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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">Tutorial</h2>
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<hr>
<dl class="page-index">
<dt><a href="#simplecase">A Very Simple Case</a>
<dt><a href="#nonintrusiveversion">Non Intrusive Version</a>
<dt><a href="#serializablemembers">Serializable Members</a>
<dt><a href="#derivedclasses">Derived Classes</a>
<dt><a href="#pointers">Pointers</a>
<dt><a href="#arrays">Arrays</a>
<dt><a href="#stl">STL Collections</a>
<dt><a href="#versioning">Class Versioning</a>
<dt><a href="#splitting">Splitting <code style="white-space: normal">serialize</code> into <code style="white-space: normal">save/load</code></a>
<dt><a href="#archives">Archives</a>
<dt><a href="#examples">List of examples</a>
</dl>
An output archive is similar to an output data stream. Data can be saved to the archive
with either the &lt;&lt; or the &amp; operator:
<pre><code>
ar &lt;&lt; data;
ar &amp; data;
</code></pre>
An input archive is similar to an input datastream. Data can be loaded from the archive
with either the &gt;&gt; or the &amp; operator.
<pre><code>
ar &gt;&gt; data;
ar &amp; data;
</code></pre>
<p>
When these operators are invoked for primitive data types, the data is simply saved/loaded
to/from the archive. When invoked for class data types, the class
<code style="white-space: normal">serialize</code> function is invoked. Each
<code style="white-space: normal">serialize</code> function is uses the above operators
to save/load its data members. This process will continue in a recursive manner until
all the data contained in the class is saved/loaded.
<h3><a name="simplecase">A Very Simple Case</a></h3>
These operators are used inside the <code style="white-space: normal">serialize</code>
function to save and load class data members.
<p>
Included in this library is a program called
<a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a> which illustrates how
to use this system. Below we excerpt code from this program to
illustrate with the simplest possible case how this library is
intended to be used.
<pre>
<code>
#include &lt;fstream&gt;
// include headers that implement a archive in simple text format
#include &lt;boost/archive/text_oarchive.hpp&gt;
#include &lt;boost/archive/text_iarchive.hpp&gt;
/////////////////////////////////////////////////////////////
// gps coordinate
//
// illustrates serialization for a simple type
//
class gps_position
{
private:
friend class boost::serialization::access;
// When the class Archive corresponds to an output archive, the
// &amp; operator is defined similar to &lt;&lt;. Likewise, when the class Archive
// is a type of input archive the &amp; operator is defined similar to &gt;&gt;.
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
ar &amp; degrees;
ar &amp; minutes;
ar &amp; seconds;
}
int degrees;
int minutes;
float seconds;
public:
gps_position(){};
gps_position(int d, int m, float s) :
degrees(d), minutes(m), seconds(s)
{}
};
int main() {
// create and open a character archive for output
std::ofstream ofs("filename");
// create class instance
const gps_position g(35, 59, 24.567f);
// save data to archive
{
boost::archive::text_oarchive oa(ofs);
// write class instance to archive
oa &lt;&lt; g;
// archive and stream closed when destructors are called
}
// ... some time later restore the class instance to its orginal state
gps_position newg;
{
// create and open an archive for input
std::ifstream ifs("filename");
boost::archive::text_iarchive ia(ifs);
// read class state from archive
ia &gt;&gt; newg;
// archive and stream closed when destructors are called
}
return 0;
}
</code>
</pre>
<p>For each class to be saved via serialization, there must exist a function to
save all the class members which define the state of the class.
For each class to be loaded via serialization, there must exist a function to
load theese class members in the same sequence as they were saved.
In the above example, these functions are generated by the
template member function <code style="white-space: normal">serialize</code>.
<h3><a name="nonintrusiveversion">Non Intrusive Version</a></h3>
<p>The above formulation is intrusive. That is, it requires
that classes whose instances are to be serialized be
altered. This can be inconvenient in some cases.
An equivalent alternative formulation permitted by the
system would be:
<pre><code>
#include &lt;boost/archive/text_oarchive.hpp&gt;
#include &lt;boost/archive/text_iarchive.hpp&gt;
class gps_position
{
public:
int degrees;
int minutes;
float seconds;
gps_position(){};
gps_position(int d, int m, float s) :
degrees(d), minutes(m), seconds(s)
{}
};
namespace boost {
namespace serialization {
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, gps_position &amp; g, const unsigned int version)
{
ar &amp; g.degrees;
ar &amp; g.minutes;
ar &amp; g.seconds;
}
} // namespace serialization
} // namespace boost
</code></pre>
<p>
In this case the generated serialize functions are not members of the
<code style="white-space: normal">gps_position</code> class. The two formulations function
in exactly the same way.
<p>
The main application of non-intrusive serialization is to permit serialization
to be implemented for classes without changing the class definition.
In order for this to be possible, the class must expose enough information
to reconstruct the class state. In this example, we presumed that the
class had <code style="white-space: normal">public</code> members - not a common occurence. Only
classes which expose enough information to save and restore the class
state will be serializable without changing the class definition.
<h3><a name="serializablemembers">Serializable Members</a></h3>
<p>
A serializable class with serializable members would look like this:
<pre><code>
class bus_stop
{
friend class boost::serialization::access;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
ar &amp; latitude;
ar &amp; longitude;
}
gps_position latitude;
gps_position longitude;
protected:
bus_stop(const gps_position &amp; lat_, const gps_position &amp; long_) :
latitude(lat_), longitude(long_)
{}
public:
bus_stop(){}
// See item # 14 in Effective C++ by Scott Meyers.
// re non-virtual destructors in base classes.
virtual ~bus_stop(){}
};
</code></pre>
<p>That is, members of class type are serialized just as
members of primitive types are.
<p>
Note that saving an instance of the class <code style="white-space: normal">bus_stop</code>
with one of the archive operators will invoke the
<code style="white-space: normal">serialize</code> function which saves
<code style="white-space: normal">latitude</code> and
<code style="white-space: normal">longitude</code>. Each of these in turn will be saved by invoking
<code style="white-space: normal">serialize</code> in the definition of
<code style="white-space: normal">gps_position</code>. In this manner the whole
data structure is saved by the application of an archive operator to
just its root item.
<h3><a name="derivedclasses">Derived Classes</a></h3>
<p>Derived classes should include serializations of their base classes.
<pre><code>
#include &lt;boost/serialization/base_object.hpp&gt;
class bus_stop_corner : public bus_stop
{
friend class boost::serialization::access;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
// serialize base class information
ar &amp; boost::serialization::base_object&lt;bus_stop&gt;(*this);
ar &amp; street1;
ar &amp; street2;
}
std::string street1;
std::string street2;
virtual std::string description() const
{
return street1 + " and " + street2;
}
public:
bus_stop_corner(){}
bus_stop_corner(const gps_position &amp; lat_, const gps_position &amp; long_,
const std::string &amp; s1_, const std::string &amp; s2_
) :
bus_stop(lat_, long_), street1(s1_), street2(s2_)
{}
};
</code>
</pre>
<p>
Note the serialization of the base classes from the derived
class. Do <b>NOT</b> directly call the base class serialize
functions. Doing so might seem to work but will bypass the code
that tracks instances written to storage to eliminate redundancies.
It will also bypass the writing of class version information into
the archive. For this reason, it is advisable to always make member
<code style="white-space: normal">serialize</code> functions private. The declaration
<code style="white-space: normal">friend boost::serialization::access</code> will grant to the
serialization library access to private member variables and functions.
<p>
<h3><a name="pointers">Pointers</a></h3>
Suppose we define a bus route as an array of bus stops. Given that
<ol>
<li>we might have several types of bus stops (remember bus_stop is
a base class)
<li>a given bus_stop might appear in more than one route.
</ol>
it's convenient to represent a bus route with an array of pointers
to <code style="white-space: normal">bus_stop</code>.
<pre>
<code>
class bus_route
{
friend class boost::serialization::access;
bus_stop * stops[10];
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
int i;
for(i = 0; i &lt 10; ++i)
ar &amp; stops[i];
}
public:
bus_route(){}
};
</code>
</pre>
Each member of the array <code style="white-space: normal">stops</code> will be serialized.
But remember each member is a pointer - so what can this really
mean? The whole object of this serialization is to permit
reconstruction of the original data structures at another place
and time. In order to accomplish this with a pointer, it is
not sufficient to save the value of the pointer, rather the
object it points to must be saved. When the member is later
loaded, a new object has to be created and a new pointer has
to be loaded into the class member.
<p>
If the same pointer is serialized more than once, only one instance
is be added to the archive. When read back, no data is read back in.
The only operation that occurs is for the second pointer is set equal to the first
<p>
Note that, in this example, the array consists of polymorphic pointers.
That is, each array element point to one of several possible
kinds of bus stops. So when the pointer is saved, some sort of class
identifier must be saved. When the pointer is loaded, the class
identifier must be read and and instance of the corresponding class
must be constructed. Finally the data can be loaded to newly created
instance of the correct type.
As can be seen in
<a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a>,
serialization of pointers to derived classes through a base
clas pointer may require explicit enumeration of the derived
classes to be serialized. This is referred to as "registration" or "export"
of derived classes. This requirement and the methods of
satisfying it are explained in detail
<a href="serialization.html#derivedpointers">here</a>
<p>
All this is accomplished automatically by the serialization
library. The above code is all that is necessary to accomplish
the saving and loading of objects accessed through pointers.
<p>
<h3><a name="arrays">Arrays</a></h3>
The above formulation is in fact more complex than necessary.
The serialization library detects when the object being
serialized is an array and emits code equivalent to the above.
So the above can be shortened to:
<pre>
<code>
class bus_route
{
friend class boost::serialization::access;
bus_stop * stops[10];
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
ar &amp; stops;
}
public:
bus_route(){}
};
</code>
</pre>
<h3><a name="stl">STL Collections</a></h3>
The above example uses an array of members. More likely such
an application would use an STL collection for such a purpose.
The serialization library contains code for serialization
of all STL classes. Hence, the reformulation below will
also work as one would expect.
<pre>
<code>
#include &lt;boost/serialization/list.hpp&gt;
class bus_route
{
friend class boost::serialization::access;
std::list&lt;bus_stop *&gt; stops;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
ar &amp; stops;
}
public:
bus_route(){}
};
</code>
</pre>
<h3><a name="versioning">Class Versioning</a></h3>
<p>
Suppose we're satisfied with our <code style="white-space: normal">bus_route</code> class, build a program
that uses it and ship the product. Some time later, it's decided
that the program needs enhancement and the <code style="white-space: normal">bus_route</code> class is
altered to include the name of the driver of the route. So the
new version looks like:
<pre>
<code>
#include &lt;boost/serialization/list.hpp&gt;
#include &lt;boost/serialization/string.hpp&gt;
class bus_route
{
friend class boost::serialization::access;
std::list&lt;bus_stop *&gt; stops;
std::string driver_name;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
ar &amp; driver_name;
ar &amp; stops;
}
public:
bus_route(){}
};
</code>
</pre>
Great, we're all done. Except... what about people using our application
who now have a bunch of files created under the previous program.
How can these be used with our new program version?
<p>
In general, the serialization library stores a version number in the
archive for each class serialized. By default this version number is 0.
When the archive is loaded, the version number under which it was saved
is read. The above code can be altered to exploit this
<pre>
<code>
#include &lt;boost/serialization/list.hpp&gt;
#include &lt;boost/serialization/string.hpp&gt;
#include &lt;boost/serialization/version.hpp&gt;
class bus_route
{
friend class boost::serialization::access;
std::list&lt;bus_stop *&gt; stops;
std::string driver_name;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int version)
{
// only save/load driver_name for newer archives
if(version &gt; 0)
ar &amp; driver_name;
ar &amp; stops;
}
public:
bus_route(){}
};
BOOST_CLASS_VERSION(bus_route, 1)
</code>
</pre>
By application of versioning to each class, there is no need to
try to maintain a versioning of files. That is, a file version
is the combination of the versions of all its constituent classes.
This system permits programs to be always compatible with archives
created by all previous versions of a program with no more
effort than required by this example.
<h3><a name="splitting">Splitting <code style="white-space: normal">serialize</code>
into <code style="white-space: normal">save/load</code></a></h3>
The <code style="white-space: normal">serialize</code> function is simple, concise, and guarantees
that class members are saved and loaded in the same sequence
- the key to the serialization system. However, there are cases
where the load and save operations are not as similar as the examples
used here. For example, this could occur with a class that has evolved through
multiple versions. The above class can be reformulated as:
<pre>
<code>
#include &lt;boost/serialization/list.hpp&gt;
#include &lt;boost/serialization/string.hpp&gt;
#include &lt;boost/serialization/version.hpp&gt;
#include &lt;boost/serialization/split_member.hpp&gt;
class bus_route
{
friend class boost::serialization::access;
std::list&lt;bus_stop *&gt; stops;
std::string driver_name;
template&lt;class Archive&gt;
void save(Archive &amp; ar, const unsigned int version) const
{
// note, version is always the latest when saving
ar &amp; driver_name;
ar &amp; stops;
}
template&lt;class Archive&gt;
void load(Archive &amp; ar, const unsigned int version)
{
if(version &gt; 0)
ar &amp; driver_name;
ar &amp; stops;
}
BOOST_SERIALIZATION_SPLIT_MEMBER()
public:
bus_route(){}
};
BOOST_CLASS_VERSION(bus_route, 1)
</code>
</pre>
The macro <code style="white-space: normal">BOOST_SERIALIZATION_SPLIT_MEMBER()</code> generates
code which invokes the <code style="white-space: normal">save</code>
or <code style="white-space: normal">load</code>
depending on whether the archive is used for saving or loading.
<h3><a name="archives">Archives</a></h3>
Our discussion here has focused on adding serialization
capability to classes. The actual rendering of the data to be serialized
is implemented in the archive class. Thus the stream of serialized
data is a product of the serialization of the class and the
archive selected. It is a key design decision that these two
components be independent. This permits any serialization specification
to be usable with any archive.
<p>
In this tutorial, we have used a particular
archive class - <code style="white-space: normal">text_oarchive</code> for saving and
<code style="white-space: normal">text_iarchive</code> for loading.
text archives render data as text and are portable across platforms. In addition
to text archives, the library includes archive class for native binary data
and xml formatted data. Interfaces to all archive classes are all identical.
Once serialization has been defined for a class, that class can be serialized to
any type of archive.
<p>
If the current set of archive classes doesn't provide the
attributes, format, or behavior needed for a particular application,
one can either make a new archive class or derive from an existing one.
This is described later in the manual.
<h3><a name="examples">List of Examples</h3>
<dl>
<dt><a href="../example/demo.cpp" target="demo_cpp">demo.cpp</a>
<dd>This is the completed example used in this tutorial.
It does the following:
<ol>
<li>Creates a structure of differing kinds of stops, routes and schedules
<li>Displays it
<li>Serializes it to a file named "testfile.txt" with one
statement
<li>Restores to another structure
<li>Displays the restored structure
</ol>
<a href="../example/demo_output.txt" target="demo_output">Output of
this program</a> is sufficient to verify that all the
originally stated requirements for a serialization system
are met with this system. The <a href="../example/demofile.txt"
target="test_file">contents of the archive file</a> can
also be displayed as serialization files are ASCII text.
<dt><a href="../example/demo_xml.cpp" target="demo_xml_cpp">demo_xml.cpp</a>
<dd>This is a variation the original demo which supports xml archives in addition
to the others. The extra wrapping macro, BOOST_SERIALIZATION_NVP(name), is
needed to associate a data item name with the corresponding xml
tag. It is importanted that 'name' be a valid xml tag, else it
will be impossible to restore the archive.
For more information see
<a target="detail" href="wrappers.html#nvp">Name-Value Pairs</a>.
<a href="../example/demo_save.xml" target="demo_save_xml">Here</a>
is what an xml archive looks like.
<dt><a href="../example/demo_xml_save.cpp" target="demo_xml_save_cpp">demo_xml_save.cpp</a>
and <a href="../example/demo_xml_load.cpp" target="demo_xml_load_cpp">demo_xml_load.cpp</a>
<dd>Note also that though our examples save and load the program data
to an archive within the same program, this merely a convenience
for purposes of illustration. In general, the archive may or may
not be loaded by the same program that created it.
</dl>
<p>
The astute reader might notice that these examples contain a subtle but important flaw.
They leak memory. The bus stops are created in the <code style="white-space: normal">
main</code> function. The bus schedules may refer to these bus stops
any number of times. At the end of the main function after the bus schedules are destroyed,
the bus stops are destroyed. This seems fine. But what about the structure
<code style="white-space: normal">new_schedule</code> data item created by the
process of loading from an archive? This contains its own separate set of bus stops
that are not referenced outside of the bus schedule. These won't be destroyed
anywhere in the program - a memory leak.
<p>
There are couple of ways of fixing this. One way is to explicitly manage the bus stops.
However, a more robust and transparent is to use
<code style="white-space: normal">shared_ptr</code> rather than raw pointers. Along
with serialization implemenations for the Standard Library, the serialization library
includes implementation of serialization for
<code style="white-space: normal">boost::shared ptr</code>. Given this, it should be
easy to alter any of these examples to eliminate the memory leak. This is left
as an excercise for the reader.
<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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