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Subject: Re: [boost] [Boost.FixedPoint] Choose notation
From: Vicente Botet (vicente.botet_at_[hidden])
Date: 20130429 13:18:51
Dmitriy Gorbel wrote
>
> Michael Marcin3 wrote
>> I dislike the names. I don't really understand what the first 2 types
>> have to do with fixedpoint math, other than maybe as an implementation
>> detail or degenerate fixedpoint type (0 fractional bits).
> I add cardinal and integral types to the proposal because
> this types required by C++1y proposal. I agree, this types
> quite illogical. Maybe, this types proposed for supporting big integers,
> but I think for big integers better use Boost.Multiprecision library.
>
> It's easy to implement integral types based on fractional,
> using template aliases, for example:
> template
> <size_t Range>
> using cardinal = nonnegative<Range, 0>;
> template
> <size_t Range>
> using integral = negatable<Range, 0>;
>
> What community think about this?
> Should library support integral fixed point types?
The implementation of fixed<Range, Resolution> needs to have as storage an
integer<RangeResolution>.
As we need it for the implementation and its implementation is simpler than
fixed<Range,0> why not provide it to the users.
Note that this is not an arbitrary precision integer but an integer with a
fixed precision which has a different semantics (overflow).
>
> Neal Becker wrote
>> Thank you. The last time we discussed this topic, the issue was not one
>> of
>> notation, but of different semantics that were desired by different end
>> users of
>> a fixedpoint type.
>>
>> I think there were several areas of disagreement  but perhaps primary
>> was
>> whether arithmetic operations would produce different types or the same
>> types,
>> and how to handle arithmetic between mixed types. For example, does
>> multiplication of 2 8bit quantities promote to 16 bits or not.
>>
>> My preference is/was that no implicit conversion was ever performed and
>> that it
>> always had to be explicit. This is partly because the primary consumer
>> for this
>> technology (I think) is those working on FPGA or ASIC hardware  and they
>> should
>> always be aware of any such conversions.
> Best way  conversion policy,
> when user can choose between implicitly/explicitly conversion.
> I will try to implement both variants.
No, this is not solved by implicit/explicit conversion, but using closed
arithmetic. That is
fixed<8>* fixed<8>> fixed<8>
The C++1y proposal uses open arithmetic and
fixed<8>* fixed<8>> fixed<16>
Using closed arithmetic the user must explicitly convert to the desired
result
fixed<8> a;
fixed<16> b = fixed<16>(a) * fixed<16>(a);
Best,
Vicente
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