Re: [Boost-users] [serialization] Polymorphic serialization withpolymorphic archives problem...

25 Feb 2010

      Robert Ramey <ramey <at> rrsd.com> writes:
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Martin Lederhilger wrote:
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Hello Robert, hello Bogdan,
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I have quite the same problem (Exception multiple_code_instantiation
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thrown in basic_serializer_map.cpp at line 48) as Bogdan. It can be
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reproduced by changing
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the pointer type from polymorphic_base to polymorphic_derived2 in the
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original test_dll_exported.cpp example, which comes with the library.
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I think the problem is that when oa << BOOST_SERIALIZATION_NVP(rb2);
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is called, that the system registers an type in oserializer.hpp, which
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leads to creation of the singletons also in the executeable.
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Ahhhh - a very useful hint.  So the rule would be that if a class
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is polymorphic is to be serialized through a pointer, it should ONLY
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be done through a base class pointer?  That is, if ALL polymorphic
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base classes are abstract - this problem will never occur?
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I'll have to think about this.
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I have this problem when serializing a shared_ptr<A> in class B in
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B.dll to an object of class A in A.dll.
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I tried to simply comment out this check, but it seems that I get
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problems with tracking later. I have a class hierarchy like this:
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C derives from B derives from A. A has a weak_ptr<A> to itself
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(something like boost::enable_shared_from_this). If I serialize an
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object of type C via a base pointer of type B the serialization walks
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like this: C::serialize, B::serialize, A::serialize, and again
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C::serialize (with a wrong this pointer - and it should not do that).
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Hmmm - why should it not do that?  You may have a cycle - but
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the library handles that.  Seems unrelated to the other problem.
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Maybe my second problem depends on the first one. I hope that my
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report can be of help.
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Thanks in advance for your answers,
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And thanks for your useful information.
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Martin Lederhilger
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Hello Robert, 

On your suggestion I have run a few tests. I hope they help:

I commented out the registration macro associated to the base class in
ExportDll.dll:

- //BOOST_CLASS_EXPORT_IMPLEMENT(polymorphic_base)

To make the example more realistic I modified the inheritance hierarchy as
follows:

class polymorphic_base{};

class polymorphic_derived1 : public polymorphic_base{};

class polymorphic_derived2 : public polymorphic_derived1{};

I added one integer data member to polymorphic_derived1 and
polymorphic_derived2.

Here are the results:

==================================================

Polymorphic serialization through a pointer to polymorphic_base WORKS AS
EXPECTED:

// Save code -----------------------------

    polymorphic_base *rb0 = new polymorphic_derived1(100);

    polymorphic_base *rb1 = new polymorphic_derived1(10);

    polymorphic_base *rb2 = new polymorphic_derived2(10,20);

    oa << BOOST_SERIALIZATION_NVP(rb0);

    oa << BOOST_SERIALIZATION_NVP(rb1);

    oa << BOOST_SERIALIZATION_NVP(rb2);

// Load code -----------------------------

    polymorphic_base *rb0 = NULL;

    polymorphic_base *rb1 = NULL;

    polymorphic_base *rb2 = NULL;

    ia >> BOOST_SERIALIZATION_NVP(rb0);

    ia >> BOOST_SERIALIZATION_NVP(rb1);

    ia >> BOOST_SERIALIZATION_NVP(rb2);

The serialized content is presented below:

- for polymorphic_xml_[io]archive

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<!DOCTYPE boost_serialization>

<boost_serialization signature="serialization::archive" version="7">

<rb0 class_id="1" class_name="polymorphic_derived1" tracking_level="1"
version="0" object_id="_0">

          <polymorphic_base class_id="0" tracking_level="0"
version="0"></polymorphic_base>

          <m_d1_>100</m_d1_>

</rb0>

<rb1 class_id_reference="1" object_id="_1">

          <polymorphic_base></polymorphic_base>

          <m_d1_>10</m_d1_>

</rb1>

<rb2 class_id="2" class_name="polymorphic_derived2" tracking_level="1"
version="0" object_id="_2">

          <polymorphic_derived1 object_id="_3">

                   <polymorphic_base></polymorphic_base>

                   <m_d1_>20</m_d1_>

          </polymorphic_derived1>

          <m_d2_>10</m_d2_>

</rb2>

</boost_serialization>

- for polymorphic_text_[io]archive

22 serialization::archive 7 1 20 polymorphic_derived1 1 0

0 0 0 100 1

1 10 2 20 polymorphic_derived2 1 0

2

3 20 10

==================================================

Polymorphic serialization through a pointer to polymorphic_derived1 (i.e.
the class in the middle of the hierarchy) FAILS WITH THE SAME EXCEPTION
thrown about multiple registration (see the beginning of this thread).

==================================================

What is worrisome though, isomorphic serialization of polymorphic_derived1
as an object followed by polymorphic serialization through a pointer to the
base class FAILS WITH ACCESS VIOLATION:

// Save code -----------------------------

    polymorphic_derived1 rb0(100);

    polymorphic_base *rb1 = new polymorphic_derived1(10);

    polymorphic_base *rb2 = new polymorphic_derived2(10,20);

    oa << BOOST_SERIALIZATION_NVP(rb0);

    oa << BOOST_SERIALIZATION_NVP(rb1);

    oa << BOOST_SERIALIZATION_NVP(rb2);

// Load code -----------------------------

    polymorphic_derived1 rb0;

    polymorphic_base *rb1 = NULL;

    polymorphic_base *rb2 = NULL;

    ia >> BOOST_SERIALIZATION_NVP(rb0);

    ia >> BOOST_SERIALIZATION_NVP(rb1);

    ia >> BOOST_SERIALIZATION_NVP(rb2);

The code above fails in basic_iarchive.cpp(line 469) with access violation
(bpis is 0).

    if(! tracking){

        bpis_ptr->load_object_ptr(ar, t, co.file_version);

    }

The serialized content is presented below:

- using polymorphic_xml_[io]archive

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<!DOCTYPE boost_serialization>

<boost_serialization signature="serialization::archive" version="7">

<rb0 class_id="0" tracking_level="0" version="0">

          <polymorphic_base class_id="1" tracking_level="0"
version="0"></polymorphic_base>

          <m_d1_>100</m_d1_>

</rb0>

<rb1 class_id_reference="0" object_id="_0">

          <polymorphic_base></polymorphic_base>

          <m_d1_>10</m_d1_>

</rb1>

<rb2 class_id="2" class_name="polymorphic_derived2" tracking_level="1"
version="0" object_id="_1">

          <polymorphic_derived1 object_id="_2">

                   <polymorphic_base></polymorphic_base>

                   <m_d1_>20</m_d1_>

          </polymorphic_derived1>

          <m_d2_>10</m_d2_>

</rb2>

</boost_serialization>

- using polymorphic_text_[io]archive

22 serialization::archive 7 0 0 0 0 100 0

0 10 2 20 polymorphic_derived2 1 0

1

2 20 10

==================================================

Isomorphic serialization of a class not related to the hierarchy as an
object followed by polymorphic serialization through a pointer to the base
class WORKS AS EXPECTED.

==================================================

I am looking forward to hearing your conclusions.

Thank you,

Bogdan