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<title>Serialization - More on Archives</title>
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  <tr> 
    <td valign="top" width="300"> 
      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
    </td>
    <td valign="top"> 
      <h1 align="center">Serialization</h1>
      <h2 align="center">Archive Class Reference</h2>
    </td>
  </tr>
</table>
<hr>
<dl class="page-index">
  <dt><a href="#trivial">Trivial Archive</a>
  <dt><a href="#implementation">More Useful Archive Classes</a>
  <dt><a href="#usage">Usage</a>
  <dt><a href="#testing">Testing</a>
  <dt><a href="#polymorphic">Polymorphic Archives</a>
</dl>

<h3><a name="trivial">Trivial Archive</a></h3>
The <a href="archives.html"><strong>Archive</strong></a> concept specifies the functions that a
class must implement in order to be used to serialize 
<a href="serialization.html"><strong>Serializable</strong></a> types.

Our discussion will focus on archives used for saving as the hierarchy is exactly analogous
for archives used for loading data.

<h4>Minimum Requirments</h4>

The simplest class which will model the <a href="archives.html"><strong>Archive</strong></a> concept specifies the functions that a
class will look like:

<pre><code>
#include &lt;cstddef&gt; // std::size_t
//////////////////////////////////////////////////////////////
// class trivial_oarchive
class trivial_oarchive {
public:
    //////////////////////////////////////////////////////////
    // public interface used by programs that use the
    // serialization library
    typedef boost::mpl::bool_&lt;true&gt; is_saving; 
    typedef boost::mpl::bool_&lt;false&gt; is_loading;
    template&lt;class T&gt; void register_type(){}
    template&lt;class T&gt; trivial_oarchive & operator&lt;&lt;(const T & t){
        return *this;
    }
    template&lt;class T&gt; trivial_oarchive & operator&(const T & t){
        return *this &lt;&lt; t;
    }
    void save_binary(void *address, std::size_t count){};
};
</code></pre>
The simplest possible input archive class is analogous to the above.
In the following discussion, only output archives will be addressed.
Input archives are exactly symmetrical to output archives.
<p>
This archive will compile and execute with any types which implement the
<a href="serialization.html"><strong>Serializable</strong></a> concept.
For an example see
<a href="../example/demo_trivial_archive.cpp" target="demo_trivial_archive">
<code style="white-space: normal">demo_trivial_archive.cpp</code></a>.
Of course this program won't produce any output as it is.  But it provides
the starting point for a simple class which can be used to log formatted
output.  See the implementation of a <a href="simple_log.html">simple
log archive</a> to how this has been done.

<h3><a name="implementation">More Useful Archive Classes</a></h3>
The above example is fine as far as it goes.  But it doesn't implement
useful features such as serialization of pointers, class versioning
and others. This library implements a family of full featured archive
classes appropriate for a variety of purposes.  

<p>
Our archives have been factored into a tree of classes in order to minimize
repetition of code.  This is shown in the accompanying 
<a target="class_diagram" href="class_diagram.html">class diagram</a>.

Any class which fulfills the following requirements will fit into
this hierarchy and implement all the features we require.  Deriving from
the base class <a href="../../../boost/archive/detail/common_oarchive.hpp" target="common_oarchive_hpp">
common_oarchive.hpp</a> provides all features we desire which
are missing from trivial_oarchive above.

<pre><code>
<a href="../../../boost/archive/detail/common_oarchive.hpp" target="common_oarchive_hpp">
#include &lt;cstddef&gt; // std::size_t
#include &lt;boost/archive/detail/common_oarchive.hpp&gt;
</a>
/////////////////////////////////////////////////////////////////////////
// class complete_oarchive
class complete_oarchive : 
    public boost::archive::detail::common_oarchive&lt;complete_oarchive&gt;
{
    // permit serialization system privileged access to permit
    // implementation of inline templates for maximum speed.
    friend class boost::archive::save_access;

    // member template for saving primitive types.
    // Specialize for any types/templates that require special treatment
    template&lt;class T&gt;
    void save(T &amp; t);

public:
    //////////////////////////////////////////////////////////
    // public interface used by programs that use the
    // serialization library

    // archives are expected to support this function
    void save_binary(void *address, std::size_t count);
};
</code></pre>

Given a suitable definitions of <code style="white-space: normal">save</code>
and <code style="white-space: normal">save_binary</code>, 
any program using serialization with a conforming C++ compiler should compile 
and run with this archive class.

<h4>Optional Overrides</h4>

The <code style="white-space: normal">detail::common_oarchive</code> class contains
a number of functions that are used by various parts of the serialization library
to help render the archive in a particular form.

<dl>

<dt><h4><code>void save_start(char const *)</code></h4></dt>
<dd>
<strong>Default</strong>:Does nothing.<br>
<strong>Purpose</strong>:To inject/retrieve an object name into the archive.  Used
by XML archive to inject "&lt;name&gt;" before data.
</dd>
<p>

<dt><h4><code>void save_end(char const *)</code></h4></dt>
<dd>
<strong>Default</strong>:Does nothing.<br>
<strong>Purpose</strong>:To inject/retrieve an object name into the archive. Used
by XML archive to inject "&lt;/name&gt;" after data.
<dd>
</dd>
<p>
<dt><h4><code>void end_preamble()</code></h4></dt>
<dd>
<strong>Default</strong>:Does nothing.<br>
<strong>Purpose</strong>:Called <strong>each time</strong> user data is saved. 
It's not called when archive bookkeeping data is saved.  This is used by XML archives 
to determine  when to inject a "&gt;" character at the end of an XML header. XML output archives 
keep their own internal flag indicating that data being written is header data. This
internal flag is reset when an object start tag is written. When 
<code style="white-space: normal">void end_preamble()</code> is invoked and this internal flag is set
a "&gt;" character is appended to the output and the internal flag is reset. The default
implementation for <code style="white-space: normal">void end_preamble()</code> is a no-op thereby permitting it
to be optimised away for archive classes that don't use it.
</dd>
<p>
<dt><h4><code>
template&lt;class T&gt;
void save_override(T & t, int);
</code></h4></dt>
<dd>
<strong>Default</strong>:Invokes <code style="white-space: normal">archive::save(Archive & ar, t)</code><br>
This is the main entry into the serialization library.<br>
<strong>Purpose</strong>:This can be specialized in cases where the data is to be written 
to the archive in some special way.  For example, XML archives implement special handling for
name-value pairs by overriding this function template for name-value pairs.  
This replaces the default name-value pair handling, which is just to throw away the name,
with one appropriate for XML which writes out the start of an XML tag with the correct object name.
</dd>

</dl>

<h4>Types used by the serialization library</h4>
The serialization library injects bookkeeping data into the serialization archive.
This data includes things like object ids, version numbers, class names etc.  Each
of these objects is included in a wrapper so that the archive class can override the
implementation of <code style="white-space: normal">void save_override(T & t, int);</code>.
For example, in the XML archive, the override for this type renders an object_id equal to 23 as
"object_id=_23".  The following table lists the types defined in the 
<code style="white-space: normal">boost::archive namespace</code>
used internally by the serialization library:
<p>
<table border>
<tr><th align=left>type</th><th align=left><code style="white-space: normal">default<br>serialized as</code></th>
<tr><td><code style="white-space: normal">version_type</code></td><td><code style="white-space: normal">unsigned int</code></td>             
<tr><td><code style="white-space: normal">object_id_type</code></td><td><code style="white-space: normal">unsigned int</code></td>   
<tr><td><code style="white-space: normal">object_id_reference_type</code></td><td><code style="white-space: normal">unsigned int</code></td>   
<tr><td><code style="white-space: normal">class_id_type</code></td><td><code style="white-space: normal">int</code></td>   
<tr><td><code style="white-space: normal">class_id_optional_type</code></td><td><code style="white-space: normal">nothing</code></td>   
<tr><td><code style="white-space: normal">class_id_reference_type</code></td><td><code style="white-space: normal">int</code></td>   
<tr><td><code style="white-space: normal">tracking_type</code></td><td><code style="white-space: normal">bool</code></td>   
<tr><td><code style="white-space: normal">classname_type</code></td><td><code style="white-space: normal">string</code></td>   
</table>
<p>
All of these are associated with a default serialization defined in terms of primitive types
so it isn't a requirement to define <code style="white-space: normal">save_override</code> 
for these types.
<p>
These are defined in
<a href="../../../boost/archive/basic_archive.hpp" target="basic_archive_hpp"><code style="white-space: normal">basic_archive.hpp</code></a>.
All of these types have been assigned an 
<a target="detail" href="traits.html#level">implementation level</a> of
<code style="white-space: normal">primitive</code> and are convertible to types such as int, unsigned int, etc. 
so that they have default implementations.  This is illustrated by
<a href="../../../boost/archive/basic_text_iarchive.hpp" target="basic_text_iarchive_hpp"><code style="white-space: normal">basic_text_iarchive.hpp</code></a>.
which relies upon the default.  However, in some cases, overrides will have to be
explicitly provided for these types. For an example see
<a href="../../../boost/archive/basic_xml_iarchive.hpp" target="basic_xml_iarchive_hpp"><code style="white-space: normal">basic_xml_iarchive.hpp</code></a>.
<p>
In real practice, we probably won't be quite done. 
One or more of the following issues may need to be addressed:
<ul>
    <li>Even if we are using a conforming compiler, we might want our new archive class
    to be portable to non-conforming compilers.
    <li>Our archive format might require extra information inserted into it.  For
    example, XML archives need &lt;name ... &gt;...&lt;/name&gt; surrounding
    all data objects.
    <li>Addressing any of the above may generate more issues to be addressed.
    <li>The archives included with the library are all templates which use a
    <code style="white-space: normal">stream</code> or
    <code style="white-space: normal">streambuf</code>
    as a template parameter rather than simple classes.
    Combined with the above, even more issues arise with non-conforming compilers.
</ul>
The attached <a target="class_diagram" href="class_diagram.html">class diagram</a>
shows the relationships between classes used to implement the serialization library.
<p>
A close examination of the archives included with the library illustrate
what it takes to make a portable archive that covers all data types.
<h3><a name="usage">Usage</a></h3>
The newly created archive will usually be stored in its own header module.  All
that is necessary is to include the header and construct an instance of the new archive.
EXCEPT for one special case.
<ul>
    <li>Instances of a derived class are serialized through a base class pointer.
    <li>Such instances are not "registered" neither implicitly nor explicitly. That
    is, the macro <code style="white-space: normal">BOOT_CLASS_EXPORT</code> is used 
    to instantiate the serialization code for the included archives.
</ul>

To make this work, the following should be included after the archive
class definition.
<pre><code>
BOOST_SERIALIZATION_REGISTER_ARCHIVE(Archive)
</code></pre>
Failure to do this will not inhibit the program from compiling, linking
and executing properly - except in one case.  If an instance of a derived
class is serialized through a pointer to its base class, the program
will throw an
<a href="exceptions.html#unregistered_class"><code style="white-space: normal">unregistered_class</code></a>
exception.
<p>

<h4><a name="testing">Testing</h4>
Exhaustive testing of the library requires testing the different aspects of object
serialization with each archive.  There are 46 different tests that can run with any archive.  
There are 5 "standard archives" included with the system. 
(3 in systems that don't support wide charactor i/o). 
<p>
In addition, there are 28 other tests which aren't related to any particular archive class.
<p>
The default <code style="white-space: normal">bjam</code> testing setup will run all
the above described tests.  This will result in as many as 46 archive tests * 5 
standard archives + 28 general tests = 258 tests. Note that a complete test of the
library would include DLL vs static library, release vs debug so the actual total 
would be closer to 1032 tests.
<p>
For each archive there is a header file in the test directory similar to the one below.
The name of this archive is passed to the test program by setting the
environmental variable <code style="white-space: normal">BOOST_ARCHIVE_TEST</code>
to the name of the header.  Here is the header file 
<code style="white-space: normal">test_archive.hpp</code> . Test header files for
other archives are similar.
<pre><code>
// text_archive test header
// include output archive header
#include &lt;boost/archive/text_oarchive.hpp&gt;
// set name of test output archive
typedef boost::archive::text_oarchive test_oarchive;
// set name of test output stream
typedef std::ofstream test_ostream;

// repeat the above for input archive
#include &lt;boost/archive/text_iarchive.hpp&gt;
typedef boost::archive::text_iarchive test_iarchive;
typedef std::ifstream test_istream;

// define open mode for streams
//   binary archives should use std::ios_base::binary
#define TEST_STREAM_FLAGS (std::ios_base::openmode)0
</code></pre>

To test a new archive, for example, portable binary archives, with the gcc compiler, 
make a header file <code style="white-space: normal">portable_binary_archive.hpp</code>
and invoke <code style="white-space: normal">bjam</code> with
<pre><code> 
-sBOOST_ARCHIVE_LIST=portable_binary_archive.hpp
</code></pre>
This process in encapsulated in the shell or cmd script 
<code style="white-space: normal">library_test</code> whose command line is
<pre><code>
library_test --toolset=gcc -sBOOST_ARCHIVE_LIST=portable_binary_archive.hpp
</code></pre>
<h3><a name="polymorphic">Polymorphic Archives</a></h3>

<h4>Motivation</h4>

All archives described so far are implemented as templates.  Code to save and load
data to archives is regenerated for each combination of archive class and data type.
Under these cirumstances, a good optimizing compiler that can expand 
<code>inline</code> functions to enough depth will generate fast code.  
However:
<ul>
<li>Much inline code may be replicated.
<li>If there are several archive classes, code will be regenerated for each archive class.
<li>If serialization code is placed in a library, that library must be rebuilt 
each time a new archive class is created.
<li>If serialization code is placed in a DLL,
  <ul>
  <li>The DLL will contain versions of code for each known archive type.  
    This would result in loading of DLLs which contain
    much code that is not used - basically defeating one of the main motivations
    for choosing to use a DLL in the first place.
  <li>If a new archive is created and an application shipped, all DLLs have to be
    rebuilt, and reshipped along with the application which uses the new archive.  Thus
    the other main motivation for using a DLL is defeated.
  </ul>
</ul>

<h4>Implementation</h4>
The solution is the pair <code>polymorphic_oarchive</code>
and <code>polymorphic_iarchive</code>.  They present a common interface of virtual
functions - no templates - that is equivalent to the standard templated one.

This is shown in the accompanying 
<a target="class_diagram" href="class_diagram.html">class diagram</a>
<p>
The accompanying demo program in files

<a target=demo_polymorphic_cp href="../example/demo_polymorphic.cpp"><code style="white-space: normal">demo_polymorphic.cpp</code></a>, 
<a target=demo_polymorphic_A_hpp href="../example/demo_polymorphic_A.hpp"><code style="white-space: normal">demo_polymorphic_A.hpp</code></a>, and 
<a target=demo_polymorphic_A_cpp href="../example/demo_polymorphic_A.cpp"><code style="white-space: normal">demo_polymorphic_A</code></a>
show how polymorphic archives are to be used. Note the following:
<ul>
  <li><a target=demo_polymorphic_A_hpp href="../example/demo_polymorphic_A.hpp"><code style="white-space: normal">demo_polymorphic_A.hpp</code></a> and
<a target=demo_polymorphic_A_cpp href="../example/demo_polymorphic_A.cpp"><code style="white-space: normal">demo_polymorphic_A.cpp</code></a>
contain no templates and no reference to any specific archive implementation.  That is, they will
only have to be compiled once for all archive implementations.  This even applies to archives classes
created in the future.
  <li>The main program <a target=demo_polymorphic_cp href="../example/demo_polymorphic.cpp"><code style="white-space: normal">demo_polymorphic.cpp</code></a>
specifies a specific archive implementation.  
</ul>
As can be seen in the
<a target="class_diagram" href="class_diagram.html">class diagram</a>
and the header files, this implementation is just a composition of the polymorphic
interface and the standard template driven implementation.  This composition is
accomplished by the templates
<a target=polymorphic_iarchive_route_hpp href="../../../boost/archive/detail/polymorphic_iarchive_route.hpp"><code style="white-space: normal">polymorphic_iarchive_route.hpp</code></a>
and
<a target=polymorphic_oarchive_route_hpp href="../../../boost/archive/detail/polymorphic_oarchive_route.hpp"><code style="white-space: normal">polymorphic_oarchive_route.hpp</code></a>
which redirect calls to the polymorphic archives to the specific archive.
As these contain no code specific to the particular implementation archive, they can be used to create
a polymorphic archive implementation from any functioning templated archive implementation.
<p>
As a convenience, small header files have been included which contain 
a <code style="white-space: normal">typedef</code> for a polymorphic implementation for each corresponding
templated one.  For example, the headers
<a target=polymorphic_text_iarchive_hpp href="../../../boost/archive/polymorphic_text_iarchive.hpp"><code style="white-space: normal">polymorphic_text_iarchive.hpp</code></a>
and
<a target=polymorphic_text_oarchive_hpp href="../../../boost/archive/polymorphic_text_oarchive.hpp"><code style="white-space: normal">polymorphic_text_oarchive.hpp</code></a>.
contain the <code style="white-space: normal">typedef</code> for the polymorphic implementation
of the standard text archive classes  
<a target=text_iarchive_hpp href="../../../boost/archive/text_iarchive.hpp"><code style="white-space: normal">text_iarchive.hpp</code></a>
and
<a target=text_oarchive_hpp href="../../../boost/archive/text_oarchive.hpp"><code style="white-space: normal">text_oarchive.hpp</code></a>
respectively. All included polymorphic archives use the same naming scheme.

<h4>Usage</h4>
Polymorphic archives address the issues raised above regarding templated implementation.
That is, there is no replicated code, and no recompilation for new archives.  This will
result in smaller executables for program which use more than one type of archive, and 
smaller DLLS. There is a penalty for calling archive functions through a virtual function 
dispatch table and there is no possibility for a compiler to <code style="white-space: normal">inline</code> 
archive functions.  This will result in a detectable degradation in performance for 
saving and loading archives.
<p>
Note that the concept of polymophic archives is fundamentally incompatible with the
serialization of new types that are marked "primitive" by the user with:
<pre><code> 
BOOST_CLASS_IMPLEMENTATION(my_primitive_type, boost::serialization::primitive_type)
</code></pre>

Code to implement serialization for these types is instantiated "on the fly" in the user's program.
But this conflicts with the whole purpose of the polymorphic archive. An attempt to
serialize such a primitive type will result in a compilation error since the common polymorhic
interface is static and cannot instantiate code for a new type.

<p>
The main utility of polymorphic archives will be to permit the building of class DLLs that will
include serialization code for all present and future archives with no redundant code.
<hr>
<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. 
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</i></p>
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