Wednesday, March 24, 2021

Digest for comp.lang.c++@googlegroups.com - 2 updates in 1 topic

"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Mar 24 03:21PM +0100

On 24.03.2021 00:22, James Lothian wrote:
>     LM lm;
>     return 0;
> }
 
Looks like a Visual C++ bug. And it looks like a bug related to
parameter pack handling, and not related to (lack of) two-phase template
compilation. I think you should report it.
 
Workaround:
 
---------------------------------------------
template< int L, int M >
class Unit {};
 
template< class U1, class U2 >
struct Mul;
 
template< int L1, int L2, int R1, int R2 >
struct Mul<Unit<L1, L2>, Unit<R1, R2> >
{
using Result = Unit<L1 + R1, L2 + R2>;
};
 
using Length = Unit<1, 0>;
using Mass = Unit<0, 1>;
 
using LM = typename Mul<Length, Mass>::Result;
 
auto main() -> int
{
LM lm;
(void) lm;
}
---------------------------------------------
 
However, to my eyes the units computation doesn't seem correct. I don't
know if Boost Units does this correctly, but I think it must. So I
recommend using Boost Units instead.
 
<url: https://www.boost.org/doc/libs/1_65_0/doc/html/boost_units.html>
 
 
- Alf
James Lothian <jameslothian1@gmail.com>: Mar 24 07:25PM

Alf P. Steinbach wrote:
 
> Looks like a Visual C++ bug. And it looks like a bug related to
> parameter pack handling, and not related to (lack of) two-phase template
> compilation. I think you should report it.
 
Thank you, that's pretty much what I thought.
> Workaround:
 
This is a much distilled-down version of something a good bit
bigger. I know I can work round this by avoiding the parameter pack and
fixing the number of dimensions in a Unit, but this means that when I
add another dimension to my units, I have to modify Mul (and Div, and
Exp, and so on). I've found that the following creeping horror works:
 
template <class U1, class U2>
struct Mul
{
private:
template <int... L, int... R>
static auto mulGuts(Unit<L...>, Unit<R...>)
{
return Unit<L + R...>();
}
 
public:
typedef decltype(mulGuts(U1(), U2())) Result;
};
 
but it does creep and it is horrible.
 
> know if Boost Units does this correctly, but I think it must. So I
> recommend using Boost Units instead.
 
> <url: https://www.boost.org/doc/libs/1_65_0/doc/html/boost_units.html>
 
But where's the fun in that :-)
 
Thanks,
James
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