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

Subject: Re: [Boost-users] Needs advices on design ( mpl or processor )
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-01-07 18:28:25

On 01/07/12 02:42, Allan Nielsen wrote:
> On Sat, Jan 7, 2012 at 1:54 AM, Larry Evans <cppljevans_at_[hidden]> wrote:
>> On 01/06/12 16:19, Allan Nielsen wrote:
>>> My problem is not to calculate the actually size, but move the
>>> calculation from the instantiation into the definition of the library.
>> I have not idea what this means.
>> Please clarify.
> 
> First of all, thanks for your input.
> 
> I will try to illustrate my design problem in a more simple example:
> 
> struct TagA {
>    enum Size { size = 4 };
>    enum E { A0  =  0, A1  =  1, A2  =  2, A3  =  3, };
> };
> 
> struct TagB {
>    enum Size { size = 4 };
>    enum E { B0  =  0, B1  =  1, B2  =  2, B3  =  3, };
> };
> 
> struct TagC {
>    enum Size { size = 4 };
>    enum E { C0  =  0, C1  =  1, C2  =  2, C3  =  3, };
> };
> 
> 
> // Simple edition of CompressedEnum, but has the same problem

Sorry, what was that problem again?

> template<typename T, int OFFSET>
> struct CompressedEnum {
>     enum O { offset = OFFSET };
>     enum S { size = T::size };
>     T get( ) { ... }
> };
> 
> // Simple edition of CompressedEnums, but has the same problem
> template < typename B0, typename B1, typename B2 >
> struct CompressedEnums: public B0, B1, B2 {
>     template < typename T> get() { ... }
> };
> 
> void example1()
> {
>     // As you see, the expresions for calculating the offset get quite long,
>     // and are not easy to maintain.

Here's where I don't understand why the calculation are done outside
the temple and then passed as args to the template.
Why not have a template do the calculations and accumulate the results,
somewhat like mpl::fold or the code I posted earlier?

>     C< CompressedEnum<tagA, 1>,
> 
>        CompressedEnum<tagB, CompressedEnum<tagA, 1>::offset *
>                             CompressedEnum<tagA, 1>::size >,
> 
>        CompressedEnum<tagC,
> 
>             CompressedEnum<tagB, CompressedEnum<tagA, 1>::offset *
>                                  CompressedEnum<tagA, 1>::size >::offset
>             *
>             CompressedEnum<tagB, CompressedEnum<tagA, 1>::offset *
>                                  CompressedEnum<tagA, 1>::size >::size
>        >
>     > c1;
> 
> 
> }
> 
[snip]

-regards,
Larry