[spanbuf.virtuals]

# 31 Input/output library [[input.output]](./#input.output)

## 31.9 Span-based streams [[span.streams]](span.streams#spanbuf.virtuals)

### 31.9.3 Class template basic_spanbuf [[spanbuf]](spanbuf#virtuals)

#### 31.9.3.5 Overridden virtual functions [spanbuf.virtuals]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10292)

[*Note [1](#note-1)*:

Because the underlying buffer is of fixed size,
neither overflow, underflow, nor pbackfail can provide useful behavior[.](#1.sentence-1)

— *end note*]

[🔗](#lib:seekoff,basic_spanbuf)

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

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10306)

*Effects*: Alters the stream position within one or both of the
controlled sequences, if possible, as follows:

- [(2.1)](#2.1)

  If ios_base​::​in is set in which, positions the input sequence;xnext is gptr(), xbeg is eback()[.](#2.1.sentence-1)

- [(2.2)](#2.2)

  If ios_base​::​out is set in which, positions the output sequence;xnext is pptr(), xbeg is pbase()[.](#2.2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10319)

If both ios_base​::​in and ios_base​::​out are set in which and way is ios_base​::​cur,
the positioning operation fails[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10324)

For a sequence to be positioned,
if its next pointer xnext (either gptr() or pptr())
is a null pointer and
the new offset newoff as computed below is nonzero,
the positioning operation fails[.](#4.sentence-1)

Otherwise, the function determines baseoff as a value of type off_type as follows:

- [(4.1)](#4.1)

0 when way is ios_base​::​beg;

- [(4.2)](#4.2)

(pptr() - pbase()) for the output sequence, or(gptr() - eback()) for the input sequence
when way is ios_base​::​cur;

- [(4.3)](#4.3)

when way is ios_base​::​end :
  * [(4.3.1)](#4.3.1)

(pptr() - pbase()) if ios_base​::​out is set in *mode* andios_base​::​in is not set in *mode*,

  * [(4.3.2)](#4.3.2)

*buf*.size() otherwise[.](#4.sentence-2)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10351)

If baseoff+off would overflow, or
if baseoff+off is less than zero, or
if baseoff+off is greater than *buf*.size(),
the positioning operation fails[.](#5.sentence-1)

Otherwise, the function computesoff_type newoff = baseoff + off; and assigns xbeg + newoff to the next pointer xnext[.](#5.sentence-2)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10362)

*Returns*: pos_type(off_type(-1)) if the positioning operation fails;pos_type(newoff) otherwise[.](#6.sentence-1)

[🔗](#lib:seekpos,basic_spanbuf)

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

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10374)

*Effects*: Equivalent to:return seekoff(off_type(sp), ios_base::beg, which);

[🔗](#lib:setbuf,basic_spanbuf)

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

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10388)

*Effects*: Equivalent to:this->span(std::span<charT>(s, n));return this;