Re: [boost] library dependencies

From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2007-05-04 16:37:02

> Emil Dotchevski wrote:
>
>>> Emil Dotchevski wrote:
>>>>  If X is usable without serialization, users shouldn't be forced to 
>>>> also
>>>>> link to the serialization library, just because of this dependency.
>>>>> (In the particular case I have in mind this dependency was controlled
>>>>> by a preprocessor macro. That's not very practical, since packagers
>>>>> surely won't provide two sets of packages for X, one with and one
>>>>> without
>>>>> this dependency.)
>>>>>
>>>>> Thus, I'd suggest to encapsulate the X-serialization functionality 
>>>>> into
>>>>> a separate library (may be header-only), such as X_serialization.hpp
>>>>> etc. Then I can still use X stand-alone, and drag in the rest whenever
>>>>> I need it.
>>>>
>>>> This has been discussed before. You don't need X_serialization.hpp, if
>>>> you
>>>> don't use BOOST_CLASS_EXPORT. See
>>>> http://www.archivesat.com/Boost_developers/thread2900871.htm.
>>>
>>> I'm not sure how relevant that is. If X.hpp contains
>>> serialization-related code, it surely needs to include serialization
>>> header files, too, to drag in any relevant declarations. Thus there is a
>>> dependency from X to serialization.
>>
>> You need no declarations because the serialization is implemented as a
>> function template:
>>
>> class foo;
>>
>> template<class Archive>
>> void serialize(Archive & ar, foo & x, const unsigned int version)
>> {
>>     ar & x.bar;
>>     ....
>> }
>>
>> The above code compiles without including any boost serialization 
>> headers.
>>
>> I have not used boost serialization (I have my own serialization 
>> library),
>> but as far as I can see the reason why foo.hpp that uses boost
>> serailization needs to include serialization headers, is to be able to 
>> use
>> BOOST_CLASS_EXPORT.
>
> If you serialize classes with base classes, you need to include
> base_object.hpp, and if you want your serialization to be
> compatible with XML archives, you have to include nvp.hpp,
> so it's not possible to completely avoid including serialization headers.
>
>> For me this is good enough reason not to use
>> BOOST_CLASS_EXPORT.
>
> If you're about to serialize derived classes via
> pointers to base classes, BOOST_CLASS_EXPORT is pretty much a must.

BOOST_CLASS_EXPORT is designed to make the necessary registrations 
automatic, but in doing so it introduces physical coupling that is 
undesirable for anyone who includes a particular class' header but does not 
need to serialize objects of that class.

I also think that having BOOST_CLASS_EXPORT deduce the persistent class name 
automatically inhibits maintainability, as any refactoring that changes 
class names would break all serialized data. I prefer having one place in my 
program where I register all classes with their persistent names, so I can 
make separate decisions when to rename classes and when to change persistent 
names.

Emil Dotchevski