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<title>Serialization - Special Considerations</title>
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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">Special Considerations</h2>
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</table>
<hr>
<dl class="page-index">
<dt><a href="#objecttracking">Object Tracking</a>
<dt><a href="#classinfo">Class Information</a>
<dt><a href="#portability">Archive Portability</a>
<dl class="page-index">
<dt><a href="#numerics">Numerics</a>
<dt><a href="#traits">Traits</a>
</dl>
<dt><a href="#binary_archives">Binary Archives</a>
<dt><a href="#xml_archives">XML Archives</a>
<dt><a href="#export">Exporting Class Serialization</a>
<dt><a href="#static_libraries">Static Libraries and Serialization</a>
<dt><a href="#dlls">DLLS - Serialization and Runtime Linking</a>
<dt><a href="#plugins">Plugins</a>
<dt><a href="#multi_threading">Multi-Threading</a>
<dt><a href="#optimizations">Optimizations</a>
<dt><a href="exceptions.html">Archive Exceptions</a>
<dt><a href="exception_safety.html">Exception Safety</a>
</dl>
<h3><a name="objecttracking">Object Tracking</a></h3>
Depending on how the class is used and other factors, serialized objects
may be tracked by memory address. This prevents the same object from being
written to or read from an archive multiple times. These stored addresses
can also be used to delete objects created during a loading process
that has been interrupted by throwing of an exception.
<p>
This could cause problems in
progams where the copies of different objects are saved from the same address.
<pre><code>
template&lt;class Archive&gt;
void save(boost::basic_oarchive &amp; ar, const unsigned int version) const
{
for(int i = 0; i &lt; 10; ++i){
A x = a[i];
ar &lt;&lt; x;
}
}
</code></pre>
In this case, the data to be saved exists on the stack. Each iteration
of the loop updates the value on the stack. So although the data changes
each iteration, the address of the data doesn't. If a[i] is an array of
objects being tracked by memory address, the library will skip storing
objects after the first as it will be assumed that objects at the same address
are really the same object.
<p>
To help detect such cases, output archive operators expect to be passed
<code style="white-space: normal">const</code> reference arguments.
<p>
Given this, the above code will invoke a compile time assertion.
The obvious fix in this example is to use
<pre><code>
template&lt;class Archive&gt;
void save(boost::basic_oarchive &amp; ar, const unsigned int version) const
{
for(int i = 0; i &lt; 10; ++i){
ar &lt;&lt; a[i];
}
}
</code></pre>
which will compile and run without problem.
The usage of <code style="white-space: normal">const</code> by the output archive operators
will ensure that the process of serialization doesn't
change the state of the objects being serialized. An attempt to do this
would constitute augmentation of the concept of saving of state with
some sort of non-obvious side effect. This would almost surely be a mistake
and a likely source of very subtle bugs.
<p>
Unfortunately, implementation issues currently prevent the detection of this kind of
error when the data item is wrapped as a name-value pair.
<p>
A similar problem can occur when different objects are loaded to and address
which is different from the final location:
<pre><code>
template&lt;class Archive&gt;
void load(boost::basic_oarchive &amp; ar, const unsigned int version) const
{
for(int i = 0; i &lt; 10; ++i){
A x;
ar &gt;&gt; x;
std::m_set.insert(x);
}
}
</code></pre>
In this case, the address of <code>x</code> is the one that is tracked rather than
the address of the new item added to the set. Left unaddressed
this will break the features that depend on tracking such as loading object through a pointer.
Subtle bugs will be introduced into the program. This can be
addressed by altering the above code thusly:
<pre><code>
template&lt;class Archive&gt;
void load(boost::basic_iarchive &amp; ar, const unsigned int version) const
{
for(int i = 0; i &lt; 10; ++i){
A x;
ar &gt;&gt; x;
std::pair&lt;std::set::const_iterator, bool&gt; result;
result = std::m_set.insert(x);
ar.reset_object_address(& (*result.first), &x);
}
}
</code></pre>
This will adjust the tracking information to reflect the final resting place of
the moved variable and thereby rectify the above problem.
<p>
If it is known a priori that no pointer
values are duplicated, overhead associated with object tracking can
be eliminated by setting the object tracking class serialization trait
appropriately.
<p>
By default, data types designated primitive by
<a target="detail" href="traits.html#level">Implementation Level</a>
class serialization trait are never tracked. If it is desired to
track a shared primitive object through a pointer (e.g. a
<code style="white-space: normal">long</code> used as a reference count), It should be wrapped
in a class/struct so that it is an identifiable type.
The alternative of changing the implementation level of a <code style="white-space: normal">long</code>
would affect all <code style="white-space: normal">long</code>s serialized in the whole
program - probably not what one would intend.
<p>
It is possible that we may want to track addresses even though
the object is never serialized through a pointer. For example,
a virtual base class need be saved/loaded only once. By setting
this serialization trait to <code style="white-space: normal">track_always</code>, we can suppress
redundant save/load operations.
<pre><code>
BOOST_CLASS_TRACKING(my_virtual_base_class, boost::serialization::track_always)
</code></pre>
<h3><a name="classinfo">Class Information</a></h3>
By default, for each class serialized, class information is written to the archive.
This information includes version number, implementation level and tracking
behavior. This is necessary so that the archive can be correctly
deserialized even if a subsequent version of the program changes
some of the current trait values for a class. The space overhead for
this data is minimal. There is a little bit of runtime overhead
since each class has to be checked to see if it has already had its
class information included in the archive. In some cases, even this
might be considered too much. This extra overhead can be eliminated
by setting the
<a target="detail" href="traits.html#level">implementation level</a>
class trait to: <code style="white-space: normal">boost::serialization::object_serializable</code>.
<p>
<i>Turning off tracking and class information serialization will result
in pure template inline code that in principle could be optimised down
to a simple stream write/read.</i> Elimination of all serialization overhead
in this manner comes at a cost. Once archives are released to users, the
class serialization traits cannot be changed without invalidating the old
archives. Including the class information in the archive assures us
that they will be readable in the future even if the class definition
is revised. A light weight structure such as display pixel might be
declared in a header like this:
<pre><code>
#include &lt;boost/serialization/serialization.hpp&gt;
#include &lt;boost/serialization/level.hpp&gt;
#include &lt;boost/serialization/tracking.hpp&gt;
// a pixel is a light weight struct which is used in great numbers.
struct pixel
{
unsigned char red, green, blue;
template&lt;class Archive&gt;
void serialize(Archive &amp; ar, const unsigned int /* version */){
ar &lt;&lt; red &lt;&lt; green &lt;&lt; blue;
}
};
// elminate serialization overhead at the cost of
// never being able to increase the version.
BOOST_CLASS_IMPLEMENTATION(pixel, boost::serialization::object_serializable);
// eliminate object tracking (even if serialized through a pointer)
// at the risk of a programming error creating duplicate objects.
BOOST_CLASS_TRACKING(pixel, boost::serialization::track_never)
</code></pre>
<h3><a name="portability">Archive Portability</a></h3>
Several archive classes create their data in the form of text or portable a binary format.
It should be possible to save such an of such a class on one platform and load it on another.
This is subject to a couple of conditions.
<h4><a name="numerics">Numerics</a></h4>
The architecture of the machine reading the archive must be able hold the data
saved. For example, the gcc compiler reserves 4 bytes to store a variable of type
<code style="white-space: normal">wchar_t</code> while other compilers reserve only 2 bytes.
So its possible that a value could be written that couldn't be represented by the loading program. This is a
fairly obvious situation and easily handled by using the numeric types in
<a target="cstding" href="../../../boost/cstdint.hpp">&lt;boost/cstdint.hpp&gt;</a>
<P>
A special integral type is <code>std::size_t</code> which is a typedef
of an integral types guaranteed to be large enough
to hold the size of any collection, but its actual size can differ depending
on the platform. The
<a href="wrappers.html#collection_size_type"><code>collection_size_type</code></a>
wrapper exists to enable a portable serialization of collection sizes by an archive.
Recommended choices for a portable serialization of collection sizes are to
use either 64-bit or variable length integer representation.
<h4><a name="traits">Traits</a></h4>
Another potential problem is illustrated by the following example:
<pre><code>
template&lt;class T&gt;
struct my_wrapper {
template&lt;class Archive&gt;
Archive & serialize ...
};
...
class my_class {
wchar_t a;
short unsigned b;
template&lt;class Archive&gt;
Archive & serialize(Archive & ar, unsigned int version){
ar & my_wrapper(a);
ar & my_wrapper(b);
}
};
</code></pre>
If <code style="white-space: normal">my_wrapper</code> uses default serialization
traits there could be a problem. With the default traits, each time a new type is
added to the archive, bookkeeping information is added. So in this example, the
archive would include such bookkeeping information for
<code style="white-space: normal">my_wrapper&lt;wchar_t&gt;</code> and for
<code style="white-space: normal">my_wrapper&lt;short_unsigned&gt;</code>.
Or would it? What about compilers that treat
<code style="white-space: normal">wchar_t</code> as a
synonym for <code style="white-space: normal">unsigned short</code>?
In this case there is only one distinct type - not two. If archives are passed between
programs with compilers that differ in their treatment
of <code style="white-space: normal">wchar_t</code> the load operation will fail
in a catastrophic way.
<p>
One remedy for this is to assign serialization traits to the template
<code style="white-space: normal">my_template</code> such that class
information for instantiations of this template is never serialized. This
process is described <a target="detail" href="traits.html#templates">above</a> and
has been used for <a target="detail" href="wrappers.html#nvp"><strong>Name-Value Pairs</strong></a>.
Wrappers would typically be assigned such traits.
<p>
Another way to avoid this problem is to assign serialization traits
to all specializations of the template <code style="white-space: normal">my_wrapper</code>
for all primitive types so that class information is never saved. This is what has
been done for our implementation of serializations for STL collections.
<h3><a name="binary_archives">Binary Archives</a></h3>
Standard stream i/o on some systems will expand linefeed characters to carriage-return/linefeed
on output. This creates a problem for binary archives. The easiest way to handle this is to
open streams for binary archives in "binary mode" by using the flag
<code style="white-space: normal">ios::binary</code>. If this is not done, the archive generated
will be unreadable.
<p>
Unfortunately, no way has been found to detect this error before loading the archive. Debug builds
will assert when this is detected so that may be helpful in catching this error.
<h3><a name="xml_archives">XML Archives</a></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 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. This is achived by requiring that all data serialized to an XML archive
be serialized as a <a target="detail" href="wrappers.html#nvp">name-value pair</a>.
The first member is the name to be used as the XML tag for the
data item while the second is a reference to the data item itself. Any attempt to serialize data
not wrapped in a in a <a target="detail" href="wrappers.html#nvp">name-value pair</a> will
be trapped at compile time. The system is implemented in such a way that for other archive classes,
just the value portion of the data is serialized. The name portion is discarded during compilation.
So by always using <a target="detail" href="wrappers.html#nvp">name-value pairs</a>, it will
be guarenteed that all data can be serialized to all archive classes with maximum efficiency.
<h3><a name="export">Exporting Class Serialization</a></h3>
<a target="detail" href="traits.html#export">Elsewhere</a> in this manual, we have described
<code style="white-space: normal">BOOST_CLASS_EXPORT</code>.
Export implies two things:
<ul>
<li>Instantiates code which is not otherwise referred to.
<li>Associates an external identifier with the class to be serialized.
The fact that the class isn't explicitly referred to implies this
requirement.
</ul>
In C++, usage of code not explicitly referred to is implemented via
virtual functions. Hence, the need for export is implied by the
usage of a derived class that is manipulated via a pointer or
reference to it's base class.
<p>
<code style="white-space: normal">BOOST_CLASS_EXPORT</code> in the same
source module that includes any of the archive class headers will
instantiate code required to serialize polymorphic pointers of
the indicated type to the all those archive classes. If no
archive class headers are included, then no code will be instantiated.
<p>
Note that the implemenation of this functionality requires
that the <code style="white-space: normal">BOOST_CLASS_EXPORT</code>
macro appear <b>after</b> and the inclusion of any archive
class headers for which code is to be instantiated.
So, code that uses <code style="white-space: normal">BOOST_CLASS_EXPORT</code>
will look like the following:
<pre><code>
#include &lt;boost/archive/text_oarchive.hpp&gt;
#include &lt;boost/archive/text_oarchive.hpp&gt;
... // other archives
#include "a.hpp" // header declaration for class a
BOOST_CLASS_EXPORT(a)
... // other class headers and exports
</code></pre>
This will be true regardless of whether the is part
of a stand alone executable, a static library or
a dyanmic or shared library.
<p>
Including
<code style="white-space: normal">BOOST_CLASS_EXPORT</code>
in the "a.hpp" header itself as one would do with
other serialization traits will make it difficult
or impossible to follow the rule above regarding
inclusion of archive headers before
<code style="white-space: normal">BOOST_CLASS_EXPORT</code>
is invoked. This can best be addressed by using
<code style="white-space: normal">BOOST_CLASS_EXPORT_KEY</code>
in the header declarations and
<code style="white-space: normal">BOOST_CLASS_EXPORT_IMPLEMENT</code>
in the class definition file.
<p>
This system has certain implications for placing code in static or shared
libraries. Placing <code style="white-space: normal">BOOST_CLASS_EXPORT</code>
in library code will have no effect unless archive class headers are
also included. So when building a library, one should include all headers
for all the archive classes which he anticipates using. Alternatively,
one can include headers for just the
<a href="archive_reference.html#polymorphic">Polymoprhic Archives</a>.
<p>
Strictly speaking, export should not be necessary if all pointer serialization
occurs through the most derived class. However, in order to detect
what would be catastophic error, the library traps ALL serializations through
a pointer to a polymorphic which are not exported or otherwise registered.
So, in practice, be prepared to register or export all classes with one
or more virtual functions which are serialized through a pointer.
<p>
Note that the implementation of this functionality depends upon vendor
specific extensions to the C++ language. So, there is no guarenteed portability
of programs which use this facility. However, all C++ compilers which
are tested with boost provide the required extensions. The library
includes the extra declarations required by each of these compilers.
It's reasonable to expect that future C++ compilers will support
these extensions or something equivalent.
<h3><a name="static_libraries">Static Libraries and Serialization</a></h3>
Code for serialization of data types can be saved in libraries
just as it can for the rest of the type implementation.
This works well, and can save huge amount of compilation time.
<ul>
<li>Only compile serialization definitions in the library.
<li>Explicitly instantiate serialization code for ALL
archive classes you intend to use in the library.
<li>For exported types, only use <code style="white-space: normal">BOOST_CLASS_EXPORT_KEY</code>
in headers.
<li>For exported types, only use <code style="white-space: normal">BOOST_CLASS_EXPORT_IMPLEMENT</code>
in definitions compiled in the library. For any particular type,
there should be only one file which contains
<code style="white-space: normal">BOOST_CLASS_EXPORT_IMPLEMENT</code>
for that type. This ensures that only one copy
of serialization code will exist within the program. It avoids
wasted space and the possibility of having different
versions of the serialization code in the same program.
Including
<code style="white-space: normal">BOOST_CLASS_EXPORT_IMPLEMENT</code>
in multiple files could result in a failure
to link due to duplicated symbols or the throwing
of a runtime exception.
<li> Code for serialization should be only in the library,
<li>Familiarize yourself with the <b>PIMPL</b> idiom.
</ul>
This is illustrated by
<a href = "../example/demo_pimpl.cpp" target="demo_pimpl">
<code style="white-space: normal">demo_pimpl.cpp</code>
</a>,
<a href = "../example/demo_pimpl_A.cpp" target="demo_pimpl">
<code style="white-space: normal">demo_pimpl_A.cpp</code>
</a>
and
<a href = "../example/demo_pimpl_A.hpp" target="demo_pimpl">
<code style="white-space: normal">demo_pimpl_A.hpp</code>
</a>
where implementation of serializaton is in a static library
completely separate from the main program.
<h3><a name="dlls">DLLS - Serialization and Runtime Linking</a></h3>
Serialization code can be placed in libraries to be linked at runtime. That is,
code can be placed in DLLS(Windows) Shared Libraries(*nix), or static libraries
as well as the main executable. The best technique is the
same as that described above for libraries. The serialization
library test suite includes the following programs
to illustrate how this works:
<p>
<a href = "../test/test_dll_simple.cpp" target="test_dll_simple">
<code style="white-space: normal">test_dll_simple</code>
</a>,
and
<a href = "../test/dll_a.cpp" target="dll_a">
<code style="white-space: normal">dll_a.cpp</code>
</a>
where implementation of serializaton is also completely separate
from the main program but the code is loaded at runtime. In this
example, this code is loaded automatically when the program which
uses it starts up, but it could just as well be loaded and unloaded
with an OS dependent API call.
<p>
Also included are
<a href = "../test/test_dll_exported.cpp" target="test_dll_exported">
<code style="white-space: normal">test_dll_exported.cpp</code>
</a>,
and
<a href = "../test/polymorphic_derived2.cpp" target="polymorphic_derived2">
<code style="white-space: normal">polymorphic_derived2.cpp</code>
</a>
which are similar to the above but include tests of the export
and no_rtti facilities in the context of DLLS.
<p>
For best results, write your code to conform to the following
guidelines:
<ul>
<li>Don't include <code>inline</code> code in classes used in DLLS.
This will generate duplicate code in the DLLS and mainline. This
needlessly duplicates code. Worse, it makes is possible for
different versions of the same code exist simultaneously. This
type of error turns out to be excruciatingly difficult to debug.
Finally, it opens the possibility that module being referred to
might be explictly unloaded which would (hopefully) result in
a runtime error. This is another bug that is not always
reproducible or easy to find. For class member templates use something like
<pre><code>
template&lt;class Archive&gt;
void serialize(Archive & ar, const unsigned int version);
</code></pre>
in the header, and
<pre><code>
template&lt;class Archive&gt;
void myclass::serialize(Archive & ar, const unsigned int version){
...
}
BOOST_EXPORT_CLASS_IMPLEMENT(my_class)
#include &lt;boost/archive/text_oarchive&gt;
#include &lt;boost/archive/text_iarchive&lt;
template myclass::serialize(boost::archive::text_oarchive & ar, const unsigned int version);
template myclass::serialize(boost::archive::text_iarchive & ar, const unsigned int version);
... // repeat for each archive class to be used.
</code></pre>
in the implementation file. This will result in generation of all code
required in only one place. The library does not detect this type of error for you.
<li>If DLLS are to be loaded an unloaded explicitly (e.g. using <code>dlopen</code> in *nix or
<code>LoadLibrary</code> in Windows). Try to arrange that they are unloaded in the reverse
sequence. This should guarentee that problems are avoided even if the
above guidline hasn't been followed.
</ul>
<h3><a name="plugins">Plugins</a></h3>
In order to implement the library, various facilities for runtime
manipulation of types are runtime were required. These
are <a target="detail" href="extended_type_info.html"><code>extended_type_info</code></a>
for associating classes with external identifying strings (<b>GUID</b>)
and <a target="detail" href="void_cast.html"><code>void_cast</code></a>
for casting between pointers of related types.
To complete the functionality of
<a target="detail" href="extended_type_info.html"><code>extended_type_info</code></a>
the ability to construct and destroy corresponding types has been
added. In order to use this functionality, one must specify
how each type is created. This should be done at the time
a class is exported. So, a more complete example of the code above would be:
<pre><code>
#include &lt;boost/archive/text_oarchive.hpp&gt;
#include &lt;boost/archive/text_oarchive.hpp&gt;
... // other archives
#include "a.hpp" // header declaration for class a
// this class has a default constructor
BOOST_SERIALIZATION_FACTORY_0(a)
// as well as one that takes one integer argument
BOOST_SERIALIZATION_FACTORY_1(a, int)
// specify the GUID for this class
BOOST_CLASS_EXPORT(a)
... // other class headers and exports
</code></pre>
With this in place, one can construct, serialize and destroy
about which only is know the <b>GUID</b> and a base class.
<h3><a name="multi_threading">Multi-Threading</a></h3>
The fundamental purpose of serialization would conflict with multiple
thread concurrently writing/reading from/to a single open archive instance.
The library implementation presumes that the application avoids such an situtation.
<p>
However, Writing/Reading different archives simultaneously
in different tasks is permitted as each archive instance is (almost)
completely independent from any other archive instance. The only shared
information are some type tables which have been implemented using a
lock-free thread-safe
<a target="detail" href="singleton.html">
<code style="white-space: normal">singleton</code>
</a>
described elsewhere in this documentation.
<p>
This singleton implementation guarentees that all of this shared
information is initialized when the code module which contains
them is loaded. The serialization library takes care to
ensure that these data structures are not subsequently
modified. The only time there could be a problem would
be if code is loaded/unloaded while another task is
serializing data. This could only occur for types whose
serialization is implemented in a dynamically loaded/unload DLL
or shared library. So if the following is avoided:
<ul>
<li>Accessing the same archive instance from different tasks.
<li>Loading/Unloading DLLS or shared libraries while any archive
instances are open.
</ul>
The library should be thread safe.
<h3><a name="optimizations">Optimizations</a></h3>
In performance critical applications that serialize large sets of contiguous data of homogeneous
types one wants to avoid the overhead of serializing each element individually, which is
the motivation for the <a href="wrappers.html#arrays"><code>array</code></a>
wrapper.
Serialization functions for data types containing contiguous arrays of homogeneous
types, such as for <code>std::vector</code>, <code>std::valarray</code> or
<code>boost::multiarray</code> should serialize them using an
<a href="wrappers.html#arrays"><code>array</code></a> wrapper to make use of
these optimizations.
Archive types that can provide optimized serialization for contiguous arrays of
homogeneous types should implement these by overloading the serialization of
the <a href="wrappers.html#arrays"><code>array</code></a> wrapper, as is done
for the binary archives.
<h3><a href="exceptions.html">Archive Exceptions</a></h3>
<h3><a href="exception_safety.html">Exception Safety</a></h3>
<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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