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Subject: Re: [boost] [1.44] Beta progress?
From: Matthias Troyer (troyer_at_[hidden])
Date: 2010-07-26 14:07:16

On 26 Jul 2010, at 12:41, Robert Ramey wrote:

> Matthias Troyer wrote:
>> On 25 Jul 2010, at 23:56, Robert Ramey wrote:
>>> Matthias Troyer wrote:
>> Yes, what is missing is the list of special types and their
>> interface. In the past they were all STRONG_TYPEDEF, and the
>> semantics of STRONG_TYPEDEF was the same as that of the underlying
>> integral type. That has changed and broken some code.
>> The basic issue is that an archive deriving from
>> detail::common_[io]archive has to support the fundamental C++ types,
>> std:: string, and that collection of special types. In order to
>> serialize those special types I need to be able to know something
>> about them! The specific list of concepts they model is less
>> important than that list being stable (as long as serialization can
>> be implemented efficiently). Besides the default constructor it would
>> be good to have a type member specifying the (current) implementation
>> type:
>> class D : public T { \
>> public: \
>> typedef T implementation_type; \
>> explicit D(const T t) : T(t){} \
>> D() : T() {} \
>> }; \
> A month or so ago I had most of these types implemented with
> STRONG_TYPEDEF. In addition to these I had a few
> implemented "by hand": tracking_type, class_name_type. So
> moving a few from the STRONG_TYPEDEF column to the
> "hand rolled" column didn't raise any red flags. I found that
> I had to add the following operations to these types:
> For T
> T:base_type // where base_type is some integer supported by binary_primitive
> and text_primitive.
> convertable to T::base_type &
> convertable to const T::base_type
> operator ==
> operator<
> This was totally ad hoc- I was just "making things work" Of course
> STRONG_TYPEDEF supports all of these except base_type which
> could easily be added to it at no cost. Soooooo we could
> define such a concept and that would solve the current
> dilema, avoid future problems, and make mpi_archives
> easier to write and more robust. In principle I see this as
> an improvement. Of course the devil is in the details.
> I see the utility of augmenting STRONG_TYPEDEF but
> I wonder about it. if you have code T:base_type and
> T is not one of the types we're using - it won't have
> this available - compiler error. Wouldn't it be better
> to specify the implicit requirements as above and just
> know that T will be converted to what one wants? and
> sizeof(T) can be applied to both the (now) more elaborate
> types as well as C++ primitives.

The only problem I see with sizeof(T) is that it cannot tell me about signedness. Otherwise I could hack it from sizeof(T)

>> and also use the implementation_type member in the other special
>> classes. That way you could change those integer types at any time
>> without causing any breakage in Boost.MPI. Right now I have to
>> manually set these types:
>> BOOST_MPI_DATATYPE(boost::archive::class_id_optional_type,
>> get_mpi_datatype(int_least16_t()), integer);
> Though it's off topic at this point, I've never understood what this macro
> is for. why do these special types need handling different than any
> other common c++ primitive types like integers. Wasn't it enough
> for these types to be convertable to integers and references to integers?

This macro specializes the get_mpi_datatype() function for the type boost::archive::class_id_optional_type and implements it to return the appropriate MPI Datatype, which here is the same as that of an int_least16_t. Do you have a way how I can class_id_optional_type to an integer, without knowing what integer type to use?


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