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67
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[stringbuf.assign]
# 31 Input/output library [[input.output]](./#input.output)
## 31.8 String-based streams [[string.streams]](string.streams#stringbuf.assign)
### 31.8.2 Class template basic_stringbuf [[stringbuf]](stringbuf#assign)
#### 31.8.2.3 Assignment and swap [stringbuf.assign]
[🔗](#lib:operator=,basic_stringbuf)
`basic_stringbuf& operator=(basic_stringbuf&& rhs);
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8343)
*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]](stringbuf.cons "31.8.2.2 Constructors"))[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8348)
*Returns*: *this[.](#2.sentence-1)
[🔗](#lib:swap,basic_stringbuf)
`void swap(basic_stringbuf& rhs) noexcept(see below);
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8359)
*Preconditions*: allocator_traits<Allocator>​::​propagate_on_container_swap​::​value is true orget_allocator() == rhs.get_allocator() is true[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8365)
*Effects*: Exchanges the state of *this and rhs[.](#4.sentence-1)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8370)
*Remarks*: The exception specification is equivalent to:
allocator_traits<Allocator>​::​propagate_on_container_swap​::​value ||
allocator_traits<Allocator>​::​is_always_equal​::​value
[🔗](#lib:swap,basic_stringbuf_)
`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](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8385)
*Effects*: Equivalent to x.swap(y)[.](#6.sentence-1)

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[stringbuf.cons]
# 31 Input/output library [[input.output]](./#input.output)
## 31.8 String-based streams [[string.streams]](string.streams#stringbuf.cons)
### 31.8.2 Class template basic_stringbuf [[stringbuf]](stringbuf#cons)
#### 31.8.2.2 Constructors [stringbuf.cons]
[🔗](#lib:basic_stringbuf,constructor)
`explicit basic_stringbuf(ios_base::openmode which);
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8150)
*Effects*: Initializes the base class withbasic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")), and*mode* with which[.](#1.sentence-1)
It isimplementation-defined
whether the sequence pointers
(eback(), gptr(), egptr(),pbase(), pptr(), epptr())
are initialized to null pointers[.](#1.sentence-2)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8163)
*Postconditions*: str().empty() is true[.](#2.sentence-1)
[🔗](#lib:basic_stringbuf,constructor_)
`explicit basic_stringbuf(
const basic_string<charT, traits, Allocator>& s,
ios_base::openmode which = ios_base::in | ios_base::out);
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8176)
*Effects*: Initializes the base class withbasic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")),*mode* with which, and*buf* with s,
then calls *init-buf-ptrs*()[.](#3.sentence-1)
[🔗](#lib:basic_stringbuf,constructor__)
`basic_stringbuf(ios_base::openmode which, const Allocator& a);
`
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8191)
*Effects*: Initializes the base class withbasic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")),*mode* with which, and*buf* with a,
then calls *init-buf-ptrs*()[.](#4.sentence-1)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8199)
*Postconditions*: str().empty() is true[.](#5.sentence-1)
[🔗](#lib:basic_stringbuf,constructor___)
`explicit basic_stringbuf(
basic_string<charT, traits, Allocator>&& s,
ios_base::openmode which = ios_base::in | ios_base::out);
`
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8212)
*Effects*: Initializes the base class with basic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")),*mode* with which, and*buf* with std​::​move(s),
then calls *init-buf-ptrs*()[.](#6.sentence-1)
[🔗](#lib:basic_stringbuf,constructor____)
`template<class SAlloc>
basic_stringbuf(
const basic_string<charT, traits, SAlloc>& s,
ios_base::openmode which, const Allocator& a);
`
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8229)
*Effects*: Initializes the base class with basic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")),*mode* with which, and*buf* with {s,a},
then calls *init-buf-ptrs*()[.](#7.sentence-1)
[🔗](#lib:basic_stringbuf,constructor_____)
`template<class SAlloc>
explicit basic_stringbuf(
const basic_string<charT, traits, SAlloc>& s,
ios_base::openmode which = ios_base::in | ios_base::out);
`
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8246)
*Constraints*: is_same_v<SAlloc, Allocator> is false[.](#8.sentence-1)
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8250)
*Effects*: Initializes the base class with basic_streambuf() ([[streambuf.cons]](streambuf.cons "31.6.3.2 Constructors")),*mode* with which, and*buf* with s,
then calls *init-buf-ptrs*()[.](#9.sentence-1)
[🔗](#lib:basic_stringbuf,constructor______)
`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](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8269)
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[.](#10.sentence-1)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8274)
*Constraints*: is_convertible_v<const T&, basic_string_view<charT, traits>> is true[.](#11.sentence-1)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8279)
*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*()[.](#12.sentence-1)
[🔗](#lib:basic_stringbuf,constructor_______)
`basic_stringbuf(basic_stringbuf&& rhs);
basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a);
`
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8296)
*Effects*: Copy constructs the base class from rhs and
initializes *mode* with rhs.mode[.](#13.sentence-1)
In the first form buf is initialized
from std​::​move(rhs).str()[.](#13.sentence-2)
In the second form *buf* is initialized
from {std​::​move(rhs).str(), a}[.](#13.sentence-3)
It isimplementation-defined whether the sequence pointers in *this (eback(), gptr(), egptr(),pbase(), pptr(), epptr()) obtain
the values which rhs had[.](#13.sentence-4)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8311)
*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.sentence-1)
- [(14.1)](#14.1)
str() == rhs_p.str()
- [(14.2)](#14.2)
gptr() - eback() == rhs_p.gptr() - rhs_p.eback()
- [(14.3)](#14.3)
egptr() - eback() == rhs_p.egptr() - rhs_p.eback()
- [(14.4)](#14.4)
pptr() - pbase() == rhs_p.pptr() - rhs_p.pbase()
- [(14.5)](#14.5)
epptr() - pbase() == rhs_p.epptr() - rhs_p.pbase()
- [(14.6)](#14.6)
if (eback()) eback() != rhs_a.eback()
- [(14.7)](#14.7)
if (gptr()) gptr() != rhs_a.gptr()
- [(14.8)](#14.8)
if (egptr()) egptr() != rhs_a.egptr()
- [(14.9)](#14.9)
if (pbase()) pbase() != rhs_a.pbase()
- [(14.10)](#14.10)
if (pptr()) pptr() != rhs_a.pptr()
- [(14.11)](#14.11)
if (epptr()) epptr() != rhs_a.epptr()
- [(14.12)](#14.12)
getloc() == rhs_p.getloc()
- [(14.13)](#14.13)
rhs is empty but usable,
as if std​::​move(rhs).str() was called.

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[stringbuf.general]
# 31 Input/output library [[input.output]](./#input.output)
## 31.8 String-based streams [[string.streams]](string.streams#stringbuf.general)
### 31.8.2 Class template basic_stringbuf [[stringbuf]](stringbuf#general)
#### 31.8.2.1 General [stringbuf.general]
[🔗](#lib:basic_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]](stringbuf.cons "31.8.2.2 Constructors"), 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); 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);
basic_stringbuf(const basic_stringbuf&) = delete;
basic_stringbuf(basic_stringbuf&& rhs);
basic_stringbuf(basic_stringbuf&& rhs, const Allocator& a); // [[stringbuf.assign]](stringbuf.assign "31.8.2.3 Assignment and swap"), 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]](stringbuf.members "31.8.2.4 Member functions"), 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); template<class T>void str(const T& t); protected:// [[stringbuf.virtuals]](stringbuf.virtuals "31.8.2.5 Overridden virtual functions"), 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*};}
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8113)
The classbasic_stringbuf is derived frombasic_streambuf to associate possibly the input sequence and possibly
the output sequence with a sequence of arbitrary[*characters*](#def:characters)[.](#1.sentence-1)
The sequence can be initialized from, or made available as, an object of classbasic_string[.](#1.sentence-2)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8124)
For the sake of exposition,
the maintained data and internal pointer initialization is presented here as:
- [(2.1)](#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[.](#2.1.sentence-1)
- [(2.2)](#2.2)
basic_string<charT, traits, Allocator> *buf* contains the underlying character sequence[.](#2.2.sentence-1)
- [(2.3)](#2.3)
*init-buf-ptrs*() sets the base class'
get area ([[streambuf.get.area]](streambuf.get.area "31.6.3.4.2 Get area access")) and
put area ([[streambuf.put.area]](streambuf.put.area "31.6.3.4.3 Put area access")) pointers
after initializing, moving from, or assigning to *buf* accordingly[.](#2.3.sentence-1)

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[stringbuf.members]
# 31 Input/output library [[input.output]](./#input.output)
## 31.8 String-based streams [[string.streams]](string.streams#stringbuf.members)
### 31.8.2 Class template basic_stringbuf [[stringbuf]](stringbuf#members)
#### 31.8.2.4 Member functions [stringbuf.members]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8392)
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[.](#1.sentence-1)
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[.](#1.sentence-2)
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)[.](#1.sentence-3)
[🔗](#itemdecl:1)
`void init-buf-ptrs();
`
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8411)
*Effects*: Initializes the input and output sequences from *buf* according to *mode*[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8416)
*Postconditions*:
- [(3.1)](#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)](#3.1.1)
in addition, if ios_base​::​ate is set in *mode*, pptr() == pbase() + *buf*.size() is true,
* [(3.1.2)](#3.1.2)
otherwise pptr() == pbase() is true[.](#3.1.sentence-1)
- [(3.2)](#3.2)
If ios_base​::​in is set in *mode*, eback() points to *buf*.front(), and (gptr() == eback() && egptr() == eback() + *buf*.size()) is true[.](#3.2.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8433)
[*Note [1](#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())[.](#4.sentence-1)
All operations retrieving a basic_string from buf ensure that the basic_string invariants hold on the returned value[.](#4.sentence-2)
— *end note*]
[🔗](#lib:get_allocator,basic_stringbuf)
`allocator_type get_allocator() const noexcept;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8451)
*Returns*: *buf*.get_allocator()[.](#5.sentence-1)
[🔗](#lib:str,basic_stringbuf)
`basic_string<charT, traits, Allocator> str() const &;
`
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8462)
*Effects*: Equivalent to:return basic_string<charT, traits, Allocator>(view(), get_allocator());
[🔗](#lib:str,basic_stringbuf_)
`template<class SAlloc>
basic_string<charT, traits, SAlloc> str(const SAlloc& sa) const;
`
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8477)
*Constraints*: SAlloc is a type that
qualifies as an allocator ([[container.reqmts]](container.reqmts "23.2.2.2 Container requirements"))[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8482)
*Effects*: Equivalent to:return basic_string<charT, traits, SAlloc>(view(), sa);
[🔗](#itemdecl:5)
`basic_string<charT, traits, Allocator> str() &&;
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8495)
*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[.](#9.sentence-1)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8503)
*Returns*: A basic_string<charT, traits, Allocator> object
move constructed from
the basic_stringbuf's underlying character sequence in buf[.](#10.sentence-1)
This can be achieved by first adjusting buf to have
the same content as view()[.](#10.sentence-2)
[🔗](#lib:view,basic_stringbuf)
`basic_string_view<charT, traits> view() const noexcept;
`
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8518)
Let sv be basic_string_view<charT, traits>[.](#11.sentence-1)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8521)
*Returns*: A sv object referring to
the basic_stringbuf's underlying character sequence in buf:
- [(12.1)](#12.1)
If ios_base​::​out is set in *mode*,
then sv(pbase(), high_mark - pbase()) is returned[.](#12.1.sentence-1)
- [(12.2)](#12.2)
Otherwise, if ios_base​::​in is set in *mode*,
then sv(eback(), egptr() - eback()) is returned[.](#12.2.sentence-1)
- [(12.3)](#12.3)
Otherwise, sv() is returned[.](#12.3.sentence-1)
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8533)
[*Note [2](#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[.](#13.sentence-1)
— *end note*]
[🔗](#lib:str,basic_stringbuf__)
`void str(const basic_string<charT, traits, Allocator>& s);
`
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8547)
*Effects*: Equivalent to:*buf* = s;*init-buf-ptrs*();
[🔗](#lib:str,basic_stringbuf___)
`template<class SAlloc>
void str(const basic_string<charT, traits, SAlloc>& s);
`
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8563)
*Constraints*: is_same_v<SAlloc, Allocator> is false[.](#15.sentence-1)
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8567)
*Effects*: Equivalent to:*buf* = s;*init-buf-ptrs*();
[🔗](#lib:str,basic_stringbuf____)
`void str(basic_string<charT, traits, Allocator>&& s);
`
[17](#17)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8582)
*Effects*: Equivalent to:*buf* = std::move(s);*init-buf-ptrs*();
[🔗](#lib:str,basic_stringbuf_____)
`template<class T>
void str(const T& t);
`
[18](#18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8598)
*Constraints*: is_convertible_v<const T&, basic_string_view<charT, traits>> is true[.](#18.sentence-1)
[19](#19)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8603)
*Effects*: Equivalent to:basic_string_view<charT, traits> sv = t;*buf* = sv;*init-buf-ptrs*();

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[stringbuf.virtuals]
# 31 Input/output library [[input.output]](./#input.output)
## 31.8 String-based streams [[string.streams]](string.streams#stringbuf.virtuals)
### 31.8.2 Class template basic_stringbuf [[stringbuf]](stringbuf#virtuals)
#### 31.8.2.5 Overridden virtual functions [stringbuf.virtuals]
[🔗](#lib:underflow,basic_stringbuf)
`int_type underflow() override;
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8621)
*Returns*: If the input sequence has a read position available,
returnstraits​::​to_int_type(*gptr())[.](#1.sentence-1)
Otherwise, returnstraits​::​eof()[.](#1.sentence-2)
Any character in the underlying buffer which has been initialized is considered
to be part of the input sequence[.](#1.sentence-3)
[🔗](#lib:pbackfail,basic_stringbuf)
`int_type pbackfail(int_type c = traits::eof()) override;
`
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8638)
*Effects*: Puts back the character designated by c to the input
sequence, if possible, in one of three ways:
- [(2.1)](#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()[.](#2.1.sentence-1)
Returns:c[.](#2.1.sentence-2)
- [(2.2)](#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()[.](#2.2.sentence-1)
Returns:c[.](#2.2.sentence-2)
- [(2.3)](#2.3)
Iftraits​::​eq_int_type(c, traits​::​eof()) returnstrue and if the input sequence has a putback position available,
assignsgptr() - 1 togptr()[.](#2.3.sentence-1)
Returns:traits​::​not_eof(c)[.](#2.3.sentence-2)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8692)
*Returns*: As specified above, ortraits​::​eof() to indicate failure[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8698)
*Remarks*: If the function can succeed in more than one of these ways, it is
unspecified which way is chosen[.](#4.sentence-1)
[🔗](#lib:overflow,basic_stringbuf)
`int_type overflow(int_type c = traits::eof()) override;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8711)
*Effects*: Appends the character designated by c to the output
sequence, if possible, in one of two ways:
- [(5.1)](#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)[.](#5.1.sentence-1)
Signals success by returning c[.](#5.1.sentence-2)
- [(5.2)](#5.2)
Iftraits​::​eq_int_type(c, traits​::​eof()) returnstrue,
there is no character to append[.](#5.2.sentence-1)
Signals success by returning a value other thantraits​::​eof()[.](#5.2.sentence-2)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8741)
*Returns*: As specified above, ortraits​::​eof() to indicate failure[.](#6.sentence-1)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8747)
*Remarks*: The function can alter the number of write positions available as a
result of any call[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8752)
The function can make a write position available only ifios_base​::​out is set in *mode*[.](#8.sentence-1)
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[.](#8.sentence-2)
If ios_base​::​in is set in *mode*,
the function alters the read end pointeregptr() to point just past the new write position[.](#8.sentence-3)
[🔗](#lib:seekoff,basic_stringbuf)
`pos_type seekoff(off_type off, ios_base::seekdir way,
ios_base::openmode which
= ios_base::in | ios_base::out) override;
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8775)
*Effects*: Alters the stream position within one of the
controlled sequences, if possible, as indicated in Table [144](#tab:stringbuf.seekoff.pos "Table 144: seekoff positioning")[.](#9.sentence-1)
Table [144](#tab:stringbuf.seekoff.pos) — seekoff positioning [[tab:stringbuf.seekoff.pos]](./tab:stringbuf.seekoff.pos)
| [🔗](#tab:stringbuf.seekoff.pos-row-1)<br>**Conditions** | **Result** |
| --- | --- |
| [🔗](#tab:stringbuf.seekoff.pos-row-2)<br>ios_base​::​in is set in which | positions the input sequence |
| [🔗](#tab:stringbuf.seekoff.pos-row-3)<br>ios_base​::​out is set in which | positions the output sequence |
| [🔗](#tab:stringbuf.seekoff.pos-row-4)<br>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 |
| [🔗](#tab:stringbuf.seekoff.pos-row-5)<br>Otherwise | the positioning operation fails[.](#tab:stringbuf.seekoff.pos-row-5-column-2-sentence-1) |
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8795)
For a sequence to be positioned,
the function determines newoff as indicated in
Table [145](#tab:stringbuf.seekoff.newoff "Table 145: newoff values")[.](#10.sentence-1)
If the sequence's next pointer
(eithergptr() orpptr())
is a null pointer and newoff is nonzero,
the positioning operation fails[.](#10.sentence-2)
Table [145](#tab:stringbuf.seekoff.newoff) — newoff values [[tab:stringbuf.seekoff.newoff]](./tab:stringbuf.seekoff.newoff)
| [🔗](#tab:stringbuf.seekoff.newoff-row-1)<br>**Condition** | **newoff Value** |
| --- | --- |
| [🔗](#tab:stringbuf.seekoff.newoff-row-2)<br>way == ios_base​::​beg | 0 |
| [🔗](#tab:stringbuf.seekoff.newoff-row-3)<br>way == ios_base​::​cur | the next pointer minus the beginning pointer (xnext - xbeg)[.](#tab:stringbuf.seekoff.newoff-row-3-column-2-sentence-1) |
| [🔗](#tab:stringbuf.seekoff.newoff-row-4)<br>way == ios_base​::​end | the high mark pointer minus the beginning pointer (high_mark - xbeg)[.](#tab:stringbuf.seekoff.newoff-row-4-column-2-sentence-1) |
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8817)
If(newoff + off) < 0,
or if newoff + off refers to an uninitialized
character ([[stringbuf.members]](stringbuf.members "31.8.2.4 Member functions")),
the positioning operation fails[.](#11.sentence-1)
Otherwise, the function assignsxbeg + newoff + off to the next pointer xnext[.](#11.sentence-2)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8827)
*Returns*: pos_type(newoff),
constructed from the resultant offset newoff (of typeoff_type),
that stores the resultant stream position, if possible[.](#12.sentence-1)
If the positioning operation fails, or
if the constructed object cannot represent the resultant stream position,
the return value ispos_type(off_type(-1))[.](#12.sentence-2)
[🔗](#lib:seekpos,basic_stringbuf)
`pos_type seekpos(pos_type sp,
ios_base::openmode which
= ios_base::in | ios_base::out) override;
`
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8848)
*Effects*: Equivalent to seekoff(off_type(sp), ios_base​::​beg, which)[.](#13.sentence-1)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8852)
*Returns*: sp to indicate success, orpos_type(off_type(-1)) to indicate failure[.](#14.sentence-1)
[🔗](#lib:setbuf,basic_streambuf)
`basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n) override;
`
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8866)
*Effects*: implementation-defined,
except thatsetbuf(0, 0) has no effect[.](#15.sentence-1)
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L8873)
*Returns*: this[.](#16.sentence-1)