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<h1><img src="../../../boost.png" alt="boost.png (6897 bytes)"
align="middle" width="277" height="86">Header <<cite><a
href="../../../boost/io/ios_state.hpp">boost/io/ios_state.hpp</a></cite>
></h1>
<p>The header <cite><a
href="../../../boost/io/ios_state.hpp">boost/io/ios_state.hpp</a></cite>
covers saving the stream state of objects in the C++ IOStreams
system.</p>
<h2><a name="contents">Contents</a></h2>
<ol>
<li><a href="#contents">Contents</a></li>
<li><a href="#rationale">Rationale</a></li>
<li><a href="#header">Header Synopsis</a></li>
<li><a href="#base_savers">Savers for Basic Standard Attributes</a></li>
<li><a href="#adv_savers">Savers for Advanced Standard Attributes</a></li>
<li><a href="#user_savers">Savers for User-Defined Attributes</a></li>
<li><a href="#combo_savers">Savers for Combined Attributes</a></li>
<li><a href="#example">Example</a></li>
<li><a href="#refer">References</a></li>
<li><a href="#credits">Credits</a>
<ul>
<li><a href="#contributors">Contributors</a></li>
<li><a href="#history">History</a></li>
</ul></li>
</ol>
<h2><a name="rationale">Rationale</a></h2>
<p>Sometimes a certain value has to change only for a limited scope.
Saver classes save a copy of the current state of some object (or an
aspect of an object), and reset the object's state at destruction time,
undoing any change the object may have gone through.</p>
<p>The saver class strategy is helpful when using I/O stream objects.
Manipulator objects can change some aspect of a stream during input or
output. The state changed by the manipulator usually sticks to its new
value after the I/O transaction. This can be a problem if manipulators
are used in a function that is not supposed to externally change a
stream's state.</p>
<blockquote><pre>#include <ostream>
#include <ios>
void hex_my_byte( std::ostream &os, char byte )
{
os << std::hex << static_cast<unsigned>(byte);
}
</pre></blockquote>
<p>The <var>os</var> stream will retain its new hexadecimal printing
mode after the call to <code>hex_my_byte</code>. The stream's printing
mode can be saved and restored with manual calls to the stream's state
inspecting and mutating member functions. The manual method becomes
unwieldy if the main functionality is complex and/or needs to be
exception safe. A saver class can implement the better "resource
acquisition is initialization" strategy.</p>
<p>See the <a href="#example">example</a> below for better code, using
saver classes.</p>
<h2><a name="header">Header Synopsis</a></h2>
<blockquote><pre>#include <iosfwd> <i>// for std::char_traits (declaration)</i>
namespace boost
{
namespace io
{
class ios_flags_saver;
class ios_precision_saver;
class ios_width_saver;
class ios_base_all_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_iostate_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_exception_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_tie_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_rdbuf_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_fill_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_locale_saver;
template < typename Ch, class Tr = ::std::char_traits<Ch> >
class basic_ios_all_saver;
typedef basic_ios_iostate_saver<char> ios_iostate_saver;
typedef basic_ios_iostate_saver<wchar_t> wios_iostate_saver;
typedef basic_ios_exception_saver<char> ios_exception_saver;
typedef basic_ios_exception_saver<wchar_t> wios_exception_saver;
typedef basic_ios_tie_saver<char> ios_tie_saver;
typedef basic_ios_tie_saver<wchar_t> wios_tie_saver;
typedef basic_ios_rdbuf_saver<char> ios_rdbuf_saver;
typedef basic_ios_rdbuf_saver<wchar_t> wios_rdbuf_saver;
typedef basic_ios_fill_saver<char> ios_fill_saver;
typedef basic_ios_fill_saver<wchar_t> wios_fill_saver;
typedef basic_ios_locale_saver<char> ios_locale_saver;
typedef basic_ios_locale_saver<wchar_t> wios_locale_saver;
typedef basic_ios_all_saver<char> ios_all_saver;
typedef basic_ios_all_saver<wchar_t> wios_all_saver;
class ios_iword_saver;
class ios_pword_saver;
class ios_all_word_saver;
}
}
</pre></blockquote>
<h2><a name="base_savers">Savers for Basic Standard Attributes</a></h2>
<p>The basic saver classes have this format:</p>
<blockquote><pre>class <var>saver_class</var>
{
typedef std::ios_base state_type;
typedef <i>implementation_defined</i> aspect_type;
explicit saver_class( state_type &s );
saver_class( state_type &s, <var>aspect_type</var> const &new_value );
~saver_class();
void restore();
};
</pre></blockquote>
<p>The <var>state_type</var> is the IOStreams base class
<code>std::ios_base</code>. The user would usually place an actual
input, output, or combined stream object for the state-type parameter,
and not a base class object. The first constructor takes a stream
object and saves a reference to the stream and the current value of a
particular stream attribute. The second constructor works like the
first, and uses its second argument to change the stream's attribute to
the new <var>aspect_type</var> value given. The destructor restores the
stream's attribute to the saved value. The restoration can be activated
early (and often) with the <code>restore</code> member function.</p>
<table border="1" align="center">
<caption>Basic IOStreams State Saver Classes</caption>
<tr>
<th>Class</th>
<th>Saved Attribute</th>
<th>Attribute Type</th>
<th>Reading Method</th>
<th>Writing Method</th>
</tr>
<tr>
<td><code>boost::io::ios_flags_saver</code></td>
<td>Format control flags</td>
<td><code>std::ios_base::fmtflags</code></td>
<td><code>flags</code></td>
<td><code>flags</code></td>
</tr>
<tr>
<td><code>boost::io::ios_precision_saver</code></td>
<td>Number of digits to print after decimal point</td>
<td><code>std::streamsize</code></td>
<td><code>precision</code></td>
<td><code>precision</code></td>
</tr>
<tr>
<td><code>boost::io::ios_width_saver</code></td>
<td>Minimum field width for printing objects</td>
<td><code>std::streamsize</code></td>
<td><code>width</code></td>
<td><code>width</code></td>
</tr>
</table>
<h2><a name="adv_savers">Savers for Advanced Standard Attributes</a></h2>
<p>The saver class templates have this format:</p>
<blockquote><pre>template < typename Ch, class Tr >
class <var>saver_class</var>
{
typedef std::basic_ios<Ch, Tr> state_type;
typedef <i>implementation_defined</i> aspect_type;
explicit saver_class( state_type &s );
saver_class( state_type &s, <var>aspect_type</var> const &new_value );
~saver_class();
void restore();
};
</pre></blockquote>
<p>The <var>state_type</var> is a version of the IOStreams base class
template <code>std::basic_ios<Ch, Tr></code>, where
<code>Ch</code> is a character type and <code>Tr</code> is a character
traits class. The user would usually place an actual input, output, or
combined stream object for the state-type parameter, and not a base
class object. The first constructor takes a stream object and saves a
reference to the stream and the current value of a particular stream
attribute. The second constructor works like the first, and uses its
second argument to change the stream's attribute to the new
<var>aspect_type</var> value given. The destructor restores the stream's
attribute to the saved value. The restoration can be activated
early (and often) with the <code>restore</code> member function.</p>
<table border="1" align="center">
<caption>Advanced IOStreams State Saver Class Templates</caption>
<tr>
<th>Class Template</th>
<th>Saved Attribute</th>
<th>Attribute Type</th>
<th>Reading Method</th>
<th>Writing Method</th>
</tr>
<tr>
<td><code>boost::io::basic_ios_iostate_saver<Ch, Tr></code></td>
<td>Failure state of the stream <a href="#Note1">[1]</a>, <a href="#Note2">[2]</a></td>
<td><code>std::ios_base::iostate</code></td>
<td><code>rdstate</code></td>
<td><code>clear</code></td>
</tr>
<tr>
<td><code>boost::io::basic_ios_exception_saver<Ch, Tr></code></td>
<td>Which failure states trigger an exception <a href="#Note1">[1]</a></td>
<td><code>std::ios_base::iostate</code></td>
<td><code>exceptions</code></td>
<td><code>exceptions</code></td>
</tr>
<tr>
<td><code>boost::io::basic_ios_tie_saver<Ch, Tr></code></td>
<td>Output stream synchronized with the stream</td>
<td><code>std::basic_ostream<Ch, Tr> *</code></td>
<td><code>tie</code></td>
<td><code>tie</code></td>
</tr>
<tr>
<td><code>boost::io::basic_ios_rdbuf_saver<Ch, Tr></code></td>
<td>Stream buffer associated with the stream <a href="#Note2">[2]</a></td>
<td><code>std::basic_streambuf<Ch, Tr> *</code></td>
<td><code>rdbuf</code></td>
<td><code>rdbuf</code></td>
</tr>
<tr>
<td><code>boost::io::basic_ios_fill_saver<Ch, Tr></code></td>
<td>Character used to pad oversized field widths</td>
<td><code>Ch</code></td>
<td><code>fill</code></td>
<td><code>fill</code></td>
</tr>
<tr>
<td><code>boost::io::basic_ios_locale_saver<Ch, Tr></code></td>
<td>Locale information associated with the stream <a href="#Note3">[3]</a></td>
<td><code>std::locale</code></td>
<td><code>getloc</code> (from <code>std::ios_base</code>)</td>
<td><code>imbue</code> (from <code>std::basic_ios<Ch, Tr></code>)</td>
</tr>
</table>
<h3>Notes</h3>
<ol>
<li>When the failure state flags and/or the failure state exception
watching flags are changed, an exception is thrown if a match
occurs among the two sets of flags. This could mean that
the <a name="Note1">constructor or destructor of these class
templates may throw</a>.</li>
<li>When the associated stream buffer is changed, the stream's
failure state set is reset to "good" if the given stream
buffer's address is non-NULL, but the "bad" failure
state is set if that address is NULL. This means that a saved
failure state of "good" may be restored as "bad"
if the stream is stripped of an associated stream buffer. Worse,
given a NULL stream buffer address, an exception is thrown if the
"bad" failure state is being watched. This could mean
that the <a name="Note2">constructor or destructor of these class
templates may throw</a>.</li>
<li>The <a name="Note3">saver for the locale uses the
<code>std::basic_ios<Ch, Tr></code> class to extract their
information</a>, although it could have used the functionality
in <code>std::ios_base</code>. The problem is that the versions
of the needed member functions in <code>ios_base</code> are not
polymorphically related to the ones in <code>basic_ios</code>.
The stream classes that will be used with the saver classes
should use the versions of the member functions closest to them
by inheritance, which means the ones in
<code>basic_ios</code>.</li>
</ol>
<h2><a name="user_savers">Savers for User-Defined Attributes</a></h2>
<p>The saver classes for user-defined formatting information have this
format:</p>
<blockquote><pre>#include <iosfwd> <i>// for std::ios_base (declaration)</i>
class <var>saver_class</var>
{
typedef std::ios_base state_type;
typedef int index_type;
typedef <i>implementation_defined</i> aspect_type;
explicit saver_class( state_type &s, index_type i );
saver_class( state_type &s, index_type i, <var>aspect_type</var> const &new_value );
~saver_class();
void restore();
};
</pre></blockquote>
<p>The index <var>i</var> differentiates between specific user-defined
formatting attributes. The index can only be determined at run-time
(most likely with the class-static <code>std::ios_base::xalloc</code>
member function).</p>
<p>The <var>state_type</var> is the base class of the IOStreams system,
<code>std::ios_base</code>. The user would usually place an actual
input, output, or combined stream object for the state-type parameter,
and not a base class object. The first constructor takes a stream
object and index and saves a reference to the stream and the current
value of a particular stream attribute. The second constructor works
like the first, and uses its third argument to change the stream's
attribute to the new <var>aspect_type</var> value given. The destructor
restores the stream's attribute to the saved value. The restoration can
be activated early (and often) with the <code>restore</code> member
function.</p>
<table border="1" align="center">
<caption>IOStream User-Defined State Saver Classes</caption>
<tr>
<th>Class</th>
<th>Saved Attribute</th>
<th>Attribute Type</th>
<th>Reference Method</th>
</tr>
<tr>
<td><code>boost::io::ios_iword_saver</code></td>
<td>Numeric user-defined format flag</td>
<td><code>long</code></td>
<td><code>iword</code></td>
</tr>
<tr>
<td><code>boost::io::ios_pword_saver</code></td>
<td>Pointer user-defined format flag</td>
<td><code>void *</code></td>
<td><code>pword</code></td>
</tr>
</table>
<h2><a name="combo_savers">Savers for Combined Attributes</a></h2>
<p>There are three class (templates) for combined attribute savers. The
<code>boost:io::ios_base_all_saver</code> saver class combines the
functionality of all the basic attribute saver classes. It has a
constructor that takes the stream to have its state preserved. The
<code>boost::io::basic_ios_all_saver</code> combines the functionality
of all the advanced attribute saver class templates and the combined
basic attribute saver class. It has a constructor that takes the stream
to have its state preserved. The
<code>boost::io::ios_all_word_saver</code> saver class combines the
saver classes that preserve user-defined formatting information. Its
constructor takes the stream to have its attributes saved and the index
of the user-defined attributes. The destructor for each class restores
the saved state. Restoration can be activated early (and often) for a
class with the <code>restore</code> member function.</p>
<h2><a name="example">Example</a></h2>
<p>The code used in the <a href="#rationale">rationale</a> can be
improved at two places. The printing function could use a saver around
the code that changes the formatting state. Or the calling function can
surround the call with a saver. Or both can be done, especially if the
user does not know if the printing function uses a state saver. If the
user wants a series of changes back & forth, without surrounding each
change within a separate block, the <code>restore</code> member function
can be called between each trial.</p>
<blockquote><pre>#include <boost/io/ios_state.hpp>
#include <ios>
#include <iostream>
#include <ostream>
void new_hex_my_byte( std::ostream &os, char byte )
{
boost::io::ios_flags_saver ifs( os );
os << std::hex << static_cast<unsigned>(byte);
}
int main()
{
using std::cout;
using std::cerr;
//...
{
boost::io::ios_all_saver ias( cout );
new_hex_my_byte( cout, 'A' );
}
//...
{
boost::io::ios_all_saver ias( cerr );
new_hex_my_byte( cerr, 'b' );
ias.restore();
new_hex_my_byte( cerr, 'C' );
}
//...
}
</pre></blockquote>
<h2><a name="refer">References</a></h2>
<ul>
<li>The I/O state saver library header itself: <cite><a
href="../../../boost/io/ios_state.hpp">boost/io/ios_state.hpp</a></cite></li>
<li>Some test/example code: <cite><a
href="../test/ios_state_test.cpp">ios_state_test.cpp</a></cite></li>
</ul>
<h2><a name="credits">Credits</a></h2>
<h3><a name="contributors">Contributors</a></h3>
<dl>
<dt><a href="http://www.boost.org/people/daryle_walker.html">Daryle Walker</a>
<dd>Started the library. Contributed the initial versions of the
format flags, precision, width, and user-defined format flags
saver classes. Contributed the initial versions of the success
state, success state exception flags, output stream tie, stream
buffer, character fill, and locale saver class templates.
Contributed the combined attribute classes and class template.
Contributed the test file <cite><a
href="../test/ios_state_test.cpp">ios_state_test.cpp</a></cite>.
</dl>
<h3><a name="history">History</a></h3>
<dl>
<dt>28 Feb 2005, Daryle Walker
<dd>Added the <code>restore</code> member functions, based on suggestions
by Gennadiy Rozental and Rob Stewart
<dt>13 Mar 2002, Daryle Walker
<dd>Initial version
</dl>
<hr>
<p>Revised: 28 February 2005</p>
<p>Copyright 2002, 2005 Daryle Walker. Use, modification, and distribution
are subject to the Boost Software License, Version 1.0. (See accompanying
file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a copy at
<<a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>>.)</p>
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