Re: [Boost-users] Needs advices on design ( mpl or processor )

Subject: Re: [Boost-users] Needs advices on design ( mpl or processor )
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-01-07 03:53:24

Le 06/01/12 17:19, Allan Nielsen a écrit :
> Hi
>
> This is not a specific questions directly related to Boost::mpl, but a request
> for help if somebody has the time...
>
> For the last couple of month I have been trying to learn to use the boost::mpl
> library.
>
> To learn to use the library I decided to use it for creating a
> "compressed-enum" library. The library should make it possible to store several
> enums in a single variable (for instance an unsigned int)
>
Hi,

I have no see yet your implementation and how I can help you in, but I 
guess that you can find your own responses in the implementation of this 
library

https://svn.boost.org/svn/boost/sandbox/SOC/2010/bit_masks/lib/integer/doc/html/index.html

In addition to what you want to do, I think that all this stuff can be 
generalized to a compressed_tuple.

About compressed_tuple.

Tuples of types can be compressed depending on the bits needed to store 
the underlying type of each one of the tuple elements. For example a 
compressed_tuple<month,day,weekday> would take 3 bytes, but month needs 
only 5 bits, day 3 bits and weekday 3 bits, that is 11 bits which can be 
represented using just 2 bytes.

This is quite close to the bitfield library. The main difference is that 
bitfield worked only with built-in types and required to state 
explicitly the number of bits for each field while compressed_tuple can 
work with UDT for which the user has stated once for all the number of 
bits needed to store the UDT.

In addition, the UDT need to DefaultConstructible  be 
ExplicitlyConstructible from its underlying type and be 
ExplicitlyConvertible to it's underlying type.

There is yet another difference. The motivation of the bitfield library 
was to make possible to work with bitfields in a portable way (endianness).
The motivation of the compressed_tuple, is to compress UDT which cannot 
be used with C-bitfields.

The compressed_tuple_traits template needs to be specialized for each 
UDT and define the width_in_bits, underlying_type ...

As the bitfield library, the total number of needed bits cannot exceed 
64 bits.

This should work also for  ordinal types. An ordinal type is a type that 
allows to get the value from an index (0..n) and retrieve the associated 
position of a value  (See 
https://svn.boost.org/svn/boost/sandbox/enums/libs/enums/doc/html/index.html 
for more details - section Tutorial/Ordinal Enums).

Let me know if you are interested.

Best,
Vicente