29 Input/output library [input.output]

29.7 Formatting and manipulators [iostream.format]

29.7.4 Input streams [input.streams]

1
#
The header <istream> defines two types and a function signature that control input from a stream buffer along with a function template that extracts from stream rvalues.

29.7.4.1 Class template basic_­istream [istream]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_istream : virtual public basic_ios<charT, traits> {
  public:
    // types (inherited from basic_­ios)
    using char_type   = charT;
    using int_type    = typename traits::int_type;
    using pos_type    = typename traits::pos_type;
    using off_type    = typename traits::off_type;
    using traits_type = traits;

    // [istream.cons], constructor/destructor
    explicit basic_istream(basic_streambuf<charT, traits>* sb);
    virtual ~basic_istream();

    // [istream.sentry], prefix/suffix
    class sentry;

    // [istream.formatted], formatted input
    basic_istream<charT, traits>&
      operator>>(basic_istream<charT, traits>& (*pf)(basic_istream<charT, traits>&));
    basic_istream<charT, traits>&
      operator>>(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));
    basic_istream<charT, traits>&
      operator>>(ios_base& (*pf)(ios_base&));

    basic_istream<charT, traits>& operator>>(bool& n);
    basic_istream<charT, traits>& operator>>(short& n);
    basic_istream<charT, traits>& operator>>(unsigned short& n);
    basic_istream<charT, traits>& operator>>(int& n);
    basic_istream<charT, traits>& operator>>(unsigned int& n);
    basic_istream<charT, traits>& operator>>(long& n);
    basic_istream<charT, traits>& operator>>(unsigned long& n);
    basic_istream<charT, traits>& operator>>(long long& n);
    basic_istream<charT, traits>& operator>>(unsigned long long& n);
    basic_istream<charT, traits>& operator>>(float& f);
    basic_istream<charT, traits>& operator>>(double& f);
    basic_istream<charT, traits>& operator>>(long double& f);

    basic_istream<charT, traits>& operator>>(void*& p);
    basic_istream<charT, traits>& operator>>(basic_streambuf<char_type, traits>* sb);

    // [istream.unformatted], unformatted input
    streamsize gcount() const;
    int_type get();
    basic_istream<charT, traits>& get(char_type& c);
    basic_istream<charT, traits>& get(char_type* s, streamsize n);
    basic_istream<charT, traits>& get(char_type* s, streamsize n, char_type delim);
    basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb);
    basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb, char_type delim);

    basic_istream<charT, traits>& getline(char_type* s, streamsize n);
    basic_istream<charT, traits>& getline(char_type* s, streamsize n, char_type delim);

    basic_istream<charT, traits>& ignore(streamsize n = 1, int_type delim = traits::eof());
    int_type                      peek();
    basic_istream<charT, traits>& read    (char_type* s, streamsize n);
    streamsize                    readsome(char_type* s, streamsize n);

    basic_istream<charT, traits>& putback(char_type c);
    basic_istream<charT, traits>& unget();
    int sync();

    pos_type tellg();
    basic_istream<charT, traits>& seekg(pos_type);
    basic_istream<charT, traits>& seekg(off_type, ios_base::seekdir);

  protected:
    // [istream.cons], copy/move constructor
    basic_istream(const basic_istream&) = delete;
    basic_istream(basic_istream&& rhs);

    // [istream.assign], assign and swap
    basic_istream& operator=(const basic_istream&) = delete;
    basic_istream& operator=(basic_istream&& rhs);
    void swap(basic_istream& rhs);
  };

  // [istream.extractors], character extraction templates
  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 charT, class traits, size_t N>
    basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&, charT(&)[N]);
  template<class traits, size_t N>
    basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, unsigned char(&)[N]);
  template<class traits, size_t N>
    basic_istream<char, traits>& operator>>(basic_istream<char, traits>&, signed char(&)[N]);
}
1
#
The class template basic_­istream defines a number of member function signatures that assist in reading and interpreting input from sequences controlled by a stream buffer.
2
#
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 are described as if they obtain (or extract) input characters by calling rdbuf()->sbumpc() or rdbuf()->sgetc().
They may use other public members of istream.
3
#
If rdbuf()->sbumpc() or rdbuf()->sgetc() returns traits​::​eof(), then the input function, except as explicitly noted otherwise, completes its actions and does setstate(eofbit), which may throw ios_­base​::​failure ([iostate.flags]), before returning.
4
#
If one of these called functions throws an exception, then unless explicitly noted otherwise, the input function sets badbit in the error state.
If badbit is set in exceptions(), the input function rethrows the exception without completing its actions, otherwise it does not throw anything and proceeds as if the called function had returned a failure indication.

29.7.4.1.1 Constructors [istream.cons]

🔗
explicit basic_istream(basic_streambuf<charT, traits>* sb);
1
#
Effects: Initializes the base class subobject with basic_­ios​::​init(sb) ([basic.ios.cons]).
2
#
Postconditions: gcount() == 0.
🔗
basic_istream(basic_istream&& rhs);
3
#
Effects: Default constructs the base class, copies the gcount() from rhs, calls basic_­ios<charT, traits>​::​move(rhs) to initialize the base class, and sets the gcount() for rhs to 0.
🔗
virtual ~basic_istream();
4
#
Remarks: Does not perform any operations of rdbuf().

29.7.4.1.2 Assignment and swap [istream.assign]

🔗
basic_istream& operator=(basic_istream&& rhs);
1
#
Effects: Equivalent to: swap(rhs).
2
#
Returns: *this.
🔗
void swap(basic_istream& rhs);
3
#
Effects: Calls basic_­ios<charT, traits>​::​swap(rhs).
Exchanges the values returned by gcount() and rhs.gcount().

29.7.4.1.3 Class basic_­istream​::​sentry [istream.sentry]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_istream<charT, traits>::sentry {
    bool ok_;                   // exposition only
  public:
    explicit sentry(basic_istream<charT, traits>& is, bool noskipws = false);
    ~sentry();
    explicit operator bool() const { return ok_; }
    sentry(const sentry&) = delete;
    sentry& operator=(const sentry&) = delete;
  };
}
1
#
The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.
🔗
explicit sentry(basic_istream<charT, traits>& is, bool noskipws = false);
2
#
Effects: If is.good() is false, calls is.setstate(failbit).
Otherwise, prepares for formatted or unformatted input.
First, if is.tie() is not a null pointer, the function calls is.tie()->flush() to synchronize the output sequence with any associated external C stream.
Except that this call can be suppressed if the put area of is.tie() is empty.
Further an implementation is allowed to defer the call to flush until a call of is.rdbuf()->underflow() occurs.
If no such call occurs before the sentry object is destroyed, the call to flush may be eliminated entirely.301
If noskipws is zero and is.flags() & ios_­base​::​skipws is nonzero, the function extracts and discards each character as long as the next available input character c is a whitespace character.
If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits​::​eof(), the function calls setstate(failbit | eofbit) (which may throw ios_­base​::​failure).
3
#
Remarks: The constructor 
explicit sentry(basic_istream<charT, traits>& is, bool noskipws = false)
uses the currently imbued locale in is, to determine whether the next input character is whitespace or not.
4
#
To decide if the character c is a whitespace character, the constructor performs as if it executes the following code fragment: 
const ctype<charT>& ctype = use_facet<ctype<charT>>(is.getloc());
if (ctype.is(ctype.space, c) != 0)
  // c is a whitespace character.
5
#
If, after any preparation is completed, is.good() is true, ok_­ != false otherwise, ok_­ == false.
During preparation, the constructor may call setstate(failbit) (which may throw ios_­base​::​​failure ([iostate.flags])).302
🔗
~sentry();
6
#
Effects: None.
🔗
explicit operator bool() const;
7
#
Returns: ok_­.
301)
This will be possible only in functions that are part of the library.
The semantics of the constructor used in user code is as specified.
⮥
302)
The sentry constructor and destructor can also perform additional implementation-dependent operations.
⮥

29.7.4.2 Formatted input functions [istream.formatted]

29.7.4.2.1 Common requirements [istream.formatted.reqmts]

1
#
Each formatted input function begins execution by constructing an object of class sentry with the noskipws (second) argument false.
If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input.
If an exception is thrown during input then ios_­base​::​badbit is turned on303 in *this's error state.
If (exceptions()&badbit) != 0 then the exception is rethrown.
In any case, the formatted input function destroys the sentry object.
If no exception has been thrown, it returns *this.
303)
This is done without causing an ios_­base​::​failure to be thrown.
⮥

29.7.4.2.2 Arithmetic extractors [istream.formatted.arithmetic]

🔗
operator>>(unsigned short& val); operator>>(unsigned int& val); operator>>(long& val); operator>>(unsigned long& val); operator>>(long long& val); operator>>(unsigned long long& val); operator>>(float& val); operator>>(double& val); operator>>(long double& val); operator>>(bool& val); operator>>(void*& val);
1
#
As in the case of the inserters, these extractors depend on the locale's num_­get<> object to perform parsing the input stream data.
These extractors behave as formatted input functions (as described in [istream.formatted.reqmts]).
After a sentry object is constructed, the conversion occurs as if performed by the following code fragment: 
using numget = num_get<charT, istreambuf_iterator<charT, traits>>;
iostate err = iostate::goodbit;
use_facet<numget>(loc).get(*this, 0, *this, err, val);
setstate(err);
In the above fragment, loc stands for the private member of the basic_­ios class.
[Note
:
The first argument provides an object of the istreambuf_­iterator class which is an iterator pointed to an input stream.
It bypasses istreams and uses streambufs directly.
— end note
]
Class locale relies on this type as its interface to istream, so that it does not need to depend directly on istream.
🔗
operator>>(short& val);
2
#
The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment): 
using numget = num_get<charT, istreambuf_iterator<charT, traits>>;
iostate err = ios_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (lval < numeric_limits<short>::min()) {
  err |= ios_base::failbit;
  val = numeric_limits<short>::min();
} else if (numeric_limits<short>::max() < lval) {
  err |= ios_base::failbit;
  val = numeric_limits<short>::max();
}  else
  val = static_cast<short>(lval);
setstate(err);
🔗
operator>>(int& val);
3
#
The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment): 
using numget = num_get<charT, istreambuf_iterator<charT, traits>>;
iostate err = ios_base::goodbit;
long lval;
use_facet<numget>(loc).get(*this, 0, *this, err, lval);
if (lval < numeric_limits<int>::min()) {
  err |= ios_base::failbit;
  val = numeric_limits<int>::min();
} else if (numeric_limits<int>::max() < lval) {
  err |= ios_base::failbit;
  val = numeric_limits<int>::max();
}  else
  val = static_cast<int>(lval);
setstate(err);

29.7.4.2.3 basic_­istream​::​operator>> [istream.extractors]

🔗
basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& (*pf)(basic_istream<charT, traits>&));
1
#
Effects: None.
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
2
#
Returns: pf(*this).304
🔗
basic_istream<charT, traits>& operator>>(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));
3
#
Effects: Calls pf(*this).
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
4
#
Returns: *this.
🔗
basic_istream<charT, traits>& operator>>(ios_base& (*pf)(ios_base&));
5
#
Effects: Calls pf(*this).305
This extractor does not behave as a formatted input function (as described in [istream.formatted.reqmts]).
6
#
Returns: *this.
🔗
template<class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in, charT (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, unsigned char (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, signed char (&s)[N]);
7
#
Effects: Behaves like a formatted input member (as described in [istream.formatted.reqmts]) of in.
After a sentry object is constructed, operator>> extracts characters and stores them into s.
If width() is greater than zero, n is min(size_­t(width()), N).
Otherwise n is N.
n is the maximum number of characters stored.
8
#
Characters are extracted and stored until any of the following occurs:
  • (8.1)
    n-1 characters are stored;
  • (8.2)
    end of file occurs on the input sequence;
  • (8.3)
    letting ct be use_­facet<ctype<charT>>(in.getloc()), ct.is(ct.space, c) is true.
9
#
operator>> then stores a null byte (charT()) in the next position, which may be the first position if no characters were extracted.
operator>> then calls width(0).
10
#
If the function extracted no characters, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
11
#
Returns: in.
🔗
template<class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in, charT& c); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, unsigned char& c); template<class traits> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in, signed char& c);
12
#
Effects: Behaves like a formatted input member (as described in [istream.formatted.reqmts]) of in.
After a sentry object is constructed a character is extracted from in, if one is available, and stored in c.
Otherwise, the function calls in.setstate(failbit).
13
#
Returns: in.
🔗
basic_istream<charT, traits>& operator>>(basic_streambuf<charT, traits>* sb);
14
#
Effects: Behaves as an unformatted input function.
If sb is null, calls setstate(failbit), which may throw ios_­base​::​failure ([iostate.flags]).
After a sentry object is constructed, 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:
  • (14.1)
    end-of-file occurs on the input sequence;
  • (14.2)
    inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • (14.3)
    an exception occurs (in which case the exception is caught).
15
#
If the function inserts no characters, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is set in exceptions() ([iostate.flags]), then the caught exception is rethrown.
16
#
Returns: *this.
304)
See, for example, the function signature ws(basic_­istream&) ([istream.manip]).
⮥
305)
See, for example, the function signature dec(ios_­base&).
⮥

29.7.4.3 Unformatted input functions [istream.unformatted]

1
#
Each unformatted input function begins execution by constructing an object of class sentry with the default argument noskipws (second) argument true.
If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input.
Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input.
In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of nonzero size as an argument shall also store a null character (using charT()) in the first location of the array.
If an exception is thrown during input then ios_­base​::​badbit is turned on306 in *this's error state.
(Exceptions thrown from basic_­ios<>​::​clear() are not caught or rethrown.)
If (exceptions()&badbit) != 0 then the exception is rethrown.
It also counts the number of characters extracted.
If no exception has been thrown it ends by storing the count in a member object and returning the value specified.
In any event the sentry object is destroyed before leaving the unformatted input function.
🔗
streamsize gcount() const;
2
#
Effects: None.
This member function does not behave as an unformatted input function (as described above).
3
#
Returns: The number of characters extracted by the last unformatted input member function called for the object.
🔗
int_type get();
4
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts a character c, if one is available.
Otherwise, the function calls setstate(failbit), which may throw ios_­base​::​failure ([iostate.flags]).
5
#
Returns: c if available, otherwise traits​::​eof().
🔗
basic_istream<charT, traits>& get(char_type& c);
6
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts a character, if one is available, and assigns it to c.307
Otherwise, the function calls setstate(failbit) (which may throw ios_­base​::​failure ([iostate.flags])).
7
#
Returns: *this.
🔗
basic_istream<charT, traits>& get(char_type* s, streamsize n, char_type delim);
8
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.308
Characters are extracted and stored until any of the following occurs:
  • (8.1)
    n is less than one or n - 1 characters are stored;
  • (8.2)
    end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit));
  • (8.3)
    traits​::​eq(c, delim) for the next available input character c (in which case c is not extracted).
9
#
If the function stores no characters, it calls setstate(failbit) (which may throw ios_­base​::​​failure ([iostate.flags])).
In any case, if n is greater than zero it then stores a null character into the next successive location of the array.
10
#
Returns: *this.
🔗
basic_istream<charT, traits>& get(char_type* s, streamsize n);
11
#
Effects: Calls get(s, n, widen('\n')).
12
#
Returns: Value returned by the call.
🔗
basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb, char_type delim);
13
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and inserts them in the output sequence controlled by sb.
Characters are extracted and inserted until any of the following occurs:
  • (13.1)
    end-of-file occurs on the input sequence;
  • (13.2)
    inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • (13.3)
    traits​::​eq(c, delim) for the next available input character c (in which case c is not extracted);
  • (13.4)
    an exception occurs (in which case, the exception is caught but not rethrown).
14
#
If the function inserts no characters, it calls setstate(failbit), which may throw ios_­base​::​​failure ([iostate.flags]).
15
#
Returns: *this.
🔗
basic_istream<charT, traits>& get(basic_streambuf<char_type, traits>& sb);
16
#
Effects: Calls get(sb, widen('\n')).
17
#
Returns: Value returned by the call.
🔗
basic_istream<charT, traits>& getline(char_type* s, streamsize n, char_type delim);
18
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and stores them into successive locations of an array whose first element is designated by s.309
Characters are extracted and stored until one of the following occurs:
  1. 1.end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit));
  2. 2.traits​::​eq(c, delim) for the next available input character c (in which case the input character is extracted but not stored);310
  3. 3.n is less than one or n - 1 characters are stored (in which case the function calls setstate(​failbit)).
19
#
These conditions are tested in the order shown.311
20
#
If the function extracts no characters, it calls setstate(failbit) (which may throw ios_­base​::​​failure ([iostate.flags])).312
21
#
In any case, if n is greater than zero, it then stores a null character (using charT()) into the next successive location of the array.
22
#
Returns: *this.
23
#
[Example
: 
#include <iostream>

int main() {
  using namespace std;
  const int line_buffer_size = 100;

  char buffer[line_buffer_size];
  int line_number = 0;
  while (cin.getline(buffer, line_buffer_size, '\n') || cin.gcount()) {
    int count = cin.gcount();
    if (cin.eof())
      cout << "Partial final line";     // cin.fail() is false
    else if (cin.fail()) {
      cout << "Partial long line";
      cin.clear(cin.rdstate() & ~ios_base::failbit);
    } else {
      count--;                          // Don't include newline in count
      cout << "Line " << ++line_number;
    }
    cout << " (" << count << " chars): " << buffer << endl;
  }
}
— end example
]
🔗
basic_istream<charT, traits>& getline(char_type* s, streamsize n);
24
#
Returns: getline(s, n, widen('\n'))
🔗
basic_istream<charT, traits>& ignore(streamsize n = 1, int_type delim = traits::eof());
25
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, extracts characters and discards them.
Characters are extracted until any of the following occurs:
  • (25.1)
    n != numeric_­limits<streamsize>​::​max() and n characters have been extracted so far
  • (25.2)
    end-of-file occurs on the input sequence (in which case the function calls setstate(eofbit), which may throw ios_­base​::​failure ([iostate.flags]));
  • (25.3)
    traits​::​eq_­int_­type(traits​::​to_­int_­type(c), delim) for the next available input character c (in which case c is extracted).
26
#
Remarks: The last condition will never occur if traits​::​eq_­int_­type(delim, traits​::​eof()).
27
#
Returns: *this.
🔗
int_type peek();
28
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, reads but does not extract the current input character.
29
#
Returns: traits​::​eof() if good() is false.
Otherwise, returns rdbuf()->sgetc().
🔗
basic_istream<charT, traits>& read(char_type* s, streamsize n);
30
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
Otherwise extracts characters and stores them into successive locations of an array whose first element is designated by s.313
Characters are extracted and stored until either of the following occurs:
  • (30.1)
    n characters are stored;
  • (30.2)
    end-of-file occurs on the input sequence (in which case the function calls setstate(failbit | eofbit), which may throw ios_­base​::​failure ([iostate.flags])).
31
#
Returns: *this.
🔗
streamsize readsome(char_type* s, streamsize n);
32
#
Effects: Behaves as an unformatted input function (as described above).
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
Otherwise extracts characters and stores them into successive locations of an array whose first element is designated by s.
If rdbuf()->in_­avail() == -1, calls setstate(eofbit) (which may throw ios_­base​::​failure ([iostate.flags])), and extracts no characters;
  • (32.1)
    If rdbuf()->in_­avail() == 0, extracts no characters
  • (32.2)
    If rdbuf()->in_­avail() > 0, extracts min(rdbuf()->in_­avail(), n)).
33
#
Returns: The number of characters extracted.
🔗
basic_istream<charT, traits>& putback(char_type c);
34
#
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit.
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
If rdbuf() is not null, calls rdbuf()->sputbackc(c).
If rdbuf() is null, or if sputbackc returns traits​::​eof(), calls setstate(badbit) (which may throw ios_­base​::​failure ([iostate.flags])).
[Note
:
This function extracts no characters, so the value returned by the next call to gcount() is 0.
— end note
]
35
#
Returns: *this.
🔗
basic_istream<charT, traits>& unget();
36
#
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit.
After constructing a sentry object, if !good() calls setstate(failbit) which may throw an exception, and return.
If rdbuf() is not null, calls rdbuf()->sungetc().
If rdbuf() is null, or if sungetc returns traits​::​eof(), calls setstate(badbit) (which may throw ios_­base​::​failure ([iostate.flags])).
[Note
:
This function extracts no characters, so the value returned by the next call to gcount() is 0.
— end note
]
37
#
Returns: *this.
🔗
int sync();
38
#
Effects: Behaves as an unformatted input function (as described above), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if rdbuf() is a null pointer, returns -1.
Otherwise, calls rdbuf()->pubsync() and, if that function returns -1 calls setstate(badbit) (which may throw ios_­base​::​failure ([iostate.flags]), and returns -1.
Otherwise, returns zero.
🔗
pos_type tellg();
39
#
Effects: Behaves as an unformatted input function (as described above), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount().
40
#
Returns: After constructing a sentry object, if fail() != false, returns pos_­type(-1) to indicate failure.
Otherwise, returns rdbuf()->pubseekoff(0, cur, in).
🔗
basic_istream<charT, traits>& seekg(pos_type pos);
41
#
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit, it does not count the number of characters extracted, and it does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos(pos, ios_­base​::​in).
In case of failure, the function calls setstate(failbit) (which may throw ios_­base​::​failure).
42
#
Returns: *this.
🔗
basic_istream<charT, traits>& seekg(off_type off, ios_base::seekdir dir);
43
#
Effects: Behaves as an unformatted input function (as described above), except that the function first clears eofbit, does not count the number of characters extracted, and does not affect the value returned by subsequent calls to gcount().
After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_­base​::​in).
In case of failure, the function calls setstate(​failbit) (which may throw ios_­base​::​failure).
44
#
Returns: *this.
306)
This is done without causing an ios_­base​::​failure to be thrown.
⮥
307)
Note that this function is not overloaded on types signed char and unsigned char.
⮥
308)
Note that this function is not overloaded on types signed char and unsigned char.
⮥
309)
Note that this function is not overloaded on types signed char and unsigned char.
⮥
310)
Since the final input character is “extracted”, it is counted in the gcount(), even though it is not stored.
⮥
311)
This allows an input line which exactly fills the buffer, without setting failbit.
This is different behavior than the historical AT&T implementation.
⮥
312)
This implies an empty input line will not cause failbit to be set.
⮥
313)
Note that this function is not overloaded on types signed char and unsigned char.
⮥

29.7.4.4 Standard basic_­istream manipulators [istream.manip]

1
#
Each instantiation of the function template specified in this subclause is a designated addressable function ([namespace.std]).
🔗
template<class charT, class traits> basic_istream<charT, traits>& ws(basic_istream<charT, traits>& is);
2
#
Effects: Behaves as an unformatted input function, except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount().
After constructing a sentry object extracts characters as long as the next available character c is whitespace or until there are no more characters in the sequence.
Whitespace characters are distinguished with the same criterion as used by sentry​::​sentry.
If ws stops extracting characters because there are no more available it sets eofbit, but not failbit.
3
#
Returns: is.

29.7.4.5 Rvalue stream extraction [istream.rvalue]

🔗
template<class Istream, class T> Istream&& operator>>(Istream&& is, T&& x);
1
#
Constraints: The expression is >> std​::​forward<T>(x) is well-formed when treated as an unevaluated operand and Istream is publicly and unambiguously derived from ios_­base.
2
#
Effects: Equivalent to: 
is >> std::forward<T>(x);
return std::move(is);

29.7.4.6 Class template basic_­iostream [iostreamclass]

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_iostream
    : public basic_istream<charT, traits>,
      public basic_ostream<charT, traits> {
  public:
    using char_type   = charT;
    using int_type    = typename traits::int_type;
    using pos_type    = typename traits::pos_type;
    using off_type    = typename traits::off_type;
    using traits_type = traits;

    // [iostream.cons], constructor
    explicit basic_iostream(basic_streambuf<charT, traits>* sb);

    // [iostream.dest], destructor
    virtual ~basic_iostream();

  protected:
    // [iostream.cons], constructor
    basic_iostream(const basic_iostream&) = delete;
    basic_iostream(basic_iostream&& rhs);

    // [iostream.assign], assign and swap
    basic_iostream& operator=(const basic_iostream&) = delete;
    basic_iostream& operator=(basic_iostream&& rhs);
    void swap(basic_iostream& rhs);
  };
}
1
#
The class template basic_­iostream inherits a number of functions that allow reading input and writing output to sequences controlled by a stream buffer.

29.7.4.6.1 Constructors [iostream.cons]

🔗
explicit basic_iostream(basic_streambuf<charT, traits>* sb);
1
#
Effects: Initializes the base class subobjects with basic_­istream<charT, traits>(sb) ([istream]) and basic_­ostream<charT, traits>(sb).
2
#
Postconditions: rdbuf() == sb and gcount() == 0.
🔗
basic_iostream(basic_iostream&& rhs);
3
#
Effects: Move constructs from the rvalue rhs by constructing the basic_­istream base class with move(rhs).

29.7.4.6.2 Destructor [iostream.dest]

🔗
virtual ~basic_iostream();
1
#
Remarks: Does not perform any operations on rdbuf().

29.7.4.6.3 Assignment and swap [iostream.assign]

🔗
basic_iostream& operator=(basic_iostream&& rhs);
1
#
Effects: Equivalent to: swap(rhs).
🔗
void swap(basic_iostream& rhs);
2
#
Effects: Calls basic_­istream<charT, traits>​::​swap(rhs).