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    copied!<p>I must admit I am not familiar with this <code>boost</code> package, but I copied and compiled the code, which produced the same result as the OP mentioned</p> <p>Realizing that we are using polymorphism, I added a <code>public: virtual ~A(){};</code> in <code>class A</code>. Also, <code>oa.register_type&lt;B&gt;();</code> is added in <code>main</code> according to the document, and output became:</p> <pre><code>A! B! </code></pre> <p>According to the specification, a class is a <code>polymorphic class</code> only when <code>declares or inherits a virtual function</code>. For non-polymorphic classed, maybe polymorphism just does not work.</p> <p>EDIT:</p> <p>Putting the <code>BOOST_CLASS_EXPORT(B);</code> in <code>B.cpp</code> instead of <code>B.h</code> seems to solve this problem of redefinition.</p> <p>EDIT:</p> <p>Checked the expansion result of <code>BOOST_CLASS_EXPORT(B)</code> (reformatted):</p> <pre><code>namespace boost { namespace serialization { template&lt;&gt; struct guid_defined&lt;B&gt; : boost::mpl::true_ {}; template&lt;&gt; inline const char * guid&lt;B&gt;(){ return "B"; } } } namespace boost { namespace archive { namespace detail { namespace { // NOTE template&lt;&gt; struct init_guid&lt; B &gt; { static guid_initializer&lt; B &gt; const &amp; g; }; guid_initializer&lt; B &gt; const &amp; init_guid&lt; B &gt;::g = ::boost::serialization::singleton&lt; guid_initializer&lt; B &gt; &gt;::get_mutable_instance().export_guid(); } } } } </code></pre> <p>And for the line marked with <code>NOTE</code>: for boost 1.42 used an anonymous namespace is used, which will be no problem if it is put into multiple cpp files or put into a header file (tested in ubuntu with g++, using the boost package come along with Ubuntu). However, in boost 1.48, <code>namespace extra_detail</code> is used, which would cause problems when put into multiple cpp files (tested in windows with VS2010, using boost downloaded from homepage).</p>
 

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