Subject: Re: [Boost-users] [serialization] Polymorphic serialization withpolymorphic archives problem...
From: Bogdan (bogdan.indy_at_[hidden])
Date: 2010-02-24 20:16:35
Robert Ramey <ramey <at> rrsd.com> writes:
>
> Martin Lederhilger wrote:
> > Hello Robert, hello Bogdan,
> >
> > I have quite the same problem (Exception multiple_code_instantiation
> > thrown in basic_serializer_map.cpp at line 48) as Bogdan. It can be
> > reproduced by changing
> > the pointer type from polymorphic_base to polymorphic_derived2 in the
> > original test_dll_exported.cpp example, which comes with the library.
> > I think the problem is that when oa << BOOST_SERIALIZATION_NVP(rb2);
> > is called, that the system registers an type in oserializer.hpp, which
> > leads to creation of the singletons also in the executeable.
>
> Ahhhh - a very useful hint. So the rule would be that if a class
> is polymorphic is to be serialized through a pointer, it should ONLY
> be done through a base class pointer? That is, if ALL polymorphic
> base classes are abstract - this problem will never occur?
>
> I'll have to think about this.
>
> > I have this problem when serializing a shared_ptr<A> in class B in
> > B.dll to an object of class A in A.dll.
> >
> > I tried to simply comment out this check, but it seems that I get
> > problems with tracking later. I have a class hierarchy like this:
> > C derives from B derives from A. A has a weak_ptr<A> to itself
> > (something like boost::enable_shared_from_this). If I serialize an
> > object of type C via a base pointer of type B the serialization walks
> > like this: C::serialize, B::serialize, A::serialize, and again
> > C::serialize (with a wrong this pointer - and it should not do that).
>
> Hmmm - why should it not do that? You may have a cycle - but
> the library handles that. Seems unrelated to the other problem.
>
> > Maybe my second problem depends on the first one. I hope that my
> > report can be of help.
> >
> > Thanks in advance for your answers,
>
> And thanks for your useful information.
>
> > Martin Lederhilger
>
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