istream(C++std)

basic_istream , istream , wistream , basic_iostream , iostream , wiostream , operator>> , ws - defines the template class that performs extractions

Synopsis

   namespace std {
   template<class E, class T = char_traits<E> >
       class basic_istream;
   typedef basic_istream<char, char_traits<char> >
       istream;
   typedef basic_istream<wchar_t, char_traits<wchar_t> >
       wistream;
   template<class E, class T = char_traits<E> >
       class basic_iostream;
   typedef basic_iostream<char, char_traits<char> >
       iostream;
   typedef basic_iostream<wchar_t, char_traits<wchar_t> >
       wiostream;

           // EXTRACTORS
   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, E *s);
   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, E& c);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char *s);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char& c);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               unsigned char *s);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               unsigned char& c);

           // MANIPULATORS
   template class<E, T>
       basic_istream<E, T>& ws(basic_istream<E, T>& is);
       };

Description

Include the iostreams standard header <istream> to define template class basic_istream, which mediates extractions for the iostreams, and the template class. basic_iostream, which mediates both insertions and extractions. The header also defines a related manipulator. (This header is typically included for you by another of the iostreams headers. You seldom have occasion to include it directly.)

basic_iostream

   template <class E, class T = char_traits<E> >
       class basic_iostream : public basic_istream<E, T>,
           public basic_ostream<E, T> {
   public:
       explicit basic_iostream(basic_streambuf<E, T>& *sb);
       virtual ~basic_iostream();
       };

The template class describes an object that controls insertions, through its base object basic_ostream<E, T>, and extractions, through its base object basic_istream<E, T>. The two objects share a common virtual base object basic_ios<E, T>. They also manage a common stream buffer, with elements of type E, whose character traits are determined by the class T. The constructor initializes its base objects via basic_istream(sb) and basic_ostream(sb).

basic_istream

basic_istream , gcount , get , getline , ignore , ipfx , isfx , operator>> , peek , putback , read , readsome , seekg , sentry , sync , tellg , unget

   template <class E, class T = char_traits<E> >
       class basic_istream
           : virtual public basic_ios<E, T> {
   public:
       typedef typename basic_ios<E, T>::char_type char_type;
       typedef typename basic_ios<E, T>::traits_type traits_type;
       typedef typename basic_ios<E, T>::int_type int_type;
       typedef typename basic_ios<E, T>::pos_type pos_type;
       typedef typename basic_ios<E, T>::off_type off_type;
       explicit basic_istream(basic_streambuf<E, T> *sb);
       class sentry;
       virtual ~istream();
       bool ipfx(bool noskip = false);
       void isfx();
       basic_istream& operator>>(
           basic_istream& (*pf)(basic_istream&));
       basic_istream& operator>>(
           ios_base& (*pf)(ios_base&));
       basic_istream& operator>>(
           basic_ios<E, T>& (*pf)(basic_ios<E, T>&));
       basic_istream& operator>>(
           basic_streambuf<E, T> *sb);
       basic_istream& operator>>(bool& n);
       basic_istream& operator>>(short& n);
       basic_istream& operator>>(unsigned short& n);
       basic_istream& operator>>(int& n);
       basic_istream& operator>>(unsigned int& n);
       basic_istream& operator>>(long& n);
       basic_istream& operator>>(unsigned long& n);
       basic_istream& operator>>(void *& n);
       basic_istream& operator>>(float& n);
       basic_istream& operator>>(double& n);
       basic_istream& operator>>(long double& n);
       streamsize gcount() const;
       int_type get();
       basic_istream& get(char_type& c);
       basic_istream& get(char_type *s, streamsize n);
       basic_istream&
           get(char_type *s, streamsize n, char_type delim);
       basic_istream&
           get(basic_streambuf<char_type, T> *sb);
       basic_istream&
           get(basic_streambuf<E, T> *sb, char_type delim);
       basic_istream& getline(char_type *s, streamsize n);
       basic_istream& getline(char_type *s, streamsize n,
           char_type delim);
       basic_istream& ignore(streamsize n = 1,
           int_type delim = traits_type::eof());
       int_type peek();
       basic_istream& read(char_type *s, streamsize n);
       streamsize readsome(char_type *s, streamsize n);
       basic_istream& putback(char_type c);
       basic_istream& unget();
       pos_type tellg();
       basic_istream& seekg(pos_type pos);
       basic_istream& seekg(off_type off,
           ios_base::seek_dir way);
       int sync();
       };

The template class describes an object that controls extraction of elements and encoded objects from a stream buffer with elements of type E, also known as char_type, whose character traits are determined by the class T, also known as traits_type.

Most of the member functions that overload operator>> are formatted input functions. They follow the pattern:

    iostate state = goodbit;
    const sentry ok(*this);
    if (ok)
        {try
            {<extract elements and convert
            accumulate flags in state
            store a successful conversion>}
        catch (...)
            {try
                {setstate(badbit); }
		catch (...)
			{}
            if ((exceptions() & badbit) != 0)
               throw; }}
    setstate(state);
    return (*this);

Many other member functions are unformatted input functions. They follow the pattern:

    iostate state = goodbit;
    count = 0;    // the value returned by gcount
    const sentry ok(*this, true);
    if (ok)
        {try
            {<extract elements and deliver
            count extracted elements in count
            accumulate flags in state>}
        catch (...)
           {try
                {setstate(badbit); }
		catch (...)
			{}
            if ((exceptions() & badbit) != 0)
                throw; }}
    setstate(state);

Both groups of functions call setstate(eofbit) if they encounter end-of-file while extracting elements.

An object of class basic_istream<E, T> stores:

a virtual public base object of class basic_ios<E, T>
an extraction count for the last unformatted input operation (called count in the code above)

basic_istream::basic_istream

   explicit basic_istream(basic_streambuf<E, T> *sb);

The constructor initializes the base class by calling init(sb). It also stores zero in the extraction count.

basic_istream::gcount

   streamsize gcount() const;

The member function returns the extraction count.

basic_istream::get

   int_type get();
   basic_istream& get(char_type& c);
   basic_istream& get(char_type *s, streamsize n);
   basic_istream& get(char_type *s, streamsize n,
       char_type delim);
   basic_istream& get(basic_streambuf<E, T> *sb);
   basic_istream& get(basic_streambuf<E, T> *sb,
       char_type delim);

The first of these unformatted input functions extracts an element, if possible, as if by returning rdbuf()->sbumpc(). Otherwise, it returns traits_type::eof(). If the function extracts no element, it calls setstate(failbit).

The second function extracts the int_type element x the same way. If x compares equal to traits_type::eof(x), the function calls setstate(failbit). Otherwise, it stores traits_type::to_char_type(x) in c. The function returns *this.

The third function returns get(s, n, widen('\n')).

The fourth function extracts up to n - 1 elements and stores them in the array beginning at s. It always stores char_type() after any extracted elements it stores. In order of testing, extraction stops:

at end of file
after the function extracts an element that compares equal to delim, in which case the element is put back to the controlled sequence
after the function extracts n - 1 elements

If the function extracts no elements, it calls setstate(failbit). In any case, it returns *this.

The fifth function returns get(sb, widen('\n')).

The sixth function extracts elements and inserts them in sb. Extraction stops on end-of-file or on an element that compares equal to delim (which is not extracted). It also stops, without extracting the element in question, if an insertion fails or throws an exception (which is caught but not rethrown). If the function extracts no elements, it calls setstate(failbit). In any case, the function returns *this.

basic_istream::getline

   basic_istream& getline(char_type *s, streamsize n);
   basic_istream& getline(char_type *s, streamsize n,
       char_type delim);

The first of these unformatted input functions returns getline(s, n, widen('\n')).

The second function extracts up to n - 1 elements and stores them in the array beginning at s. It always stores char_type() after any extracted elements it stores. In order of testing, extraction stops:

at end of file
after the function extracts an element that compares equal to delim, in which case the element is neither put back nor appended to the controlled sequence
after the function extracts n - 1 elements

If the function extracts no elements or n - 1 elements, it calls setstate(failbit). In any case, it returns *this.

basic_istream::ignore

   basic_istream& ignore(streamsize n = 1,
       int_type delim = traits_type::eof());

The unformatted input function extracts up to n elements and discards them. If n equals numeric_limits<int>::max(), however, it is taken as arbitrarily large. Extraction stops early on end-of-file or on an element x such that traits_type::to_int_type(x) compares equal to delim (which is also extracted). The function returns *this.

basic_istream::ipfx

   bool ipfx(bool noskip = false);

The member function prepares for formatted or unformatted input. If good() is true, the function:

calls tie-> flush() if tie() is not a null pointer
effectively calls ws(*this) if flags() & skipws is nonzero

If, after any such preparation, good() is false, the function calls setstate(failbit). In any case, the function returns good().

You should not call ipfx directly. It is called as needed by an object of class sentry.

basic_istream::isfx

   void isfx();

The member function has no official duties, but an implementation may depend on a call to isfx by a formatted or unformatted input function to tidy up after an extraction. You should not call isfx directly. It is called as needed by an object of class sentry.

basic_istream::operator>>

   basic_istream& operator>>(
       basic_istream& (*pf)(basic_istream&));
   basic_istream& operator>>(
       ios_base& (*pf)(ios_base&));
   basic_istream& operator>>(
       basic_ios<E, T>& (*pf)(basic_ios<E, T>&));
   basic_istream& operator>>(
       basic_streambuf<E, T> *sb);
   basic_istream& operator>>(bool& n);
   basic_istream& operator>>(short& n);
   basic_istream& operator>>(unsigned short& n);
   basic_istream& operator>>(int& n);
   basic_istream& operator>>(unsigned int& n);
   basic_istream& operator>>(long& n);
   basic_istream& operator>>(unsigned long& n);
   basic_istream& operator>>(void *& n);
   basic_istream& operator>>(float& n);
   basic_istream& operator>>(double& n);
   basic_istream& operator>>(long double& n);

The first member function ensures that an expression of the form istr >> ws calls ws(istr), then returns *this. The second and third functions ensure that other manipulators, such as hex behave similarly. The remaining functions constitute the formatted input functions.

The function:

   basic_istream& operator>>(
       basic_streambuf<E, T> *sb);

extracts elements, if sb is not a null pointer, and inserts them in sb. Extraction stops on end-of-file. It also stops, without extracting the element in question, if an insertion fails or throws an exception (which is caught but not rethrown). If the function extracts no elements, it calls setstate(failbit). In any case, the function returns *this.

The function:

   basic_istream& operator>>(bool& n);

extracts a field and converts it to a boolean value by calling use_facet<num_get<E, InIt>(getloc()). get(InIt( rdbuf()), Init(0), *this, getloc(), n). Here, InIt is defined as istreambuf_iterator<E, T>. The function returns *this.

The functions:

   basic_istream& operator>>(short& n);
   basic_istream& operator>>(unsigned short& n);
   basic_istream& operator>>(int& n);
   basic_istream& operator>>(unsigned int& n);
   basic_istream& operator>>(long& n);
   basic_istream& operator>>(unsigned long& n);
   basic_istream& operator>>(void *& n);

each extract a field and convert it to a numeric value by calling use_facet<num_get<E, InIt>(getloc()). get(InIt( rdbuf()), Init(0), *this, getloc(), x). Here, InIt is defined as istreambuf_iterator<E, T>, and x has type long, unsigned long, or void * as needed.

If the converted value cannot be represented as the type of n, the function calls setstate(failbit). In any case, the function returns *this.

The functions:

   basic_istream& operator>>(float& n);
   basic_istream& operator>>(double& n);
   basic_istream& operator>>(long double& n);

If the converted value cannot be represented as the type of n, the function calls setstate(failbit). In any case, it returns *this.

basic_istream::peek

   int_type peek();

The unformatted input function extracts an element, if possible, as if by returning rdbuf()->sgetc(). Otherwise, it returns traits_type::eof().

basic_istream::putback

   basic_istream& putback(char_type c);

The unformatted input function puts back c, if possible, as if by calling rdbuf()->sputbackc(). If rdbuf() is a null pointer, or if the call to sputbackc returns traits_type::eof(), the function calls setstate(badbit). In any case, it returns *this.

basic_istream::read

   basic_istream& read(char_type *s, streamsize n);

The unformatted input function extracts up to n elements and stores them in the array beginning at s. Extraction stops early on end-of-file, in which case the function calls setstate(failbit). In any case, it returns *this.

basic_istream::readsome

   streamsize readsome(char_type *s, streamsize n);

The member function extracts up to n elements and stores them in the array beginning at s. If rdbuf() is a null pointer, the function calls setstate(failbit). Otherwise, it assigns the value of rdbuf()->in_avail() to N. if N < 0, the function calls setstate(eofbit). Otherwise, it replaces the value stored in N with the smaller of n and N, then calls read(s, N). In any case, the function returns gcount().

basic_istream::seekg

   basic_istream& seekg(pos_type pos);
   basic_istream& seekg(off_type off,
       ios_base::seek_dir way);

If fail() is false, the first member function calls rdbuf()-> pubseekpos(pos). If fail() is false, the second function calls rdbuf()-> pubseekoff(off, way). Both functions return *this.

basic_istream::sentry

   class sentry {
   public:
       explicit sentry(basic_istream& is,
           bool noskip = false);
       operator bool() const;
       };

The nested class describes an object whose declaration structures the formatted input functions and the unformatted input functions. The constructor effectively calls is.ipfx(noskip) and stores the return value. operator bool() delivers this return value. The destructor effectively calls is.isfx().

basic_istream::sync

   int sync();

If rdbuf() is a null pointer, the function returns -1. Otherwise, it calls rdbuf()->pubsync(). If that returns -1, the function calls setstate(badbit) and returns -1. Otherwise, the function returns zero.

basic_istream::tellg

   pos_type tellg();

If fail() is false, the member function returns rdbuf()-> pubseekoff(0, cur, in). Otherwise, it returns pos_type(-1).

basic_istream::unget

   basic_istream& unget();

The unformatted input function puts back the previous element in the stream, if possible, as if by calling rdbuf()->sungetc(). If rdbuf() is a null pointer, or if the call to sungetc returns traits_type::eof(), the function calls setstate(badbit). In any case, it returns *this.

iostream

   typedef basic_iostream<char, char_traits<char> > iostream;

The type is a synonym for template class basic_iostream, specialized for elements of type char with default character traits.

istream

   typedef basic_istream<char, char_traits<char> > istream;

The type is a synonym for template class basic_istream, specialized for elements of type char with default character traits.

operator>>

   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, E *s);
   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, E& c);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char *s);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char& c);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               unsigned char *s);
   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               unsigned char& c);

The template function:

   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, E *s);

extracts up to n - 1 elements and stores them in the array beginning at s. If is.width() is greater than zero, n is is.width(); otherwise it is the largest array of E that can be declared. The function always stores E() after any extracted elements it stores. Extraction stops early on end-of-file or on any element (which is not extracted) that would be discarded by ws. If the function extracts no elements, it calls is.setstate(failbit). In any case, it calls is.width(0) and returns is.

The template function:

   template<class E, class T>
       basic_istream<E, T>&
           operator>>(basic_istream<E, T>& is, char& c);

extracts an element, if possible, and stores it in c. Otherwise, it calls is.setstate(failbit). In any case, it returns is.

The template function:

   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char *s);

returns is >> (char *)s.

The template function:

   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               signed char& c);

returns is >> (char&)c.

The template function:

   template<class T>
       basic_istream<char, T>&
          operator>>(basic_istream<char, T>& is,
              unsigned char *s);

returns is >> (char *)s.

The template function:

   template<class T>
       basic_istream<char, T>&
           operator>>(basic_istream<char, T>& is,
               unsigned char& c);

returns is >> (char&)c.

wiostream

   typedef basic_iostream<wchar_t, char_traits<wchar_t> >
       wiostream;

The type is a synonym for template class basic_iostream, specialized for elements of type wchar_t with default character traits.

wistream

   typedef basic_istream<wchar_t, char_traits<wchar_t> >
       wistream;

The type is a synonym for template class basic_istream, specialized for elements of type wchar_t with default character traits.

ws

   template class<E, T>
       basic_istream<E, T>& ws(basic_istream<E, T>& is);

The manipulator extracts and discards any elements x for which use_facet< ctype<E> >( getloc()). is( ctype<E>::space, x) is true.

The function calls setstate(eofbit) if it encounters end-of-file while extracting elements. It returns is.

References

ios(C++std) , iterator(C++std) , limits(C++std) , locale(C++std) , ostream(C++std) , streambuf(C++std) , string(C++std)