Re: [boost] [integer] Promotion of endian library code from vault

From: Neil Mayhew (neil_mayhew_at_[hidden])
Date: 2008-05-28 19:50:16

On 28/05/08 04:25 PM Beman Dawes wrote:
> Neil Mayhew wrote:
>> ... have a base class that contains everything except the 
>> constructors, and a derived class that adds the constructors
>
> There are some issues with the base class approach. For it to be a 
> POD, it can't have constructors, base classes, or private members.

Strictly speaking, yes, although we are only concerned about memory 
layout here, so the class doesn't have to be a true POD. The other 
concern is for the class to be able to live inside a union, and I think 
the constructors are the the only thing stopping that. (I #if'd the 
constructors and I was then able to use endian in a union.)

> It might be better to just have two separate types, endian and 
> old_endian. (old_endian isn't very clear as a name, but calling it 
> pod_endian becomes invalid the moment C++0x ships.)
>
>> The typedefs to big32_t etc. would then be #if'd to correspond to the 
>> POD or non-POD classes as desired. I think this is better than 
>> putting the #if's around the constructors themselves.
>
> Hummm... Seems too obscure. Better to have two sets of typedefs, with 
> the C++03 set prefixed by old_ (or whatever better name anyone can 
> come up with.)

I don't like "old_", at least not if it has to appear in my code! :-)

I would like my client code to remain essentially the same when I 
upgrade to a C++0x compiler. This means having the same type names, 
which is why I suggested conditional typedefs. I thought this was 
cleaner than putting the conditionals inside the endian class itself. 
However, it doesn't make a lot of difference.

Perhaps the simplest and best solution is therefore:

    class endian< ... >
      : cover_operators< ... >
    {
      public:
#if defined(CXX_0X) || !defined(ENDIANS_IN_UNIONS)
        endian() {}
        endian(T val) { ... }
#endif

--Neil