29 Input/output library [input.output]

29.8 String-based streams [string.streams]

29.8.2 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
    basic_stringbuf() : basic_stringbuf(ios_base::in | ios_base::out) {}
    explicit basic_stringbuf(ios_base::openmode which);
    explicit basic_stringbuf(
      const basic_string<charT, traits, Allocator>& s,
      ios_base::openmode which = ios_base::in | ios_base::out);
    explicit basic_stringbuf(const Allocator& a)
      : basic_stringbuf(ios_base::in | ios_base::out, a) {}
    basic_stringbuf(ios_base::openmode which, const Allocator& a);
    explicit basic_stringbuf(
      basic_string<charT, traits, Allocator>&& s,
      ios_base::openmode which = ios_base::in | ios_base::out);
    template<class SAlloc>
      basic_stringbuf(
        const basic_string<charT, traits, SAlloc>& s, const Allocator& a)
        : basic_stringbuf(s, ios_base::in | ios_base::out, a) {}
    template<class SAlloc>
      basic_stringbuf(
        const basic_string<charT, traits, SAlloc>& s,
        ios_base::openmode which, const Allocator& a);
    template<class SAlloc>
      explicit basic_stringbuf(
        const basic_string<charT, traits, SAlloc>& s,
        ios_base::openmode which = ios_base::in | ios_base::out);
    basic_stringbuf(const basic_stringbuf& rhs) = delete;
    basic_stringbuf(basic_stringbuf&& rhs);
    basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a);

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

    // [stringbuf.members], getters and setters
    allocator_type get_allocator() const noexcept;

    basic_string<charT, traits, Allocator> str() const &;
    template<class SAlloc>
      basic_string<charT,traits,SAlloc> str(const SAlloc& sa) const;
    basic_string<charT, traits, Allocator> str() &&;
    basic_string_view<charT, traits> view() const noexcept;

    void str(const basic_string<charT, traits, Allocator>& s);
    template<class SAlloc>
      void str(const basic_string<charT, traits, SAlloc>& s);
    void str(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
    basic_string<charT, traits, Allocator> buf;    // exposition only
    void init_buf_ptrs();                          // exposition only
  };

  template<class charT, class traits, class Allocator>
    void swap(basic_stringbuf<charT, traits, Allocator>& x,
              basic_stringbuf<charT, traits, Allocator>& y) noexcept(noexcept(x.swap(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 and internal pointer initialization is presented here as:
  • ios_­base​::​openmode mode, has in set if the input sequence can be read, and out set if the output sequence can be written.
  • basic_­string<charT, traits, Allocator> buf contains the underlying character sequence.
  • init_­buf_­ptrs() sets the base class' get area ([streambuf.get.area]) and put area ([streambuf.put.area]) pointers after initializing, moving from, or assigning to buf accordingly.

29.8.2.1 Constructors [stringbuf.cons]

explicit basic_stringbuf(ios_base::openmode which);
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), and mode with which.
It is implementation-defined whether the sequence pointers (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) are initialized to null pointers.
Ensures: str().empty() is true.
explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), mode with which, and buf with s, then calls init_­buf_­ptrs().
basic_stringbuf(ios_base::openmode which, const Allocator &a);
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), mode with which, and buf with a, then calls init_­buf_­ptrs().
Ensures: str().empty() is true.
explicit basic_stringbuf( basic_string<charT, traits, Allocator>&& s, ios_base::openmode which = ios_base::in | ios_base::out);
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), mode with which, and buf with std​::​move(s), then calls init_­buf_­ptrs().
template<class SAlloc> basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which, const Allocator &a);
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), mode with which, and buf with {s,a}, then calls init_­buf_­ptrs().
template<class SAlloc> explicit basic_stringbuf( const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which = ios_base::in | ios_base::out);
Constraints: is_­same_­v<SAlloc,Allocator> is false.
Effects: Initializes the base class with basic_­streambuf() ([streambuf.cons]), mode with which, and buf with s, then calls init_­buf_­ptrs().
basic_stringbuf(basic_stringbuf&& rhs); basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a);
Effects: Copy constructs the base class from rhs and initializes mode with rhs.mode.
In the first form buf is initialized from std​::​move(rhs).str().
In the second form buf is initialized from {std​::​move(rhs).str(), a}.
It is implementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(), pbase(), pptr(), epptr()) obtain the values which rhs had.
Ensures: 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.
  • str() == rhs_­p.str()
  • gptr() - eback() == rhs_­p.gptr() - rhs_­p.eback()
  • egptr() - eback() == rhs_­p.egptr() - rhs_­p.eback()
  • pptr() - pbase() == rhs_­p.pptr() - rhs_­p.pbase()
  • epptr() - pbase() == rhs_­p.epptr() - rhs_­p.pbase()
  • if (eback()) eback() != rhs_­a.eback()
  • if (gptr()) gptr() != rhs_­a.gptr()
  • if (egptr()) egptr() != rhs_­a.egptr()
  • if (pbase()) pbase() != rhs_­a.pbase()
  • if (pptr()) pptr() != rhs_­a.pptr()
  • if (epptr()) epptr() != rhs_­a.epptr()
  • getloc() == rhs_­p.getloc()
  • rhs is empty but usable, as if std​::​move(rhs).str() was called.

29.8.2.2 Assignment 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) noexcept(see below);
Expects: allocator_­traits<Allocator>​::​propagate_­on_­container_­swap​::​value is true or get_­allocator() == s.get_­allocator() is true.
Effects: Exchanges the state of *this and rhs.
Remarks: The expression inside noexcept is equivalent to:
allocator_­traits<Allocator>​::​propagate_­on_­container_­swap​::​value ||
allocator_­traits<Allocator>​::​is_­always_­equal​::​value.
template<class charT, class traits, class Allocator> void swap(basic_stringbuf<charT, traits, Allocator>& x, basic_stringbuf<charT, traits, Allocator>& y) noexcept(noexcept(x.swap(y)));
Effects: As if by x.swap(y).

29.8.2.3 Member functions [stringbuf.members]

The member functions getting the underlying character sequence all refer to a high_­mark value, 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 passing a basic_­string argument, or by calling one of the str member functions passing a basic_­string as an argument.
In the latter case, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_­string).
void init_buf_ptrs(); // exposition only
Effects: Initializes the input and output sequences from buf according to mode.
Ensures:
  • If ios_­base​::​out is set in mode, pbase() points to buf.front() and epptr() >= pbase() + buf.size() is true;
    • in addition, if ios_­base​::​ate is set in mode, pptr() == pbase() + buf.size() is true,
    • otherwise pptr() == pbase() is true.
  • If ios_­base​::​in is set in mode, eback() points to buf.front(), and (gptr() == eback() && egptr() == eback() + buf.size()) is true.
Note
:
For efficiency reasons, stream buffer operations might violate invariants of buf while it is held encapsulated in the basic_­stringbuf, e.g., by writing to characters in the range [buf.data() + buf.size(), buf.data() + buf.capacity()).
All operations retrieving a basic_­string from buf ensure that the basic_­string invariants hold on the returned value.
— end note
 ]
allocator_type get_allocator() const noexcept;
Returns: buf.get_­allocator().
basic_string<charT, traits, Allocator> str() const &;
Effects: Equivalent to:
return basic_string<charT, traits, Allocator>(view(), get_allocator());
template<class SAlloc> basic_string<charT, traits, SAlloc> str(const SAlloc& sa) const;
Constraints: SAlloc is a type that qualifies as an allocator ([container.requirements.general]).
Effects: Equivalent to:
return basic_string<charT, traits, SAlloc>(view(), sa);
basic_string<charT, traits, Allocator> str() &&;
Returns: A basic_­string<charT, traits, Allocator> object move constructed from the basic_­stringbuf's underlying character sequence in buf.
This can be achieved by first adjusting buf to have the same content as view().
Ensures: The underlying character sequence buf is empty and pbase(), pptr(), epptr(), eback(), gptr(), and egptr() are initialized as if by calling init_­buf_­ptrs() with an empty buf.
basic_string_view<charT, traits> view() const noexcept;
Let sv be basic_­string_­view<charT, traits>.
Returns: A sv object referring to the basic_­stringbuf's underlying character sequence in buf:
  • If ios_­base​::​out is set in mode, then sv(pbase(), high_­mark-pbase()) is returned.
  • Otherwise, if ios_­base​::​in is set in mode, then sv(eback(), egptr()-eback()) is returned.
  • Otherwise, sv() is returned.
Note
:
Using the returned sv object after destruction or invalidation of the character sequence underlying *this is undefined behavior, unless sv.empty() is true.
— end note
 ]
void str(const basic_string<charT, traits, Allocator>& s);
Effects: Equivalent to:
buf = s;
init_buf_ptrs();
template<class SAlloc> void str(const basic_string<charT, traits, SAlloc>& s);
Constraints: is_­same_­v<SAlloc,Allocator> is false.
Effects: Equivalent to:
buf = s;
init_buf_ptrs();
void str(basic_string<charT, traits, Allocator>&& s);
Effects: Equivalent to:
buf = std::move(s);
init_buf_ptrs();

29.8.2.4 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:
  • 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.
  • 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.
  • 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: As specified above, or 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:
  • 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.
  • 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: As specified above, or 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 123.
Table 123: seekoff positioning   [tab:stringbuf.seekoff.pos]
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 either
way == ios_­base​::​beg or
way == ios_­base​::​end
positions both the input and the output sequences
Otherwise
the positioning operation fails.
For a sequence to be positioned, the function determines newoff as indicated in Table 124.
If the sequence's next pointer (either gptr() or pptr()) is a null pointer and newoff is nonzero, the positioning operation fails.
Table 124: newoff values   [tab:stringbuf.seekoff.newoff]
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.