

The implementation file str.cpp of the string class functions is:

/******************* str.cpp *********************/ 
/* implementation for toy C++ string package */ 

#include <iostream.h>
#include "str.h" 

string::string(char *aStr) 
{ 
   if (aStr == NULL) 
      size = 0; 
   else 
      size = strlen(aStr); 

   if (size == 0) 
      data = NULL; 
   else { 
      data = new char [size+ 1]; 
      strcpy(data, aStr); 
   } 
}
void string::append(char *app) 
{ 
   size_t appsize = app ? strlen(app) :0; 
   char *holder = new char [size + appsize + 1]; 

   if (size) 
      strcpy(holder, data); 
   else 
      holder[0] = 0; 

   if (app) { 
      strcpy(& holder[size], app); 
      size += appsize; 
   } 
   delete [] data; 
   data = holder; 
} 
string string::operator+(
      const string& second) const 
{ 
      string temp; 
      temp. data = new char[size + second. size + 1] 

      if (size) 
         strcpy(temp. data, data); 
      else 
         temp. data[0] = 0; 

      if (second. size) 
	strcpy(& temp. data[size], second. data); 
	temp. size = size + second. size; 
	return temp; 
} 
const string& string::operator=(
	const string& second) 
{ 
   if (this == &second) 
      return *this; /* in case string = string */ 

   delete [] data; 
   if (second. size) { 
      data = new char[second. size+ 1]; 
      size = second. size; 
      strcpy(data, second. data); 
   } 
   else { 
      data = NULL; 
      size = 0; 
   } 
   return *this; 
} 
const string& string::operator=(const char* str) 
{ 
   delete [] data; 

   if (str && str[0]) { 
      size = strlen(str); 
      data = new char[size+ 1]; 
      strcpy(data, str); 
   } 
   else { 
      data = NULL; 
      size = 0; 
   } 
   return *this; 
} 
ostream& operator<< (
      ostream& ostr, const string& output) 
{ 
   return ostr << output. data; 
} 
istream& operator>> (
      istream& istr, string& input) 
{ 
   const int maxline = 256; 
   char holder[maxline]; 
   istr. get(holder, maxline, '\n'); 
   input = holder; return istr; 
} 

/******************* testr.cpp********************/ 
/* test code for toy C++ strings package */ 

#include <iostream.h> 
#include "str.h" 

main() 
{ 
   string first ("This is a string."); 
   cout << "first: " << first <<"\n"; 

   string sec; 
   sec = first; 
   cout << "sec = first: " << sec << "\n"; 

   sec = "And this is another."; 
   cout << "another sec: " << sec << "\n"; 

   cout <<" Type in a string ....."; 
   cin >> sec; 
   cout << "sec: "<< sec << "\n" 

   string third; 
   third = sec+ first; 
   cout << "sec + first: " << third << "\n"; 

   third = sec+ sec; 
   cout << "sec + sec: " << third << "\n"; 

   third. append("plus"); 
   cout << "with append:" << third << "\n"; 

   third = third + sec; 
   cout << "added to itself:" << third << "\n"; 

   return 0; 
} 

sc -cpp -o testr testr.cpp str.cpp 

cout << setw(8) << val1 << " " << setw(4) \
   << val2 << endl; 

ostream& tab(ostream& s) 
{ 
   s << '\t'; 
   return s; 
} 

	smanip_long setiosflags(long); 

// set the flags bitvector according to the bits
// set in b static ios& sios(ios& i, long b) 
// private apply function 
{
   i. setf(b); 
   return i; 
} 
smanip_long setiosflags(long b) 
// public manip function { 
   return smanip_long(sios, b); 
} 

Xstream(); 
Xstream(int f); 
Xstream(const char *name, int mode= ios::in, 
   int prot= filebuf::openprot); 
Xstream(int f, char *ptr, int len); 

ios(sbptr) 
ios() 		// protected 
init(sbptr) 	// protected 
ios(iosref) 	// private 

if(cout ) ... // next output should succeed
if(cin >> x ) ... // input to x succeeded 

if(! cout ) ... // output will not succeed
if(! (cin >> x) ) ... // input to x failed 

ios(streambuf* sbptr);

ios(); 					// protected 
void init(streambuf* sbptr); 		// protected 

ios(iosref); 				// private 
void operator=(ios&); 			// private; 

class istream : virtual public ios { ... }; 
istream::istream(streambuf* s) 
{ 
   ios::init(s); 
   // ... 
} 

	long oldadjust = cout. setf(ios::left, \
	   ios::adjustfield); 
	cout << data; 
	cout. setf(oldadjust, ios::adjustfield); 

	cout. setf(0, ios::basefield);

	#include <iostream.h>
	main() 
	{ // copy cin to cout 
	  cin. tie(0); 
	  cout. tie(0); 
	cin >> cout. rdbuf(); // see ios for 
	                      // rdbuf return 0; 
	}

	#include <iostream.h> 
	main() 
	{ // copy cin to cout 
	  cin. tie(0); 
          cout. tie(0); 
	cout << cin. rdbuf(); 
	  // see ios::rdbuf 0; 
	}