boost_1_45_0/libs/serialization/doc/wrappers.html

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      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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      <h1 align="center">Serialization</h1>
      <h2 align="center">Serialization Wrappers</h2>
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<dl class="page-index">
  <dt><a href="#binaryobjects">Binary Objects</a>
  <dt><a href="#arrays">Arrays</a>
  <dt><a href="#strong_type"><code style="white-space: normal">BOOST_STRONG_TYPEDEF</code></a>
  <dt><a href="#collection_size_type">Collection Sizes</a>
  <dt><a href="#nvp">Name-Value Pairs</a>
  <dt><a href="#composition">Composition</a>
</dl>
Sometimes it convenient to create a temporary object just to support serialization
of some underlying data. This permits an archive class to define special
handling of this type.  The library includes several such types for varying 
purposes.
<p>
Wrappers need to be treated in a special way by some archives, and hence
the <A href="traits.html#wrappers"><code>is_wrapper</code></a> trait for 
these wrapper classes is set to true.

<h3><a name="binaryobjects">Binary Objects</a></h3>
A binary object is just an sequence of bytes stored as raw
binary data.  This would most likely be used for a large amount
of "light weight" data such as a pixel map or embedded binary file.
The header file 
<a href="../../../boost/serialization/binary_object.hpp" target="binary_object_hpp">
binary_object.hpp
</a>
includes the constructors:
<pre><code>
boost::serialization::binary_object(void * t, size_t size);
boost::serialization::make_binary_object(void * t, size_t size);
</code></pre>
which will construct a temporary binary object that can be serialized just like any other object.
Its default serialization is to use archive class primitives 
<code style="white-space: normal">save_binary</code> and <code>load_binary</code>.  
Note that it doesn't allocated any storage or create any objects.
Its sole purpose is to pass the data size and address as pair to the archive class.  


<h3><a name="arrays">Arrays</a></h3>
An array is a contiguous sequence of homogeneous data types, such as a builtin
C-array, a <code>boost::array&lt;T></code> or a <code>std::vector&lt;T></code>. 
The purpose of this wrapper is to support archive types (such as binary
archives) that provide optimized serialization for contiguous sequences of 
objects of the same type. 

The header file 
<a href="../../../boost/serialization/array.hpp" target="array_hpp">
array.hpp
</a>
includes the function
<pre><code>
template &lt;T>
boost::serialization::make_array(T* t, std::size_t size);
</code></pre>
which will construct a temporary <code>array</code> object
<pre><code>
template<class T>
class array
{
public:    
    typedef T value_type;
    array(value_type* t, std::size_t s);
    value_type* address() const;
    std::size_t count() const;
};
</code></pre>
that can be serialized just like any other object.
Its default serialization is to use serialize each array element.  
Note that it doesn't allocated any storage or create any objects.
Its sole purpose is to pass the data type, size and address to the archive class.

Archive types that can provide optimized implementations for contiguous
arrays of homogeneous data types should overload the serialization of
<code>array</code>.

 
<h3><a name="strong_type"><code style="white-space: normal">BOOST_STRONG_TYPEDEF</code></h3>
Another example of a serialization wrapper is the 
<a href="strong_typedef.html"><code style="white-space: normal">BOOST_STRONG_TYPEDEF</code></a> template.
The serialization libraries uses these to pass particular kinds of integers such
as object_id, version, etc. to an archive class. Given that these integers
are now distinguishable according to their type,  XML archives can apply
special handling to these types.  For example, a version number is rendered
as an XML attribute in the form "version=12".  In the absence of any specific override,
these types are automatically converted to the underlying integer type so the
special overrides used for XML archives aren't needed for other archives.



<h3><a name="collection_size_type">Collection Sizes</h3>
An example of a strong typedef is the <code>collection_size_type</code> in the
header file
<a href="../../../boost/serialization/collection_size_type.hpp" target="collection_size_type_hpp">
collection_size_type.hpp
</a>. This type should be used for serializaing the size of a C++ collection, so
that the archive can pick the best integral representation for the serialization
of collection sizes. This is necessary since, although <code>std::size_t</code>
is guaranteed to be an integral type large enough to represent the size of
a collection on a specific platform, the archive might want to serialize
the size differently than this type. For example, the <code>collection_size_type</code>
might be serialized as a variable length integer in a portable binary archive.






<h3><a name="nvp">Name-Value Pairs</h3>
XML archives present a somewhat special case. XML format has a nested
structure that maps well to the "recursive class member visitor" pattern used
by the serialization system.  However, XML differs from other formats
in that it requires a name for each class data member.  Our goal is to
add this information to the class serialization specification while
still permiting the the serialization code to be used with any archive.
<p>
Our solution is to wrap class members to be serialized in a
<strong>name-value-pair</strong>. This structure is defined in
<a href="../../../boost/serialization/nvp.hpp" target="nvp_hpp">nvp.hpp</a>.
It is just a reference to the data member coupled with a pointer to
to a <code style="white-space: normal">const char *</code> which 
corresponds to the XML name.  It implements the default
serialization functions for a name-value pair.  This default
action is to just ignore the item name and serialize the
data value in the normal manner.  For archive classes that
don't make any special provision for name-value pairs, this
is the action which will be invoked when the name-value pair
is serialized.  Hence, wrapping a data value into a name-value
pair will have no effect when used with archives which
make no special provision for this wrapper.
<p>
The xml archive classes contain code similar to:
<pre><code>
// special treatment for name-value pairs.
template&lt;class T&gt;
xml_oarchive & operator&(const boost::serialization::nvp<T> & t)
{
    // write an xml start tag
    start_tag(t.name());

    // serialize the data as usual
    *this & t.value();

    // write an xml end tag
    end_tag(t.name());
}
</code></pre>
The most obvious and convient name to assign to as the XML data item name
is - surpise! - the name of the C++ class data member.  So our serialization
code will look like:
<pre><code>
ar & make_nvp("my_variable", my_variable);
</code></pre>
To simplify typing and enhance readability a macro is defined so we can write:
<pre><code>
ar & BOOST_SERIALIZATION_NVP(my_variable);
</code></pre>
Similarly there exists a macro definition that permits us to write:
<pre><code>
BOOST_SERIALIZATION_BASE_OBJECT_NVP(my_base_class)
</code></pre>

Note that these macros must be used in the namespace of the class, 
and without qualifying the namespace in the argument. 

<p>
<a href="../example/demo_gps.hpp" target="demo_gps_hpp">demo_gps.hpp<a>
includes NVP wrappers or all data members.
<a href="../example/demo_xml.cpp" target="demo_xml_cpp">demo_xml.cpp<a>
saves and loads data to an XML archive.
<a href="../example/demo_save.xml" target="demo_save_xml">Here</a>
is example of the XML Archive corresponding to our tutorial example.

<h3><a name="composition">Composition</h3>
Wrappers should be designed so that they can be composed as necessary.
For example, to pass binary data as a name value pair use:
<pre><code>
ar & make_nvp("named_binary_object", make_binary_object(address, size));
</code></pre>
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<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. 
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