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Subject: Re: [boost] [Review Request] Multiprecision Arithmetic Library
From: Neal Becker (ndbecker2_at_[hidden])
Date: 2012-04-03 07:13:21

Vicente J. Botet Escriba wrote:

> Le 02/04/12 10:53, John Maddock a écrit :
>>> I'm wondering if the following compiles and what is the type of c
>>> mp_uint128_t a;
>>> mp_uint256_t b;
>>> auto c = a + b;
>>> From the documentation I have the impression that we can not mix
>>> back-ends.
>> Correct, it's a compiler error.
>> Allowing mixed type arithmetic opens up a whole can of worms it seems
>> to me, plus I don't even want to think about how complex the operator
>> overloads would have to be to support that!
>> So for now, if you want to conduct mixed arithmetic, you must
>> explicitly cast one of the types.
> Humm, casting seems to be not the good option for fixed point
> arithmetic. The type of a fixed_point operation depends on the
> quantization of its arguments.
> With fixed_point arithmetic the type of fixed_point<7,0> +
> fixed_point<8,0> is fixed_point<9,0>. Note that no overflow is needed as
> the resulting type has enough range.
> Another example
> fixed_point<10,2> a;
> fixed_point<2,3> b;
> fixed_point<2,2> c;
> auto d = (a + b) * c;
I have implemented my own fixed_point, and I debated whether the result of
operations should automatically be a wider type. In the end, I decided it was
best to use explicit casting. Explicit is better than implicit. It's a bit
more verbose, but I can live with that.

Would you advocate that in C++, int8 * int8 -> int16??

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