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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">PIMPL</h2>
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<hr>
PIMPL is a C++ programming idiom described by Herb Sutter <a href="bibliography.html#10">[10]</a>
which stands for "Private Implementation".  It is also referred to as
the "Handle Body Idiom". Included in this library is a program called 
<a href="../example/demo_pimpl.cpp" target="demo_impl_cpp">demo_pimpl.cpp</a> 
which illustrates how this is used. The file 
<a href="../example/demo_pimpl_A.cpp" target="demo_impl__Acpp">demo_pimpl_A.hpp</a>
contains the declaration of the A class that hides its implementation
by including a pointer to struct B that is only defined as a pointer.
<pre><code>
// class whose declaration is hidden by a pointer
struct B;

struct A {
    // class a contains a pointer to a "hidden" declaration
    B *pimpl;
    template&lt;class Archive&gt;
    void serialize(Archive & ar, const unsigned int file_version);
    A();
};
</code></pre>
Serialization of A requires access to the definition of B. But that doesn't mean
that it requires the this access from the header file.  Since B is a pointer,
a declaration of class B is sufficient.  The implemenation of the serialization
of A includes the definition of class B defined in the separately compiled module:

<a href="../example/demo_pimpl_A.cpp" target="demo_impl_A_cpp">demo_pimpl_A.cpp</a>
by:
<pre><code>
#include "demo_pimpl_A.hpp"

// "hidden" definition of class B
struct B {
    int b;
    template&lt;class Archive&gt;
    void serialize(Archive & ar, const unsigned int file_version){
        ar & b;
    }
};

A::A() :
    pimpl(new B)
{}

A::~A(){
    delete pimpl;
}

// now we can define the serialization for class A
template&lt;class Archive&gt;
void A::serialize(Archive & ar, const unsigned int file_version){
    ar & pimpl;
}
</code></pre>
As described in <a href="bibliography.html#10">[10]</a> this brings the
following advantages:
<ul>
    <li>type "B" can be used without using its header.
    <li>implementation of B can be changed without generating a 
    cascade of recompilations.
</ul>
So, we compile the modules and everything is fine.  However when we
link, we get an error.  Two symbols are undefined:
<pre><code>
void A::serialize(boost::archive::text_oarchive & ar, const unsigned int file_version);
void A::serialize(boost::archive::text_iarchive & ar, const unsigned int file_version);
</code></pre>
The problem is that when compiling the above code, 
there is no instantiation of the <code style="white-space: normal">serialize</code> template.
There can't be as it's not "known" what types of archives
the serialization is going to be used with.  So these functions are "missing"
when an attempt to link is made.  The solution is to explicitly instantiate
serialization code for those archives which are going to be used.  In this
example, including the the following code in any *.cpp file does just that:
<pre></code>
#include &lt;boost/archive/text_iarchive.hpp&gt;
#include &lt;boost/archive/text_oarchive.hpp&gt;

template void A::serialize&lt;boost::archive::text_iarchive&gt;(
    boost::archive::text_iarchive & ar, 
    const unsigned int file_version
);
template void A::serialize&lt;boost::archive::text_oarchive&gt;(
    boost::archive::text_oarchive & ar, 
    const unsigned int file_version
);
</code></pre>
The program should now link as well as compile.
<p>
The downside of this is that one has to know which archives are going 
to be used with hidden serializations.  This is an effect of using template
driven code.  One can invoke explicity instantiation for all known templates and
presume that the linker will exclude unreferenced code.  This should
work but is platform dependent.
<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)
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