Cannot serialize protected and private base classes

[Eric Moyer <eric_moyer@…>]

boost::serialization will not compile code that serializes a class that has non-public base classes. It complains about an inaccessible base -- despite the friend definition for serialization::access.

There is a workaround for private base classes, since those can be emulated by private member variables, but I can't think of any way to work around the protected inheritance problem.

I am using g++ 4.3.3 Running on Ubuntu 9.04

I have attached two source files that cause the problem, one minimal and one slightly less minimal (with some preprocessor macros I used to trace down the problem).

I have also included the compiler output for the minimal code and the Makefile I use to build them.

[Eric Moyer <eric_moyer@…>]

Minimal code to reproduce the problem

[Eric Moyer <eric_moyer@…>]

Compiler output when I try to compile the minimal code

[Eric Moyer <eric_moyer@…>]

Minimal code to reproduce the problem + some macros to test serializing through base pointer and public vs. private vs. protected access

[Eric Moyer <eric_moyer@…>]

Makefile I use to build the bug-reproducing code

[Eric Moyer <eric_moyer@…>]

I removed one of the classes -- making the minimal even smaller

[Eric Moyer <eric_moyer@…>]

Updated makefile including _minimal_2

[Eric Moyer <eric_moyer@…>]

Compiler output when I try to compile the smaller minimal code

[Robert Ramey]

Fixed in trunk. Should be included with 1.40

[robtoth@…]

I just tried this on gcc 4.7 and the minimal example still doesn't compile. Was this ever fixed? Is there a workaround? The error is still "boost/serialization/void_cast.hpp:191: error: Base1 is an inaccessible base of Derived".

[Robert Ramey]

Replying to robtoth@…:

I made this program compile by adding

  friend class Derived;   // make Derived a friend
...
  virtual void print() const { // add "const" here
      cout << "Derived" << endl;
  }
...
  const Derived d;  // add "const" here

The addition of "const" is pretty obvious.

The addition of "friend" is not obvious and wouldn't seem necessary. On the other hand, I looked at the code and don't see an easy way to fix this.

Robert Ramey

[robtoth@…]

Sorry but I still can't compile this. The const is fine and I understand it, but the "friend" doesn't help.

boost_serialize_protected_bug_minimal_2.cc:

...
class Base1{
friend class boost::serialization::access;
friend class Derived; // Added this
...

I still get the error:

include/boost/serialization/smart_cast.hpp:204: error: 'Base1' is an inaccessible base of 'Derived'

This is on gcc svn trunk (2013-01-22, version 4.8) and boost svn trunk (as of 2013-01-22, version 1.53.0).

Did you do something different from what I did to make Derived serialize?

I can attach the full error output if it would help.

[Robert Ramey]

serialization of private base class

[Robert Ramey]

cannot serialize through a private base class pointer

[Robert Ramey]

OK _ looked at this more carefully.

There is more going on here than meets the eye.

test_private_base.cpp demonstrates that one can serialize a private base class with no problem.

test_private_base.cpp demonstrates the problem which occurs when one tries to serialized through a pointer to a private base class. The program fails to compile at line 90. Basically one cannot do this in C++. I've searched in vain for a way to do this but haven't found any. The BOOST_CLASS_EXPORT uses the same functionality to implement this facility so it has the same problem.

Bottom line - serializatiion of private/protected base classes is fine - but one cannot serialize such classes through a base class pointer.