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??