[Boost-users] Re: Guide to using serialization macro BOOST_CLASS_EXPORT

From: Robert Ramey (ramey_at_[hidden])
Date: 2005-01-07 13:58:13

Bill Lear wrote:
> I am having some trouble understanding just where I am supposed to
> put BOOST_CLASS_EXPORT macros for classes I write --- in class
> headers (.h), in class implementation files (.cc), in applications
> that plan to save/load through base pointers?
>

The whole concept behind BOOST_CLASS_EXPORT has been problematic for me for
a couple of reasons.  As a result, the confusion you cite is a real one.

My intention was that:

a) BOOST_CLASS_EXPORT be part of the header file for a particular class.
b) A typical program would look like:

#include <boost/archive/text_oarchive.hpp>
// other archives used.

#inlcude "my_class.hpp"
// other classes from my library. many of which will use EXPORT

main(){
    ...
    // the BOOST_CLASS_EXPORT will guarentee that code required for
serialization for derived classes is
    // included in the program even though it is not explicitly referred to
by name.  Without
    // BOOST_CLASS_EXPORT, a smart compiler/linker will conclude this code
is never called
    // and throw it away on linking.
    ...
}

c) BOOST_CLASS_EXPORT emits code for each archive template for the class
specified.  This could contribute to code bloat

d) This system requires that the #include for archives preceed those for
serialization implemenations.  A violation of this requirement should result
in a compile time failure with and error message and point to code with a
comment explaining the problem.

I've toyed with re-implementing this to get around problems created by
two-phase lookup but managed to address these in another way so I don't
expect this explanation to change in the near future.

> I have base class B, from which I derive lots of classes.  I tried
> putting the BOOST_CLASS_EXPORT in the header files of each class, but
> soon found that this resulted in link-time errors due to multiple
> definitions.

I'll look into this. It might be fixable.  It was my intention to permit
this.

Note that including BOOST_CLASS_EXPORT to programs which don't in fact
serialize through a base class pointer may instanciate code that is in fact
never called.  I'll look into this as well.

Robert Ramey