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VMS Help CXXLSTD, IOStreams, istream *Conan The Librarian |
Standard C++ Library
Copyright 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:
help cxxl
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, ios_base, basic_ios,
basic_streambuf, basic_iostream
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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