From: Beman Dawes (bdawes_at_[hidden])
Date: 2008-05-28 18:25:51
Neil Mayhew wrote:
> On 24/05/08 05:45 AM Beman Dawes wrote:
>> Neil Mayhew wrote:
>>> The areas that I am interested in seeing more work done on are (1)
>>> making constructors "conditional" and (2) adding a bit-range access
>>> method.
>>>
>> * Should there be a way (#ifdef? separate class?) to provide an
>> interoperable version that meets C++03 POD requirements, so endian
>> objects can be put in unions? Is that what you mean by (1)?
>
> Yes, this is what I meant. I was thinking that the most elegant solution
> would be to have a base class that contains everything except the
> constructors, and a derived class that adds the constructors (I think
> this was in a post in 2006).
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. No base
classes means no arithmetic operations unless they are 100% supplied by
the class itself. No private members means the non-POD version would
have to use private inheritance and then forward all operations to the
private operations.
Messy, but I can't think of any other approach. 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.)
>
>> Do you have a proposed design for bit-range access?
>
> I was thinking of adding methods to read and write the given bits within
> the value:
>
> value_type endian::bits(std::size_t offset, std::size_t length) const;
> value_type endian::bits(std::size_t offset, std::size_t length,
> value_type value);
>
> Making these template methods with offset and length being template
> arguments might allow for more compile-time unrolling, but since these
> methods just do shifting and masking, the compiler will probably do all
> the needed optimization anyway.
>
> A better design might be to specialize Bitfield to accept endians,
> although I haven't studied Bitfield enough yet to see how well this
> would work.
>
>> What is the use case or motivation for bit-range access?
>
> A packed binary structure like the following. The numbers are lengths in
> bits, and the total length is 24 bits (ie 3 bytes):
>
> +------------+-----------+------------+
> | field_a:6 | field_b:4 | field_c:14 |
> +------------+-----------+------------+
>
> This may look contrived, but I have seen plenty of things similar to
> this in protocols and data files.
>
> As part of a bigger struct, this would be implemented using endian::bits
> as follows:
>
> struct packet
> {
> ubig24_t abc;
>
> uint_least32_t field_a() const
> {
> return abc.bits(0, 6);
> }
> uint_least32_t field_b() const
> {
> return abc.bits(6, 4);
> }
> uint_least32_t field_b() const
> {
> return abc.bits(10, 14);
> }
> uint_least32_t field_a(uint_least32_t v)
> {
> return abc.bits(0, 6, v);
> }
> uint_least32_t field_b(uint_least32_t v)
> {
> return abc.bits(6, 4, v);
> }
> uint_least32_t field_b(uint_least32_t v)
> {
> return abc.bits(10, 14, v);
> }
> };
>
> If the design used Bitfield instead, there would be one access method
> for both read and write, which would return by value a Bitfield on abc.
> However, the method would need to have both const and non-const
> versions, returning a const and non-const Bitfield respectively.
> Unfortunately, the Bitfield can't be made a part of the struct because
> it would take up space, breaking the mapping of the memory. So there
> would be an inconsistency with the syntax for accessing other (non-bit)
> fields within the structure, ie with and without parens, but I can live
> with that.
Interesting. I need to think about that some more.
Thanks,
--Beman