Vladimir Prus wrote:
Robert Ramey wrote:
Upon (re) reading this, I'm not really sure what is being suggested here. That's why I didn't respond to the original post. I think my view that there is no problem probably reflects my lack of understanding of the original question. Anyway - here it is.
Robert Ramey
Why not make include polymorphic_archive automatic? And make other archive types resort to polymorphic_archive when no serialization for that archive type was BOOST_CLASS_EXPORTED. As you say, the only cost is a "teensy bit of execution time", in most cases users won't even notice, but the code size will be reduced.
I am generally not in favor of adding code not requested by the library user and not required. I much prefer that the library reflect what the user has said he wants. So my view is there are a couple of options.
a) if one want's to use a polymorphic archive, include that in a (possibly precompiled) module.
What I'm suggesting is that if no export code is registered for current arhive/class combination, the libraries tries polymorphic_archive/class combination.
That at least assume that if I create a library the registers class with the polymorphic_archive, it will always work, even if client application does not explicitly use polymorphic_archive
In order for the polymorphic_archive to work, the client application MUST use a polymorphic implementation. E.G. polymorphic_text_iarchive. Only the polymorphic implementations can link to the polymorphic_?archive headers and use the corresponding code. So if one want's to be sure that things will "always" work there is only one method that I can see. a) use polymorphic_?archive in pimple like headers for all classes. b) insist that all users use polymorphic_????_?archive headers. This will work even if newer polymorphic_????_?archive types are created and used.
b) if one want's to use the template archive implementations, specify the ones you want.
Let me ask a simple question: if I do
#include <boost/archive/text_iarchive.hpp> #include <boost/archive/binary_iarchive.hpp> BOOST_CLASS_EXPORT(MyClass);
what's exactly get instantiated for two archive types?
You'll get code generated for each type of archive.
If that's pretty much all of serialization library, then it's very much of a code bloat.
So it is.
Now, if BOOST_CLASS_EXPORT registers with polymorphic archive by default, I can omit the #includes, and I'll get instantiations only for one type, plus some small forwarding function in polymorphic archive. The price is a small performance reduction but: - it's very small compared to iostreams overhead - it's not clear if there will be any -- smaller code size might even increase performance due to less cache misses.
For those that hold this view, the polymorphic_?archive was invented so the above becomes:
#include <boost/archive/polymorphic_iarchive.hpp> BOOST_CLASS_EXPORT(MyClass);
and it achieves exactly the benefits you describe above. This no surprise as it was invented for just this purpose. On the other hand, there are users which strongly argue that compile time instantiation can/will lead to faster code and code bloat is not a problem in their context. In theory, this should be true. But only recently have some people started to investigate whether it is in fact true with today's compilers.
c) if this generates too much code bloat, make a library which includes any or all of the above. Its fairly easy to do this. Then link against your library of serialization implementations. Only ones actually used will be included in your program.
You're making some assumptions that are true only for certain types of libraries built on certain platform with certain compiler.
Hmm - I suppose I am. For what compilers/platforms/libraries would the above not be true? Robert Ramey