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[filebuf.virtuals]
# 31 Input/output library [[input.output]](./#input.output)
## 31.10 File-based streams [[file.streams]](file.streams#filebuf.virtuals)
### 31.10.3 Class template basic_filebuf [[filebuf]](filebuf#virtuals)
#### 31.10.3.5 Overridden virtual functions [filebuf.virtuals]
[🔗](#lib:showmanyc,basic_filebuf)
`streamsize showmanyc() override;
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11392)
*Effects*: Behaves the same asbasic_streambuf​::​showmanyc() ([[streambuf.virtuals]](streambuf.virtuals "31.6.3.5 Virtual functions"))[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11398)
*Remarks*: An
implementation may provide an overriding definition for this function
signature if it can determine whether more characters can be read from the input
sequence[.](#2.sentence-1)
[🔗](#lib:underflow,basic_filebuf)
`int_type underflow() override;
`
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11412)
*Effects*: Behaves according to the description ofbasic_streambuf<charT, traits>​::​underflow(),
with the specialization that a sequence of characters is read from the input
sequence as if by reading from the associated file
into an internal buffer (extern_buf)
and then as if by doing:char extern_buf[XSIZE];char* extern_end;
charT intern_buf[ISIZE];
charT* intern_end;
codecvt_base::result r = a_codecvt.in(state, extern_buf, extern_buf+XSIZE, extern_end,
intern_buf, intern_buf+ISIZE, intern_end);
This shall be done in such a way that the class can recover the
position
(fpos_t)
corresponding to each character betweenintern_buf andintern_end[.](#3.sentence-2)
If the value ofr indicates thata_codecvt.in() ran out of space inintern_buf,
retry with a largerintern_buf[.](#3.sentence-3)
[🔗](#lib:uflow,basic_filebuf)
`int_type uflow() override;
`
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11453)
*Effects*: Behaves according to the description ofbasic_streambuf<charT, traits>​::​uflow(),
with the specialization that a sequence of characters is read from the input
with the same method as used byunderflow[.](#4.sentence-1)
[🔗](#lib:pbackfail,basic_filebuf)
`int_type pbackfail(int_type c = traits::eof()) override;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11468)
*Effects*: Puts back the character designated by c to the input
sequence, if possible, in one of three ways:
- [(5.1)](#5.1)
Iftraits​::​eq_int_type(c, traits​::​eof()) returnsfalse and
if the function makes a putback position available
and iftraits​::​eq(to_char_type(c), gptr()[-1]) returnstrue,
decrements the next pointer for the input sequence,gptr()[.](#5.1.sentence-1)
Returns:c[.](#5.1.sentence-2)
- [(5.2)](#5.2)
Iftraits​::​eq_int_type(c, traits​::​eof()) returnsfalse and
if the function makes a putback position available
and if the function is permitted to assign to the putback position,
decrements the next pointer for the input sequence,
and stores c there[.](#5.2.sentence-1)
Returns:c[.](#5.2.sentence-2)
- [(5.3)](#5.3)
Iftraits​::​eq_int_type(c, traits​::​eof()) returnstrue,
and if either the input sequence has a putback position available or
the function makes a putback position available,
decrements the next pointer for the input sequence,gptr()[.](#5.3.sentence-1)
Returns:traits​::​not_eof(c)[.](#5.3.sentence-2)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11518)
*Returns*: As specified above, ortraits​::​eof() to indicate failure[.](#6.sentence-1)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11524)
*Remarks*: Ifis_open() == false,
the function always fails[.](#7.sentence-1)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11530)
The function does not put back a character directly to the input sequence[.](#8.sentence-1)
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11533)
If the function can succeed in more than one of these ways, it is
unspecified which way is chosen[.](#9.sentence-1)
The function can alter the number of putback positions available as a result of any call[.](#9.sentence-2)
[🔗](#lib:overflow,basic_filebuf)
`int_type overflow(int_type c = traits::eof()) override;
`
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11545)
*Effects*: Behaves according to the description ofbasic_streambuf<charT, traits>​::​overflow(c),
except that the behavior of “consuming characters” is performed by first
converting as if by:charT* b = pbase();
charT* p = pptr();
charT* end;char xbuf[XSIZE];char* xbuf_end;
codecvt_base::result r = a_codecvt.out(state, b, p, end, xbuf, xbuf+XSIZE, xbuf_end); and then
- [(10.1)](#10.1)
If r == codecvt_base​::​error then fail[.](#10.1.sentence-1)
- [(10.2)](#10.2)
If r == codecvt_base​::​noconv then output characters fromb up to (and not including) p[.](#10.2.sentence-1)
- [(10.3)](#10.3)
If r == codecvt_base​::​partial then output to the file characters fromxbuf up to xbuf_end, and repeat using characters fromend to p[.](#10.3.sentence-1)
If output fails, fail (without repeating)[.](#10.3.sentence-2)
- [(10.4)](#10.4)
Otherwise output from xbuf to xbuf_end, and fail if output fails[.](#10.4.sentence-1)
At this point if b != p and b == end (xbuf isn't large
enough) then increase XSIZE and repeat from the beginning[.](#10.4.sentence-2)
Then establishes an observable checkpoint ([[intro.abstract]](intro.abstract "4.1.2 Abstract machine"))[.](#10.sentence-2)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11574)
*Returns*: traits​::​not_eof(c) to indicate success, andtraits​::​eof() to indicate failure[.](#11.sentence-1)
Ifis_open() == false,
the function always fails[.](#11.sentence-2)
[🔗](#lib:setbuf,basic_filebuf)
`basic_streambuf* setbuf(char_type* s, streamsize n) override;
`
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11591)
*Effects*: Ifsetbuf(0, 0) is called on a stream before any I/O has occurred on that stream, the
stream becomes unbuffered[.](#12.sentence-1)
Otherwise the results are implementation-defined[.](#12.sentence-2)
“Unbuffered” means thatpbase() andpptr() always return null
and output to the file should appear as soon as possible[.](#12.sentence-3)
[🔗](#lib:seekoff,basic_filebuf)
`pos_type seekoff(off_type off, ios_base::seekdir way,
ios_base::openmode which
= ios_base::in | ios_base::out) override;
`
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11615)
*Effects*: Letwidth denotea_codecvt.encoding()[.](#13.sentence-1)
Ifis_open() == false,
oroff != 0 && width <= 0,
then the positioning operation fails[.](#13.sentence-2)
Otherwise, ifway != basic_ios​::​cur oroff != 0,
and if the last operation was output,
then update the output sequence and write any unshift sequence[.](#13.sentence-3)
Next, seek to the new position: ifwidth > 0,
callfseek(file, width * off, whence),
otherwise callfseek(file, 0, whence)[.](#13.sentence-4)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11639)
*Returns*: A newly constructedpos_type object that stores the resultant
stream position, if possible[.](#14.sentence-1)
If the positioning operation fails, or
if the object cannot represent the resultant stream position,
returnspos_type(off_type(-1))[.](#14.sentence-2)
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11650)
*Remarks*: “The last operation was output” means either
the last virtual operation was overflow or
the put buffer is non-empty[.](#15.sentence-1)
“Write any unshift sequence” means,
ifwidth is less than zero then calla_codecvt.unshift(state, xbuf, xbuf+XSIZE, xbuf_end) and output the resulting unshift sequence[.](#15.sentence-2)
The function determines one of three values for the
argument whence, of typeint,
as indicated in Table [147](#tab:filebuf.seekoff "Table 147: seekoff effects")[.](#15.sentence-3)
Table [147](#tab:filebuf.seekoff) — seekoff effects [[tab:filebuf.seekoff]](./tab:filebuf.seekoff)
| [🔗](#tab:filebuf.seekoff-row-1)<br>**way Value** | **stdio Equivalent** |
| --- | --- |
| [🔗](#tab:filebuf.seekoff-row-2)<br>basic_ios​::​beg | SEEK_SET |
| [🔗](#tab:filebuf.seekoff-row-3)<br>basic_ios​::​cur | SEEK_CUR |
| [🔗](#tab:filebuf.seekoff-row-4)<br>basic_ios​::​end | SEEK_END |
[🔗](#lib:seekpos,basic_filebuf)
`pos_type seekpos(pos_type sp,
ios_base::openmode which
= ios_base::in | ios_base::out) override;
`
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11682)
Alters the file position, if possible, to correspond to the position
stored in sp (as described below)[.](#16.sentence-1)
Altering the file position performs as follows:
| [1.](#16.1) | if (om & ios_base​::​out) != 0, then update the output sequence and write any unshift sequence; |
| --- | --- |
| [2.](#16.2) | set the file position to sp as if by a call to fsetpos; |
| [3.](#16.3) | if (om & ios_base​::​in) != 0, then update the input sequence; |
where om is the open mode passed to the last call toopen()[.](#16.sentence-2)
The operation fails ifis_open() returns false[.](#16.sentence-3)
[17](#17)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11705)
If sp is an invalid stream position, or if the function positions
neither sequence, the positioning operation fails[.](#17.sentence-1)
If sp has not been obtained by a previous successful call to one of
the positioning functions
(seekoff orseekpos)
on the same file the effects are undefined[.](#17.sentence-2)
[18](#18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11715)
*Returns*: sp on success[.](#18.sentence-1)
Otherwise returnspos_type(off_type(-1))[.](#18.sentence-2)
[🔗](#lib:sync,basic_filebuf)
`int sync() override;
`
[19](#19)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11729)
*Effects*: If a put area exists, callsfilebuf​::​overflow to write the characters to the file,
then flushes the file as if by calling fflush(file)[.](#19.sentence-1)
If a get area exists, the effect is implementation-defined[.](#19.sentence-2)
[🔗](#lib:imbue,basic_filebuf)
`void imbue(const locale& loc) override;
`
[20](#20)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11745)
*Preconditions*: If the file is not positioned at its beginning and the encoding of the current
locale as determined bya_codecvt.encoding() is state-dependent ([[locale.codecvt.virtuals]](locale.codecvt.virtuals "28.3.4.2.5.3 Virtual functions"))
then that facet is the same as
the corresponding facet of loc[.](#20.sentence-1)
[21](#21)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11754)
*Effects*: Causes characters inserted or extracted after this call
to be converted according to loc until another call ofimbue[.](#21.sentence-1)
[22](#22)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11760)
*Remarks*: This may require reconversion of previously converted characters[.](#22.sentence-1)
This in turn may require the implementation to be able to reconstruct
the original contents of the file[.](#22.sentence-2)