31 Input/output library [input.output]

31.8 String-based streams [string.streams]

31.8.2 Class template basic_stringbuf [stringbuf]

31.8.2.1 General [stringbuf.general]

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&) = delete; basic_stringbuf(basic_stringbuf&& rhs); basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a); // [stringbuf.assign], assignment and swap basic_stringbuf& operator=(const basic_stringbuf&) = 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 }; }

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:

(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.
(2.2)
basic_string<charT, traits, Allocator> buf contains the underlying character sequence.
(2.3)
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.

31.8.2.2 Constructors [stringbuf.cons]

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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.

Postconditions: str().empty() is true.

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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().

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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().

Postconditions: str().empty() is true.

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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().

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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().

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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().

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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.

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.

(11.1)
str() == rhs_p.str()
(11.2)
gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
(11.3)
egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
(11.4)
pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
(11.5)
epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
(11.6)
if (eback()) eback() != rhs_a.eback()
(11.7)
if (gptr()) gptr() != rhs_a.gptr()
(11.8)
if (egptr()) egptr() != rhs_a.egptr()
(11.9)
if (pbase()) pbase() != rhs_a.pbase()
(11.10)
if (pptr()) pptr() != rhs_a.pptr()
(11.11)
if (epptr()) epptr() != rhs_a.epptr()
(11.12)
getloc() == rhs_p.getloc()
(11.13)
rhs is empty but usable, as if std::move(rhs).str() was called.

31.8.2.3 Assignment and swap [stringbuf.assign]

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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.

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void swap(basic_stringbuf& rhs) noexcept(see below);

Preconditions: allocator_traits<Allocator>::propagate_on_container_swap::value is true or get_allocator() == rhs.get_allocator() is true.

Effects: Exchanges the state of *this and rhs.

Remarks: The exception specification is equivalent to:

allocator_traits<Allocator>::propagate_on_container_swap::value ||

allocator_traits<Allocator>::is_always_equal::value.

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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: Equivalent to: x.swap(y).

31.8.2.4 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).

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void init_buf_ptrs(); // exposition only

Effects: Initializes the input and output sequences from buf according to mode.

Postconditions:

(3.1)
If ios_base::out is set in mode, pbase() points to buf.front() and epptr() >= pbase() + buf.size() is true;
- (3.1.1)
  in addition, if ios_base::ate is set in mode, pptr() == pbase() + buf.size() is true,
- (3.1.2)
  otherwise pptr() == pbase() is true.
(3.2)
If ios_base::in is set in mode, eback() points to buf.front(), and (gptr() == eback() && egptr() == eback() + buf.size()) is true.

[Note 1:

For efficiency reasons, stream buffer operations can 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]

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allocator_type get_allocator() const noexcept;

Returns: buf.get_allocator().

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

Effects: Equivalent to: return basic_string<charT, traits, Allocator>(view(), get_allocator());

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

Constraints: SAlloc is a type that qualifies as an allocator ([container.reqmts]).

Effects: Equivalent to: return basic_string<charT, traits, SAlloc>(view(), sa);

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

Postconditions: 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.

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().

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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:

(12.1)
If ios_base::out is set in mode, then sv(pbase(), high_mark-pbase()) is returned.
(12.2)
Otherwise, if ios_base::in is set in mode, then sv(eback(), egptr()-eback()) is returned.
(12.3)
Otherwise, sv() is returned.

[Note 2:

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]

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

Effects: Equivalent to: buf = s; init_buf_ptrs();

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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();

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

Effects: Equivalent to: buf = std::move(s); init_buf_ptrs();

31.8.2.5 Overridden virtual functions [stringbuf.virtuals]

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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.

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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: 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:

(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().

Returns: As specified above, or traits::eof() to indicate failure.

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

The function can make a write position available only if ios_base::out is set in mode.

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 ios_base::in is set in mode, 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 126 .

Table 126: seekoff positioning [tab:stringbuf.seekoff.pos]

🔗	Conditions	Result
🔗	ios_base::in is set in which	positions the input sequence
🔗	ios_base::out is set in which	positions the output sequence
🔗	both ios_base::in and ios_base::out are set in which 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 127 .

If the sequence's next pointer (either gptr() or pptr()) is a null pointer and newoff is nonzero, the positioning operation fails.

Table 127: 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.

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

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

Returns: this.