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<P>The protected virtual member function endeavors to put back an element
into the
<A HREF="streambu.html#input buffer">input buffer</A>,
then make it the current element (pointed to
by the next pointer). If <CODE>meta ==
traits_type::<A HREF="string2.html#char_traits::eof">eof</A>()</CODE>,
the element to push back is effectively the one already in the stream
before the current element. Otherwise, that element is replaced by
<CODE>ch =
traits_type::<A HREF="string2.html#char_traits::to_char_type">to_char_type</A>(meta)</CODE>.
The function can put back an element in various ways:</P>

<UL>
<LI>If a <A HREF="streambu.html#putback position">putback position</A>
is available, and the element stored there compares equal to <CODE>ch</CODE>,
it can simply decrement the next pointer for the input buffer.</LI>

<LI>If the function can make a putback position available,
it can do so, set the next pointer to point at that position,
and store <CODE>ch</CODE> in that position.</LI>

<LI>If the function can push back an element onto the input stream,
it can do so, such as by calling
<CODE><A HREF="stdio.html#ungetc">ungetc</A></CODE> for an element
of type <I>char.</I></LI>
</UL>

<P>If the function cannot succeed, it returns
<CODE>traits_type::eof()</CODE>. Otherwise, it returns
<CODE>traits_type::<A HREF="string2.html#char_traits::not_eof">not_eof</A>(meta)</CODE>.</P>

<H3><A NAME="basic_filebuf::pos_type"><CODE>basic_filebuf::pos_type</CODE></A></H3>

<PRE>typedef typename traits_type::pos_type <B>pos_type</B>;</PRE>

<P>The type is a synonym for
<CODE>traits_type::<A HREF="string2.html#char_traits::pos_type">pos_type</A></CODE>.</P>

<H3><A NAME="basic_filebuf::seekoff"><CODE>basic_filebuf::seekoff</CODE></A></H3>

<PRE>virtual pos_type <B>seekoff</B>(off_type off,
    ios_base::seekdir way,
    ios_base::openmode which =
        ios_base::in | ios_base::out);</PRE>

<P>The protected virtual member function endeavors to alter the current
positions for the controlled streams. For an object of class
<CODE>basic_filebuf&lt;Elem, Tr&gt;</CODE>, a stream position can be represented
by an object of type
<CODE><A HREF="stdio.html#fpos_t">fpos_t</A></CODE>, which stores an
offset and any state information needed to parse a
<A HREF="lib_file.html#wide stream">wide stream</A>.
Offset zero designates the first element of the stream.
(An object of type
<CODE><A HREF="streambu.html#basic_streambuf::pos_type">pos_type</A></CODE>
stores at least an <CODE>fpos_t</CODE> object.)</P>

<P>For a file opened for both reading and writing,
both the input and output streams are positioned in tandem. To
<A HREF="lib_file.html#Stream States">switch</A>
between inserting and extracting, you must call either
<CODE><A HREF="streambu.html#basic_streambuf::pubseekoff">pubseekoff</A></CODE>
or
<CODE><A HREF="streambu.html#basic_streambuf::pubseekpos">pubseekpos</A></CODE>.
Calls to <CODE>pubseekoff</CODE> (and hence to <CODE>seekoff</CODE>)
have various limitations for
<A HREF="lib_file.html#text stream">text streams</A>,
<A HREF="lib_file.html#binary stream">binary streams</A>, and
<A HREF="lib_file.html#wide stream">wide streams</A>.</P>

<P>If the
<A HREF="#file pointer">file pointer</A> <CODE>fp</CODE>
is a null pointer, the function fails. Otherwise, it endeavors
to alter the stream position by calling
<CODE><A HREF="stdio.html#fseek">fseek</A>(fp, off, way)</CODE>.
If that function succeeds and the resultant position
<CODE>fposn</CODE> can be determined by calling
<CODE><A HREF="stdio.html#fgetpos">fgetpos</A>(fp, &amp;fposn)</CODE>,
the function succeeds. If the function succeeds, it returns
a value of type <CODE>pos_type</CODE> containing <CODE>fposn</CODE>.
Otherwise, it returns an invalid stream position.</P>

<H3><A NAME="basic_filebuf::seekpos"><CODE>basic_filebuf::seekpos</CODE></A></H3>

<PRE>virtual pos_type <B>seekpos</B>(pos_type pos,
    ios_base::openmode which =
        ios_base::in | ios_base::out);</PRE>

<P>The protected virtual member function endeavors to alter the current
positions for the controlled streams. For an object of class
<CODE>basic_filebuf&lt;Elem, Tr&gt;</CODE>, a stream position can be represented
by an object of type
<CODE><A HREF="stdio.html#fpos_t">fpos_t</A></CODE>, which stores an
offset and any state information needed to parse a
<A HREF="lib_file.html#wide stream">wide stream</A>.
Offset zero designates the first element of the stream.
(An object of type
<CODE><A HREF="streambu.html#basic_streambuf::pos_type">pos_type</A></CODE>
stores at least an <CODE>fpos_t</CODE> object.)</P>

<P>For a file opened for both reading and writing,
both the input and output streams are positioned in tandem. To
<A HREF="lib_file.html#Stream States">switch</A>
between inserting and extracting, you must call either
<CODE><A HREF="streambu.html#basic_streambuf::pubseekoff">pubseekoff</A></CODE>
or
<CODE><A HREF="streambu.html#basic_streambuf::pubseekpos">pubseekpos</A></CODE>.
Calls to <CODE>pubseekoff</CODE> (and hence to <CODE>seekoff</CODE>)
have various limitations for
<A HREF="lib_file.html#text stream">text streams</A>,
<A HREF="lib_file.html#binary stream">binary streams</A>, and
<A HREF="lib_file.html#wide stream">wide streams</A>.</P>

<P>For a wide stream,
if any insertions have occured since the stream was opened, or since the last
call to <CODE>streampos</CODE>, the function calls
<CODE><A HREF="#basic_filebuf::overflow">overflow</A>()</CODE>.
It also inserts any sequence needed to restore the
<A HREF="charset.html#initial conversion state">initial conversion state</A>,
by using the
<A HREF="#file conversion facet">file conversion facet</A>
<CODE>fac</CODE> to call
<CODE>fac.<A HREF="locale2.html#codecvt::unshift">unshift</A></CODE>
as needed. Each element <CODE>byte</CODE> of type <I>char</I> thus
produced is written to the associated stream designated by the
file pointer <CODE>fp</CODE> as if by successive calls of the form
<CODE><A HREF="stdio.html#fputc">fputc</A>(byte, fp)</CODE>.
If the call to <CODE>fac.unshift</CODE> or any write fails,
the function does not succeed.</P>

<P>If the
<A HREF="#file pointer">file pointer</A> <CODE>fp</CODE>
is a null pointer, the function fails. Otherwise, it endeavors
to alter the stream position by calling
<CODE><A HREF="stdio.html#fsetpos">fsetpos</A>(fp, &amp;fposn)</CODE>,
where <CODE>fposn</CODE> is the <CODE>fpos_t</CODE> object stored
in <CODE>pos</CODE>. If that function succeeds, the function returns
<CODE>pos</CODE>. Otherwise, it returns an invalid stream position.</P>

<H3><A NAME="basic_filebuf::setbuf"><CODE>basic_filebuf::setbuf</CODE></A></H3>

<PRE>virtual basic_streambuf&lt;Elem, Tr&gt;
    *<B>setbuf</B>(Elem *buffer, streamsize count);</PRE>

<P>The protected member function returns zero if the
<A HREF="#file pointer">file pointer</A> <CODE>fp</CODE>
is a null pointer. Otherwise, it calls
<CODE><A HREF="stdio.html#setvbuf">setvbuf</A>(fp, (char *)buffer,
<A HREF="stdio.html#_IOFBF">_IOFBF</A>, count * sizeof (Elem))</CODE>
to offer the array of <CODE>count</CODE> elements beginning at <CODE>buffer</CODE>
as a buffer for the stream. If that function returns a nonzero value,
the function returns a null pointer. Otherwise, it returns <CODE>this</CODE>
to signal success.</P>

<H3><A NAME="basic_filebuf::sync"><CODE>basic_filebuf::sync</CODE></A></H3>

<PRE>int <B>sync</B>();</PRE>

<P>The protected member function returns zero if the
<A HREF="#file pointer">file pointer</A> <CODE>fp</CODE>
is a null pointer. Otherwise, it returns zero only if calls to both
<CODE><A HREF="#basic_filebuf::overflow">overflow</A>()</CODE> and
<CODE><A HREF="stdio.html#fflush">fflush</A>(fp)</CODE>
succeed in flushing any pending output to the stream.</P>

<H3><A NAME="basic_filebuf::traits_type"><CODE>basic_filebuf::traits_type</CODE></A></H3>

<PRE>typedef Tr <B>traits_type</B>;</PRE>

<P>The type is a synonym for the template parameter <CODE>Tr</CODE>.</P>

<H3><A NAME="basic_filebuf::underflow"><CODE>basic_filebuf::underflow</CODE></A></H3>

<PRE>virtual int_type <B>underflow</B>();</PRE>

<P>The protected virtual member function endeavors to extract the current
element <CODE>ch</CODE> from the input stream, and return the element as
<CODE>traits_type::<A HREF="string2.html#char_traits::to_int_type">to_int_type</A>(ch)</CODE>.
It can do so in various ways:</P>

<UL>
<LI>If a <A HREF="streambu.html#read position">read position</A> is available,
it takes <CODE>ch</CODE> as the element stored in the read position
and advances the next pointer for the
<A HREF="streambu.html#input buffer">input buffer</A>.

<LI>It can read one or more elements of type <I>char,</I>
as if by successive calls of the form
<CODE><A HREF="stdio.html#fgetc">fgetc</A>(fp)</CODE>,
and convert them to an element <CODE>ch</CODE> of type <CODE>Elem</CODE>
by using the
<A HREF="#file conversion facet">file conversion facet</A>
<CODE>fac</CODE> to call
<CODE>fac.<A HREF="locale2.html#codecvt::in">in</A></CODE>
as needed.
If any read or conversion fails,
the function does not succeed.</LI>
</UL>

<P>If the function cannot succeed, it returns
<CODE>traits_type::<A HREF="string2.html#char_traits::eof">eof</A>()</CODE>. Otherwise,
it returns <CODE>ch</CODE>, converted as described above.</P>

<H2><A NAME="basic_fstream"><CODE>basic_fstream</CODE></A></H2>

<PRE>template &lt;class Elem, class Tr = char_traits&lt;Elem&gt; &gt;
    class <B>basic_fstream</B> : public basic_iostream&lt;Elem, Tr&gt; {
public:
    <B><A HREF="#basic_fstream::basic_fstream">basic_fstream</A></B>();
    explicit <B><A HREF="#basic_fstream::basic_fstream">basic_fstream</A></B>(const char *filename,
        ios_base::openmode mode =
            ios_base::in | ios_base::out);
    basic_filebuf&lt;Elem, Tr&gt; *<B><A HREF="#basic_fstream::rdbuf">rdbuf</A></B>() const;
    bool <B><A HREF="#basic_fstream::is_open">is_open</A></B>() const;
    void <B><A HREF="#basic_fstream::open">open</A></B>(const char *filename,
        ios_base::openmode mode =
            ios_base::in | ios_base::out);
    void <B><A HREF="#basic_fstream::close">close</A></B>();
    };</PRE>

<P>The template class describes an object that controls
insertion and extraction of elements and encoded objects using a
<A HREF="streambu.html#stream buffer">stream buffer</A> of class
<CODE><A HREF="#basic_filebuf">basic_filebuf</A>&lt;Elem, Tr&gt;</CODE>,
with elements of type <CODE>Elem</CODE>, whose
<A HREF="string2.html#character traits">character traits</A> are determined
by the class <CODE>Tr</CODE>. The object stores an object of class
<CODE>basic_filebuf&lt;Elem, Tr&gt;</CODE>.</P>

<H3><A NAME="basic_fstream::basic_fstream"><CODE>basic_fstream::basic_fstream</CODE></A></H3>

<PRE><B>basic_fstream</B>();
explicit <B>basic_fstream</B>(const char *filename,
    ios_base::openmode mode =
        ios_base::in | ios_base::out);</PRE>

<P>The first constructor initializes the base class by calling
<CODE><A HREF="istream.html#basic_iostream">basic_iostream</A>(sb)</CODE>,
where <CODE>sb</CODE> is the stored object of class
<CODE><A HREF="#basic_filebuf">basic_filebuf</A>&lt;Elem, Tr&gt;</CODE>.
It also initializes <CODE>sb</CODE> by calling
<CODE><A HREF="#basic_filebuf::basic_filebuf">basic_filebuf</A>&lt;Elem,
Tr&gt;()</CODE>.</P>

<P>The second constructor initializes the base class by calling
<CODE>basic_iostream(sb)</CODE>.
It also initializes <CODE>sb</CODE> by calling
<CODE><A HREF="#basic_filebuf::basic_filebuf">basic_filebuf</A>&lt;Elem,
Tr&gt;()</CODE>,
then <CODE>sb.<A HREF="#basic_filebuf::open">open</A>(filename, mode)</CODE>.
If the latter function returns a null pointer, the constructor calls
<CODE><A HREF="ios.html#basic_ios::setstate">setstate</A>(failbit)</CODE>.</P>

<H3><A NAME="basic_fstream::close"><CODE>basic_fstream::close</CODE></A></H3>

<PRE> void<B>close</B>();</PRE>

<P>The member function calls
<CODE><A HREF="#basic_fstream::rdbuf">rdbuf</A>()-&gt;
<A HREF="#basic_filebuf::close">close</A>()</CODE>.</P>

<H3><A NAME="basic_fstream::is_open"><CODE>basic_fstream::is_open</CODE></A></H3>

<PRE>bool <B>is_open</B>();</PRE>

<P>The member function returns
<CODE><A HREF="#basic_fstream::rdbuf">rdbuf</A>()-&gt;
<A HREF="#basic_filebuf::is_open">is_open</A>()</CODE>.</P>