120 lines
3.6 KiB
Markdown
120 lines
3.6 KiB
Markdown
[spanbuf.virtuals]
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# 31 Input/output library [[input.output]](./#input.output)
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## 31.9 Span-based streams [[span.streams]](span.streams#spanbuf.virtuals)
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### 31.9.3 Class template basic_spanbuf [[spanbuf]](spanbuf#virtuals)
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#### 31.9.3.5 Overridden virtual functions [spanbuf.virtuals]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10292)
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[*Note [1](#note-1)*:
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Because the underlying buffer is of fixed size,
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neither overflow, underflow, nor pbackfail can provide useful behavior[.](#1.sentence-1)
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â *end note*]
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[ð](#lib:seekoff,basic_spanbuf)
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`pos_type seekoff(off_type off, ios_base::seekdir way,
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ios_base::openmode which = ios_base::in | ios_base::out) override;
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10306)
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*Effects*: Alters the stream position within one or both of the
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controlled sequences, if possible, as follows:
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- [(2.1)](#2.1)
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If ios_base::in is set in which, positions the input sequence;xnext is gptr(), xbeg is eback()[.](#2.1.sentence-1)
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- [(2.2)](#2.2)
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If ios_base::out is set in which, positions the output sequence;xnext is pptr(), xbeg is pbase()[.](#2.2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10319)
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If both ios_base::in and ios_base::out are set in which and way is ios_base::cur,
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the positioning operation fails[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10324)
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For a sequence to be positioned,
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if its next pointer xnext (either gptr() or pptr())
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is a null pointer and
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the new offset newoff as computed below is nonzero,
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the positioning operation fails[.](#4.sentence-1)
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Otherwise, the function determines baseoff as a value of type off_type as follows:
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- [(4.1)](#4.1)
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0 when way is ios_base::beg;
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- [(4.2)](#4.2)
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(pptr() - pbase()) for the output sequence, or(gptr() - eback()) for the input sequence
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when way is ios_base::cur;
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- [(4.3)](#4.3)
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when way is ios_base::end :
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* [(4.3.1)](#4.3.1)
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(pptr() - pbase()) if ios_base::out is set in *mode* andios_base::in is not set in *mode*,
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* [(4.3.2)](#4.3.2)
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*buf*.size() otherwise[.](#4.sentence-2)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10351)
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If baseoff+off would overflow, or
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if baseoff+off is less than zero, or
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if baseoff+off is greater than *buf*.size(),
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the positioning operation fails[.](#5.sentence-1)
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Otherwise, the function computesoff_type newoff = baseoff + off; and assigns xbeg + newoff to the next pointer xnext[.](#5.sentence-2)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10362)
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*Returns*: pos_type(off_type(-1)) if the positioning operation fails;pos_type(newoff) otherwise[.](#6.sentence-1)
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[ð](#lib:seekpos,basic_spanbuf)
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`pos_type seekpos(pos_type sp, ios_base::openmode which = ios_base::in | ios_base::out) override;
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`
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10374)
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*Effects*: Equivalent to:return seekoff(off_type(sp), ios_base::beg, which);
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[ð](#lib:setbuf,basic_spanbuf)
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`basic_streambuf<charT, traits>* setbuf(charT* s, streamsize n) override;
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`
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L10388)
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*Effects*: Equivalent to:this->span(std::span<charT>(s, n));return this;
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