30 Input/output library [input.output]

30.8 String-based streams [string.streams]

30.8.2 Class template basic_stringbuf [stringbuf]

namespace std {
  template <class charT, class traits = char_traits<charT>,
            class Allocator = allocator<charT>>
  class basic_stringbuf : public basic_streambuf<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;
    using allocator_type = Allocator;

    // [stringbuf.cons], constructors
    explicit basic_stringbuf(
      ios_base::openmode which = ios_base::in | ios_base::out);
    explicit basic_stringbuf(
      const basic_string<charT, traits, Allocator>& str,
      ios_base::openmode which = ios_base::in | ios_base::out);
    basic_stringbuf(const basic_stringbuf& rhs) = delete;
    basic_stringbuf(basic_stringbuf&& rhs);

    // [stringbuf.assign], assign and swap
    basic_stringbuf& operator=(const basic_stringbuf& rhs) = delete;
    basic_stringbuf& operator=(basic_stringbuf&& rhs);
    void swap(basic_stringbuf& rhs);

    // [stringbuf.members], get and set
    basic_string<charT, traits, Allocator> str() const;
    void str(const basic_string<charT, traits, Allocator>& s);

  protected:
    // [stringbuf.virtuals], overridden virtual functions
    int_type underflow() override;
    int_type pbackfail(int_type c = traits::eof()) override;
    int_type overflow (int_type c = traits::eof()) override;
    basic_streambuf<charT, traits>* setbuf(charT*, streamsize) override;

    pos_type seekoff(off_type off, ios_base::seekdir way,
                     ios_base::openmode which
                      = ios_base::in | ios_base::out) override;
    pos_type seekpos(pos_type sp,
                     ios_base::openmode which
                      = ios_base::in | ios_base::out) override;

  private:
    ios_base::openmode mode;  // exposition only
  };

  template <class charT, class traits, class Allocator>
    void swap(basic_stringbuf<charT, traits, Allocator>& x,
              basic_stringbuf<charT, traits, Allocator>& y);
}

The class basic_stringbuf is derived from basic_streambuf to associate possibly the input sequence and possibly the output sequence with a sequence of arbitrary characters.

The sequence can be initialized from, or made available as, an object of class basic_string.

For the sake of exposition, the maintained data is presented here as:

(2.1)
ios_base::openmode mode, has in set if the input sequence can be read, and out set if the output sequence can be written.

30.8.2.1 basic_stringbuf constructors [stringbuf.cons]

explicit basic_stringbuf(
  ios_base::openmode which = ios_base::in | ios_base::out);

Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() ([streambuf.cons]), and initializing mode with which.

Postconditions: str() == "".

explicit basic_stringbuf(
  const basic_string<charT, traits, Allocator>& s,
  ios_base::openmode which = ios_base::in | ios_base::out);

Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() ([streambuf.cons]), and initializing mode with which.

Then calls str(s).

basic_stringbuf(basic_stringbuf&& rhs);

Effects: Move constructs from the rvalue rhs.

It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.

Whether they do or not, *this and rhs reference separate buffers (if any at all) after the construction.

The openmode, locale and any other state of rhs is also copied.

Postconditions: Let rhs_p refer to the state of rhs just prior to this construction and let rhs_a refer to the state of rhs just after this construction.

(5.1)
str() == rhs_p.str()
(5.2)
gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
(5.3)
egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
(5.4)
pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
(5.5)
epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
(5.6)
if (eback()) eback() != rhs_a.eback()
(5.7)
if (gptr()) gptr() != rhs_a.gptr()
(5.8)
if (egptr()) egptr() != rhs_a.egptr()
(5.9)
if (pbase()) pbase() != rhs_a.pbase()
(5.10)
if (pptr()) pptr() != rhs_a.pptr()
(5.11)
if (epptr()) epptr() != rhs_a.epptr()

30.8.2.2 Assign and swap [stringbuf.assign]

basic_stringbuf& operator=(basic_stringbuf&& rhs);

Effects: After the move assignment *this has the observable state it would have had if it had been move constructed from rhs (see [stringbuf.cons]).

Returns: *this.

void swap(basic_stringbuf& rhs);

Effects: Exchanges the state of *this and rhs.

template <class charT, class traits, class Allocator>
  void swap(basic_stringbuf<charT, traits, Allocator>& x,
            basic_stringbuf<charT, traits, Allocator>& y);

Effects: As if by x.swap(y).

30.8.2.3 Member functions [stringbuf.members]

basic_string<charT, traits, Allocator> str() const;

Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence.

If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()).

If the basic_stringbuf was created with which & ios_base::out being nonzero then the resultant basic_string contains the character sequence in the range [pbase(), high_mark), where high_mark represents the position one past the highest initialized character in the buffer.

Characters can be initialized by writing to the stream, by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function.

In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string).

Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.

void str(const basic_string<charT, traits, Allocator>& s);

Effects: Copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to mode.

Postconditions: If mode & ios_base::out is nonzero, pbase() points to the first underlying character and epptr() >= pbase() + s.size() holds; in addition, if mode & ios_base::ate is nonzero, pptr() == pbase() + s.size() holds, otherwise pptr() == pbase() is true.

If mode & ios_base::in is nonzero, eback() points to the first underlying character, and both gptr() == eback() and egptr() == eback() + s.size() hold.

30.8.2.4 Overridden virtual functions [stringbuf.virtuals]

int_type underflow() override;

Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()).

Otherwise, returns traits::eof().

Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.

int_type pbackfail(int_type c = traits::eof()) override;

Effects: Puts back the character designated by c to the input sequence, if possible, in one of three ways:

(2.1)
If traits::eq_int_type(c, traits::eof()) returns false and if the input sequence has a putback position available, and if traits::eq(to_char_type(c), gptr()[-1]) returns true, assigns gptr() - 1 to gptr().
Returns: c.
(2.2)
If traits::eq_int_type(c, traits::eof()) returns false and if the input sequence has a putback position available, and if mode & ios_base::out is nonzero, assigns c to *--gptr().
Returns: c.
(2.3)
If traits::eq_int_type(c, traits::eof()) returns true and if the input sequence has a putback position available, assigns gptr() - 1 to gptr().
Returns: traits::not_eof(c).

Returns: traits::eof() to indicate failure.

Remarks: If the function can succeed in more than one of these ways, it is unspecified which way is chosen.

int_type overflow(int_type c = traits::eof()) override;

Effects: Appends the character designated by c to the output sequence, if possible, in one of two ways:

(5.1)
If traits::eq_int_type(c, traits::eof()) returns false and if either the output sequence has a write position available or the function makes a write position available (as described below), the function calls sputc(c).
Signals success by returning c.
(5.2)
If traits::eq_int_type(c, traits::eof()) returns true, there is no character to append.
Signals success by returning a value other than traits::eof().

Remarks: The function can alter the number of write positions available as a result of any call.

Returns: traits::eof() to indicate failure.

The function can make a write position available only if (mode & ios_base::out) != 0.

To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.

If (mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.

pos_type seekoff(off_type off, ios_base::seekdir way,
                 ios_base::openmode which
                   = ios_base::in | ios_base::out) override;

Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table 110 .

Table 110 — seekoff positioning

Conditions	Result
(which & ios_base::in) == ios_base::in	positions the input sequence
(which & ios_base::out) == ios_base::out	positions the output sequence
(which & (ios_base::in \| ios_base::out)) == (ios_base::in) \| ios_base::out)) and way == either ios_base::beg or ios_base::end	positions both the input and the output sequences
Otherwise	the positioning operation fails.

For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.

Otherwise, the function determines newoff as indicated in Table 111 .

Table 111 — newoff values

Condition	newoff Value
way == ios_base::beg	0
way == ios_base::cur	the next pointer minus the beginning pointer (xnext - xbeg).
way == ios_base::end	the high mark pointer minus the beginning pointer (high_mark - xbeg).

If (newoff + off) < 0, or if newoff + off refers to an uninitialized character ([stringbuf.members]), the positioning operation fails.

Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext.

Returns: pos_type(newoff), constructed from the resultant offset newoff (of type off_type), that stores the resultant stream position, if possible.

If the positioning operation fails, or if the constructed object cannot represent the resultant stream position, the return value is pos_type(off_type(-1)).

pos_type seekpos(pos_type sp,
                 ios_base::openmode which
                   = ios_base::in | ios_base::out) override;

Effects: Equivalent to seekoff(off_type(sp), ios_base::beg, which).

Returns: sp to indicate success, or pos_type(off_type(-1)) to indicate failure.

basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n);

Effects: implementation-defined, except that setbuf(0, 0) has no effect.

Returns: this.