cppdraft_translate/cppdraft/stringbuf.md

[stringbuf]

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, class Allocator = allocator>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 basic_stringbuf(const basic_string<charT, traits, SAlloc>& s, const Allocator& a): basic_stringbuf(s, ios_base::in | ios_base::out, a) {}template basic_stringbuf(const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which, const Allocator& a); templateexplicit basic_stringbuf(const basic_string<charT, traits, SAlloc>& s, ios_base::openmode which = ios_base::in | ios_base::out); templateexplicit basic_stringbuf(const T& t, ios_base::openmode which = ios_base::in | ios_base::out); template basic_stringbuf(const T& t, const Allocator& a); template basic_stringbuf(const T& t, ios_base::openmode which, const Allocator& a); 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 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); templatevoid str(const basic_string<charT, traits, SAlloc>& s); void str(basic_string<charT, traits, Allocator>&& s); templatevoid str(const T& t); 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 onlyvoid init-buf-ptrs(); // exposition only};}

1

#

The classbasic_stringbuf is derived frombasic_streambuf to associate possibly the input sequence and possibly the output sequence with a sequence of arbitrarycharacters.

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

2

#

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]

🔗

explicit basic_stringbuf(ios_base::openmode which);

1

#

Effects: Initializes the base class withbasic_streambuf() ([streambuf.cons]), andmode with which.

It isimplementation-defined whether the sequence pointers (eback(), gptr(), egptr(),pbase(), pptr(), epptr()) are initialized to null pointers.

2

#

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

🔗

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

3

#

Effects: Initializes the base class withbasic_streambuf() ([streambuf.cons]),mode with which, andbuf with s, then calls init-buf-ptrs().

🔗

basic_stringbuf(ios_base::openmode which, const Allocator& a);

4

#

Effects: Initializes the base class withbasic_streambuf() ([streambuf.cons]),mode with which, andbuf with a, then calls init-buf-ptrs().

5

#

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

🔗

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

6

#

Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]),mode with which, andbuf 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);

7

#

Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]),mode with which, andbuf 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);

8

#

Constraints: is_same_v<SAlloc, Allocator> is false.

9

#

Effects: Initializes the base class with basic_streambuf() ([streambuf.cons]),mode with which, andbuf with s, then calls init-buf-ptrs().

🔗

template<class T> explicit basic_stringbuf(const T& t, ios_base::openmode which = ios_base::in | ios_base::out); template<class T> basic_stringbuf(const T& t, const Allocator& a); template<class T> basic_stringbuf(const T& t, ios_base::openmode which, const Allocator& a);

10

#

Let which be ios_base​::​in | ios_base​::​out for the overload with no parameter which, anda be Allocator() for the overload with no parameter a.

11

#

Constraints: is_convertible_v<const T&, basic_string_view<charT, traits>> is true.

12

#

Effects: Creates a variable sv as if bybasic_string_view<charT, traits> sv = t, then value-initializes the base class, initializes mode with which, and direct-non-list-initializes buf with sv, a, then calls init-buf-ptrs().

🔗

basic_stringbuf(basic_stringbuf&& rhs); basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a);

13

#

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

14

#

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

• (14.1)

str() == rhs_p.str()

• (14.2)

gptr() - eback() == rhs_p.gptr() - rhs_p.eback()

• (14.3)

egptr() - eback() == rhs_p.egptr() - rhs_p.eback()

• (14.4)

pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()

• (14.5)

epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()

• (14.6)

if (eback()) eback() != rhs_a.eback()

• (14.7)

if (gptr()) gptr() != rhs_a.gptr()

• (14.8)

if (egptr()) egptr() != rhs_a.egptr()

• (14.9)

if (pbase()) pbase() != rhs_a.pbase()

• (14.10)

if (pptr()) pptr() != rhs_a.pptr()

• (14.11)

if (epptr()) epptr() != rhs_a.epptr()

• (14.12)

getloc() == rhs_p.getloc()

• (14.13)

rhs is empty but usable, as if std​::​move(rhs).str() was called.

31.8.2.3 Assignment and swap [stringbuf.assign]

🔗

basic_stringbuf& operator=(basic_stringbuf&& rhs);

1

#

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]).

2

#

Returns: *this.

🔗

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

3

#

Preconditions: allocator_traits​::​propagate_on_container_swap​::​value is true orget_allocator() == rhs.get_allocator() is true.

4

#

Effects: Exchanges the state of *this and rhs.

5

#

Remarks: The exception specification is equivalent to:

allocator_traits​::​propagate_on_container_swap​::​value ||

allocator_traits​::​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)));

6

#

Effects: Equivalent to x.swap(y).

31.8.2.4 Member functions [stringbuf.members]

1

#

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();

2

#

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

3

#

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.

4

#

[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]

🔗

allocator_type get_allocator() const noexcept;

5

#

Returns: buf.get_allocator().

🔗

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

6

#

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;

7

#

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

8

#

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

🔗

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

9

#

Postconditions: The underlying character sequence buf is empty andpbase(), pptr(), epptr(), eback(),gptr(), and egptr() are initialized as if by calling init-buf-ptrs() with an empty buf.

10

#

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().

🔗

basic_string_view<charT, traits> view() const noexcept;

11

#

Let sv be basic_string_view<charT, traits>.

12

#

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.

13

#

[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]

🔗

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

14

#

Effects: Equivalent to:buf = s;init-buf-ptrs();

🔗

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

15

#

Constraints: is_same_v<SAlloc, Allocator> is false.

16

#

Effects: Equivalent to:buf = s;init-buf-ptrs();

🔗

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

17

#

Effects: Equivalent to:buf = std::move(s);init-buf-ptrs();

🔗

template<class T> void str(const T& t);

18

#

Constraints: is_convertible_v<const T&, basic_string_view<charT, traits>> is true.

19

#

Effects: Equivalent to:basic_string_view<charT, traits> sv = t;buf = sv;init-buf-ptrs();

31.8.2.5 Overridden virtual functions [stringbuf.virtuals]

🔗

int_type underflow() override;

1

#

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

Otherwise, returnstraits​::​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;

2

#

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

• (2.1)

  Iftraits​::​eq_int_type(c, traits​::​eof()) returnsfalse and if the input sequence has a putback position available, and iftraits​::​eq(to_char_type(c), gptr()[-1]) returnstrue, assignsgptr() - 1 togptr(). Returns:c.

• (2.2)

  Iftraits​::​eq_int_type(c, traits​::​eof()) returnsfalse 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)

  Iftraits​::​eq_int_type(c, traits​::​eof()) returnstrue and if the input sequence has a putback position available, assignsgptr() - 1 togptr(). Returns:traits​::​not_eof(c).

3

#

Returns: As specified above, ortraits​::​eof() to indicate failure.

4

#

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;

5

#

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

• (5.1)

  Iftraits​::​eq_int_type(c, traits​::​eof()) returnsfalse and if either the output sequence has a write position available or the function makes a write position available (as described below), the function callssputc(c). Signals success by returning c.

• (5.2)

  Iftraits​::​eq_int_type(c, traits​::​eof()) returnstrue, there is no character to append. Signals success by returning a value other thantraits​::​eof().

6

#

Returns: As specified above, ortraits​::​eof() to indicate failure.

7

#

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

8

#

The function can make a write position available only ifios_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 pointeregptr() 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;

9

#

Effects: Alters the stream position within one of the controlled sequences, if possible, as indicated in Table 144.

Table 144 — 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.

10

#

For a sequence to be positioned, the function determines newoff as indicated in Table 145.

If the sequence's next pointer (eithergptr() orpptr()) is a null pointer and newoff is nonzero, the positioning operation fails.

Table 145 — 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).

11

#

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

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

12

#

Returns: pos_type(newoff), constructed from the resultant offset newoff (of typeoff_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 ispos_type(off_type(-1)).

🔗

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

13

#

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

14

#

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

🔗

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

15

#

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

16

#

Returns: this.