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A header library for calculations with units

License: MIT License

C++ 100.00%

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trass3r graydon

units-cxx14's Issues

Unit definitions can be changed, fix creates a new problem

Right now example.cpp doesn't define units as const or constexpr. This means that they themselves can't be defined in header files without some ugly tricks, and physical constants can't be either.

There is no direct technical problem with declaring them constexpr, which solves the above problem, but there is an indirect problem in it, because now to declare mutable unit variables, you'd have to write std::remove_const<decltype(_x)>::type instead of just decltype(x).

I have two ideas on how to fix this:

Create macros, something like CONST_UNIT(u) and VAR_UNIT(u), that hide decltype and std::remove_const
Don't fix it in the header and instead change the example to create typedefs like typedef std::remove_const<decltype(_V)>::type voltage, so the user then has voltage and const voltage as types.

I'm not sure about 2., it might give the illusion that you can manage different physical dimensions with the same unit with units-cxx14 (distinguish between potential and voltage [both V], or a dimensionless value and an angle [both unitless]). But I can't say I like macros either. What do you think?

degrees/radians

Currently radians are commented out: // skipping radian: rad = m / m
and degrees result in a primitive type losing type safety.

void foo(decltype(deg) a)
{}
void unitstest()
{
    foo(5 * rad);
}

Definition of Hz as 1/s doesn't work

I added it to example.cpp as a very simple testcase.

Code:

auto _Hz = 1 / _s;

g++ 4.9.2 error message:

In file included from example.cpp:26:0:
units.hpp: In instantiation of ‘constexpr typename std::enable_if<std::is_arithmetic<_Tp>::value, unitscxx::quantity<NT, N, D> >::type unitscxx::operator/(MulType, unitscxx::quantity<NT, N, D>) [with MulType = int; NT = double; N = detail::sequence<units, (units)1>; D = detail::sequence<units>; typename std::enable_if<std::is_arithmetic<_Tp>::value, unitscxx::quantity<NT, N, D> >::type = unitscxx::quantity<double, detail::sequence<units, (units)1>, detail::sequence<units> >]’:
example.cpp:43:16:   required from here
units.hpp:367:36: error: could not convert ‘unitscxx::quantity<double, detail::sequence<units>, detail::sequence<units> >((double)left).unitscxx::quantity<NumericType, Numerator, Denominator>::operator/<double, detail::sequence<units, (units)1>, detail::sequence<units> >(right)’ from ‘unitscxx::quantity<double, detail::sequence<units>, detail::sequence<units, (units)1> >’ to ‘std::enable_if<true, unitscxx::quantity<double, detail::sequence<units, (units)1>, detail::sequence<units> > >::type {aka unitscxx::quantity<double, detail::sequence<units, (units)1>, detail::sequence<units> >}’
   return unitless_quantity(left) / right;
                                    ^
units.hpp:368:2: error: body of constexpr function ‘constexpr typename std::enable_if<std::is_arithmetic<_Tp>::value, unitscxx::quantity<NT, N, D> >::type unitscxx::operator/(MulType, unitscxx::quantity<NT, N, D>) [with MulType = int; NT = double; N = detail::sequence<units, (units)1>; D = detail::sequence<units>; typename std::enable_if<std::is_arithmetic<_Tp>::value, unitscxx::quantity<NT, N, D> >::type = unitscxx::quantity<double, detail::sequence<units, (units)1>, detail::sequence<units> >]’ not a return-statement
  }
  ^

clang++ 3.5.0 error message:

In file included from example.cpp:26:
./units.hpp:367:10: error: no viable conversion from 'quantity<[...], sequence<[...], (no argument)>, sequence<[...], 1>>' to 'quantity<[...],
      sequence<[...], 1>, sequence<[...], (no argument)>>'
                return unitless_quantity(left) / right;
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
example.cpp:43:14: note: in instantiation of function template specialization 'unitscxx::operator/<int, double, detail::sequence<units, 1>,
      detail::sequence<units> >' requested here
auto _Hz = 1 / _s;
             ^
./units.hpp:228:13: note: candidate constructor not viable: no known conversion from 'unitscxx::quantity<double, detail::sequence<units>,
      detail::sequence<units, 1> >' to 'const unitscxx::quantity<double, detail::sequence<units, 1>, detail::sequence<units> > &' for 1st argument
                constexpr quantity(const quantity&) = default;
                          ^
./units.hpp:229:13: note: candidate constructor not viable: no known conversion from 'unitscxx::quantity<double, detail::sequence<units>,
      detail::sequence<units, 1> >' to 'unitscxx::quantity<double, detail::sequence<units, 1>, detail::sequence<units> > &&' for 1st argument
                constexpr quantity(quantity&&) = default;
                          ^
1 error generated.

Also, for everything else to work with g++ 4.9, I first had to make a small change to the header file (see the second revision of my gist). Anyway, I don't really understand what this error message says.

Should we allow operators to work with quantities of different NumericType's?

This is already working for the overloads of * and / for two quantities, but + and - for two quantities, as well as pretty much every operator with a quantity on one side and a primitive numeric type on the other requires equal numeric types.

If you don't see a problem with generalizing these functions to everything where std::is_arithmetic<NT>::value is true, I could make a PR.

units.hpp:38:20: error: ‘size_t’ does not name a type

Pretty self-explanatory, simply a missing include of <cstddef>.

Also, ~~two~~ three other small things:

Is there a reason you don't have include guards? I guess you don't strictly need them, as your code is 100% templates, but it still seems to be reasonable to have them.
Is there a reason the quantity constructors and const member functions aren't constexpr? It's the same thing for the non-member operator* and operator/.
Why are quantity& operator+=(quantity that) and quantity& operator-=(quantity that) const?

Here's an updated version (I don't like PRs, just create your own commits, you can select individual changes using -p / --patch with git add): https://gist.github.com/jplatte/96cf29cba575d6e8de53

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