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cppdraft/spanbuf/virtuals.md

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+[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;