istream (3C++std) - Tru64 UNIX
Standard C++ LibraryCopyright 1996, Rogue Wave Software, Inc.
NAME
basic_istream,istream
This page describes the ANSI basic_istream class. If you would like
information on the pre-ANSI istream class, use the command:
man 3C++ istream or man istream.3C++
SYNOPSIS
#include <istream>
template<class charT, class traits = char_traits<charT> >
class basic_istream
: virtual public basic_ios<charT, traits>
DESCRIPTION
The class basic_istream defines a number of member function signatures that
assist in reading and interpreting input from sequences controlled by a
stream buffer.
Two groups of member function signatures share common properties: the
formatted input functions (or extractors) and the unformatted input
functions. Both groups of input functions obtain (or extract) input
characters by calling basic_streambuf member functions. They both begin by
constructing an object of class basic_istream::sentry and, if this object
is in good state after construction, the function endeavors to obtain the
requested input. The sentry object performs exception safe initialization,
such as controlling the status of the stream or locking it in multithread
environment.
Some formatted input functions parse characters extracted from the input
sequence, converting the result to a value of some scalar data type, and
storing the converted value in an object of that scalar type. The
conversion behavior is locale dependent, and directly depend on the locale
object imbued in the stream.
INTERFACE
template<class charT, class traits = char_traits<charT> >
class basic_istream
: virtual public basic_ios<charT, traits> {
public:
typedef basic_istream<charT, traits> istream_type;
typedef basic_ios<charT, traits> ios_type;
typedef basic_streambuf<charT, traits> streambuf_type;
typedef traits traits_type;
typedef charT char_type;
typedef typename traits::int_type int_type;
typedef typename traits::pos_type pos_type;
typedef typename traits::off_type off_type;
explicit basic_istream(basic_streambuf<charT, traits> *sb);
virtual ~basic_istream();
class sentry
{
public:
inline explicit sentry(basic_istream<charT,traits>&,
bool noskipws = 0);
~sentry();
operator bool ();
};
istream_type& operator>>(istream_type& (*pf)(istream_type&));
istream_type& operator>>(ios_base& (*pf)(ios_base&));
istream_type& operator>>(ios_type& (*pf)(ios_type&));
istream_type& operator>>(bool& n);
istream_type& operator>>(short& n);
istream_type& operator>>(unsigned short& n);
istream_type& operator>>(int& n);
istream_type& operator>>(unsigned int& n);
istream_type& operator>>(long& n);
istream_type& operator>>(unsigned long& n);
istream_type& operator>>(float& f);
istream_type& operator>>(double& f);
istream_type& operator>>(long double& f);
istream_type& operator>>(void*& p);
istream_type& operator>>(streambuf_type& sb);
istream_type& operator>>(streambuf_type *sb);
int_type get();
istream_type& get(char_type *s, streamsize n, char_type delim);
istream_type& get(char_type *s, streamsize n);
istream_type& get(char_type& c);
istream_type& get(streambuf_type& sb,char_type delim);
istream_type& get(streambuf_type& sb);
istream_type& getline(char_type *s, streamsize n,char_type delim);
istream_type& getline(char_type *s, streamsize n);
istream_type& ignore(streamsize n = 1,
int_type delim = traits::eof());
istream_type& read(char_type *s, streamsize n);
streamsize readsome(char_type *s, streamsize n);
int peek();
pos_type tellg();
istream_type& seekg(pos_type&);
istream_type& seekg(off_type&, ios_base::seekdir);
istream_type& putback(char_type c);
istream_type& unget();
streamsize gcount() const;
int sync();
protected:
explicit basic_istream( );
};
//global function
template<class charT, class traits>
basic_istream<charT, traits>&
ws(basic_istream<charT, traits>&);
template<class charT, class traits>
basic_istream<charT, traits>&
operator>> (basic_istream<charT, traits>&, charT&);
template<class charT, class traits>
basic_istream<charT, traits>&
operator>> (basic_istream<charT, traits>&, charT*);
template<class traits>
basic_istream<char, traits>&
operator>> (basic_istream<char, traits>&, unsigned char&);
template<class traits>
basic_istream<char, traits>&
operator>> (basic_istream<char, traits>&, signed char&);
template<class traits>
basic_istream<char, traits>&
operator>> (basic_istream<char, traits>&, unsigned char*);
template<class traits>
basic_istream<char, traits>&
operator>> (basic_istream<char, traits>&, signed char*);
TYPES
char_type
The type char_type is a synonym for them template parameter charT.
int_type
The type int_type is a synonym of type traits::in_type.
ios_type
The type ios_type is a synonym for basic_ios<charT, traits> .
istream
The type istream is an instantiation of class basic_istream on type
char:
typedef basic_istream<char> istream;
istream_type
The type istream_type is a synonym for basic_istream<charT, traits>.
off_type
The type off_type is a synonym of type traits::off_type.
pos_type
The type pos_type is a synonym of type traits::pos_type.
streambuf_type
The type streambuf_type is a synonym for basic_streambuf<charT, traits>
.
traits_type
The type traits_type is a synonym for the template parameter traits.
wistream
The type wistream is an instantiation of class basic_istream on type
wchar_t:
typedef basic_istream<wchar_t> wistream;
PUBLIC CONSTRUCTOR
explicit basic_istream(basic_streambuf<charT, traits>* sb);
Constructs an object of class basic_istream, assigning initial values to
the base class by calling basic_ios::init(sb).
PUBLIC DESTRUCTOR
virtual ~basic_istream();
Destroys an object of class basic_istream.
SENTRY CLASS
explicit sentry(basic_istream<charT,traits>&, bool noskipws=0);
Prepares for formatted or unformatted input. First if the basic_ios
member function tie() is not a null pointer, the function synchronizes
the output sequence with any associated stream. If noskipws is zero and
the ios_base member function flags() & skipws is nonzero, the function
extracts and discards each character as long as the next available input
character is a white space character. If after any preparation is
completed the basic_ios member function good() is true, the sentry
conversion function operator bool() will return true. Otherwise it will
return false. In multithread environment the sentry object constructor
is responsible for locking the stream and the stream buffer associated
with the stream.
~sentry();
Destroys an object of class sentry. In multithread environment, the
sentry object destructor is responsible for unlocking the stream and the
stream buffer associated with the stream.
operator bool();
If after any preparation is completed the basic_ios member function
good() is true, the sentry conversion function operator bool() will
return true else it will return false.
EXTRACTORS
istream_type&
operator>>(istream_type&
(*pf) (istream_type&));
Calls pf(*this), then return *this. See, for example, the
function signature ws(basic_istream&).
istream_type&
operator>>(ios_type& (*pf) (ios_type&));
Calls pf(*this), then return *this.
istream_type&
operator>>(ios_base& (*pf) (ios_base&));
Calls pf(*this), then return *this. See, for example, the function
signature dec(ios_base&).
istream_type&
operator>>(bool& n);
Converts a Boolean value, if one is available, and stores it in n. If
the ios_base member function flag() & ios_base::boolalpha is false it
tries to read an integer value, which if found must be 0 or 1. If the
boolalpha flag is true, it reads characters until it determines whether
the characters read are correct according to the locale function
numpunct<>::truename() or numpunct<>::falsename(). If no match is found,
it calls the basic_ios member function setstate(failbit), which may
throw ios_base::failure.
istream_type&
operator>>(short& n);
Converts a signed short integer, if one is available, and stores it in
n, then returns *this.
istream_type&
operator>>(unsigned short& n);
Converts an unsigned short integer, if one is available, and stores it
in n, then return *this.
istream_type&
operator>>(int& n);
Converts a signed integer, if one is available, and stores it in n, then
return *this.
istream_type&
operator>>(unsigned int& n);
Converts an unsigned integer, if one is available, and stores it in n,
then return *this.
istream_type&
operator>>(long& n);
Converts a signed long integer, if one is available, and stores it in n,
then returns *this.
istream_type&
operator>>(unsigned long& n);
Converts an unsigned long integer, if one is available, and stores it in
n, then returns *this.
istream_type&
operator>>(float& f);
Converts a float, if one is available, and stores it in f, then returns
*this.
istream_type&
operator>>(double& f);
Converts a double, if one is available, and stores it in f, then returns
*this.
istream_type&
operator>>(long double& f);
Converts a long double, if one is available, and stores it in f, then
returns *this.
istream_type&
operator>>(void*& p);
Extracts a void pointer, if one is available, and stores it in p, then
return *this.
istream_type&
operator>>(streambuf_type* sb);
If sb is null, calls the basic_ios member function setstate(badbit),
which may throw ios_base::failure. Otherwise extracts characters from
*this and inserts them in the output sequence controlled by sb.
Characters are extracted and inserted until any of the following occurs:
• end-of-file occurs on the input sequence
• inserting in the output sequence fails
• an exception occurs
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. If
failure was due to catching an exception thrown while extracting
characters from sb and failbit is on in exception(), then the caught
exception is rethrown.
istream_type&
operator>>(streambuf_type& sb);
Extracts characters from *this and inserts them in the output sequence
controlled by sb. Characters are extracted and inserted until any of the
following occurs:
• end-of-file occurs on the input sequence
• inserting in the output sequence fails
• an exception occurs
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. If
failure was due to catching an exception thrown while extracting
characters from sb and failbit is on in exception(), then the caught
exception is rethrown.
UNFORMATTED FUNCTIONS
streamsize
gcount() const;
Returns the number of characters extracted by the last unformatted input
member function called.
int_type
get();
Extracts a character, if one is available. Otherwise, the function calls
the basic_ios member function setstate(failbit), which may throw
ios_base::failure. Returns the character extracted or traits::eof() if
none is available.
istream_type&
get(char_type& c);
Extracts a character, if one is available, and assigns it to c.
Otherwise, the function calls the basic_ios member function
setstate(failbit), which may throw ios_base::failure.
istream_type&
get(char_type* s, streamsize n,char_type delim);
Extracts characters and stores them into successive locations of an
array whose first element is designated by s. Characters are extracted
and stored until any of the following occurs:
• n-1 characters are stored
• end-of-file occurs on the input sequence
• the next available input character == delim.
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. In any
case, it stores a null character into the next successive location of
the array.
istream_type&
get(char_type* s, streamsize n);
Calls get(s,n,widen('0)).
istream_type&
get(streambuf_type& sb,char_type delim);
Extracts characters and inserts them in the output sequence controlled
by sb. Characters are extracted and inserted until any of the following
occurs:
• end-of-file occurs on the input sequence
• inserting in the output sequence fails
• the next available input character == delim.
• an exception occurs
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. If
failure was due to catching an exception thrown while extracting
characters from sb and failbit is on in exception(), then the caught
exception is rethrown.
istream_type&
get(streambuf_type& sb);
Calls get(sb,widen('0)).
istream_type&
getline(char_type* s, streamsize n, char_type delim);
Extracts characters and stores them into successive locations of an
array whose first element is designated by s. Characters are extracted
and stored until any of the following occurs:
• n-1 characters are stored
• end-of-file occurs on the input sequence
• the next available input character == delim.
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. In any
case, it stores a null character into the next successive location of
the array.
istream_type&
getline(char_type* s, streamsize n);
Calls getline(s,n,widen('0)).
istream_type&
ignore(streamsize n=1, int_type delim=traits::eof());
Extracts characters and discards them. Characters are extracted until
any of the following occurs:
• n characters are extracted
• end-of-file occurs on the input sequence
• the next available input character == delim.
int_type
peek();
Returns traits::eof() if the basic_ios member function good() returns
false. Otherwise, returns the next available character. Does not
increment the current get pointer.
istream_type&
putback(char_type c);
Insert c in the putback sequence.
istream_type&
read(char_type* s, streamsize n);
Extracts characters and stores them into successive locations of an
array whose first element is designated by s. Characters are extracted
and stored until any of the following occurs:
• n characters are stored
• end-of-file occurs on the input sequence
If the function does not store n characters, it calls the basic_ios
member function setstate(failbit), which may throw ios_base::failure.
streamsize
readsome(char_type* s, streamsize n);
Extracts characters and stores them into successive locations of an
array whose first element is designated by s. If rdbuf()->in_avail() ==
-1, calls the basic_ios member function setstate(eofbit).
• If rdbuf()->in_avail() == 0, extracts no characters
• If rdbuf()->in_avail() > 0, extracts min( rdbuf()->in_avail(),
n)
In any case the function returns the number of characters extracted.
istream_type&
seekg(pos_type& pos);
If the basic_ios member function fail() returns false, executes
rdbuf()->pubseekpos(pos), which will position the current pointer of the
input sequence at the position designated by pos.
istream_type&
seekg(off_type& off, ios_base::seekdir dir);
If the basic_ios member function fail() returns false, executes
rdbuf()->pubseekpos(off,dir), which will position the current pointer of
the input sequence at the position designated by off and dir.
int
sync();
If rdbuf() is a null pointer, return -1. Otherwise, calls rdbuf()-
>pubsync() and if that function returns -1 calls the basic_ios member
function setstate(badbit). The purpose of this function is to
synchronize the internal input buffer, with the external sequence of
characters.
pos_type
tellg();
If the basic_ios member function fail() returns true, tellg() returns
pos_type(off_type(-1)) to indicate failure. Otherwise it returns the
current position of the input sequence by calling rdbuf()-
>pubseekoff(0,cur,in).
istream_type&
unget();
If rdbuf() is not null, calls rdbuf()->sungetc(). If rdbuf() is null or
if sungetc() returns traits::eof(), calls the basic_ios member function
setstate(badbit).
NON MEMBER FUNCTIONS
template<class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, charT& c);
Extracts a character if one is available, and stores it in c. Otherwise
the function calls the basic_ios member function setstate(failbit),
which may throw ios_base::failure.
template<class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, charT* s);
Extracts characters and stores them into successive locations of an
array whose first element is designated by s. If the ios_base member
function is.width() is greater than zero, then is.width() is the maximum
number of characters stored. Characters are extracted and stored until
any of the following occurs:
• if is.witdh()>0, is.witdh()-1 characters are extracted
• end-of-file occurs on the input sequence
• the next available input character is a white space.
If the function stores no characters, it calls the basic_ios member
function setstate(failbit), which may throw ios_base::failure. In any
case, it then stores a null character into the next successive location
of the array and calls width(0).
Template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is, unsigned char& c);
Returns is >> (char&)c.
Template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is, signed char& c);
Returns is >> (char&)c.
Template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is, unsigned char* c);
Returns is >> (char*)c.
Template<class traits>
basic_istream<char, traits>&
operator>>(basic_istream<char, traits>& is, signed char* c);
Returns is >> (char*)c.
template<class charT, class traits>
basic_istream<charT, traits>&
ws(basic_istream<charT, traits>& is);
Skips any white space in the input sequence and returns is.
EXAMPLES
//
// stdlib/examples/manual/istream1.cpp
//
#include<iostream>
#include<istream>
#include<fstream>
void main ( )
{
using namespace std;
float f= 3.14159;
int i= 3;
char s[200];
// open a file for read and write operations
ofstream out("example", ios_base::in | ios_base::out
| ios_base::trunc);
// tie the istream object to the ofstream filebuf
istream in (out.rdbuf());
// output to the file
out << "Annie is the Queen of porting" << endl;
out << f << endl;
out << i << endl;
// seek to the beginning of the file
in.seekg(0);
f = i = 0;
// read from the file using formatted functions
in >> s >> f >> i;
// seek to the beginning of the file
in.seekg(0,ios_base::beg);
// output the all file to the standard output
cout << in.rdbuf();
// seek to the beginning of the file
in.seekg(0);
// read the first line in the file
// "Annie is the Queen of porting"
in.getline(s,100);
cout << s << endl;
// read the second line in the file
// 3.14159
in.getline(s,100);
cout << s << endl;
// seek to the beginning of the file
in.seekg(0);
// read the first line in the file
// "Annie is the Queen of porting"
in.get(s,100);
// remove the newline character
in.ignore();
cout << s << endl;
// read the second line in the file
// 3.14159
in.get(s,100);
cout << s << endl;
// remove the newline character
in.ignore();
// store the current file position
istream::pos_type position = in.tellg();
out << "replace the int" << endl;
// move back to the previous saved position
in.seekg(position);
// output the remain of the file
// "replace the int"
// this is equivalent to
// cout << in.rdbuf();
while( !char_traits<char>::eq_int_type(in.peek(),
char_traits<char>::eof()) )
cout << char_traits<char>::to_char_type(in.get());
cout << "0 << flush;
}
//
// istream example two
//
#include <iostream>
void main ( )
{
using namespace std;
char p[50];
// remove all the white spaces
cin >> ws;
// read characters from stdin until a newline
// or 49 characters have been read
cin.getline(p,50);
// output the result to stdout
cout << p;
}
SEE ALSO
char_traits(3C++std), ios_base(3C++std), basic_ios(3C++std),
basic_streambuf(3C++std), basic_iostream(3C++std)
Working Paper for Draft Proposed International Standard for Information
Systems--Programming Language C++, Section 27.6.1
STANDARDS CONFORMANCE
ANSI X3J16/ISO WG21 Joint C++ Committee
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