2020-03-09 18:55:41 +01:00
|
|
|
|
FAQ
|
|
|
|
|
|
===
|
|
|
|
|
|
|
2020-03-19 10:49:09 +01:00
|
|
|
|
.. contents:: Questions:
|
|
|
|
|
|
:local:
|
|
|
|
|
|
|
|
|
|
|
|
|
2020-03-09 18:55:41 +01:00
|
|
|
|
General
|
|
|
|
|
|
-------
|
|
|
|
|
|
|
|
|
|
|
|
Why all UDLs are prefixed with ``q_`` instead of just using unit symbol?
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
|
|
|
|
|
Usage of only unit symbols in UDLs would be a preferred approach (less to type,
|
|
|
|
|
|
easier to understand and maintain). However, while increasing the coverage for
|
|
|
|
|
|
the library we learned that there are a lot unit symbols that conflict with
|
|
|
|
|
|
built-in types or numeric extensions. A few of those are: ``F`` (farad),
|
|
|
|
|
|
``J`` (joule), ``W`` (watt), ``K`` (kelvin), ``d`` (day),
|
|
|
|
|
|
``l`` or ``L`` (litre), ``erg``, ``ergps``. For a while for those we used ``_``
|
|
|
|
|
|
prefix to make the library work at all, but at some point we had to unify the
|
|
|
|
|
|
naming and we came up with ``q_`` prefix which results in a creation of
|
|
|
|
|
|
quantity of a provided unit.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Why dimensions depend on units and not vice versa?
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
|
|
|
|
|
Most of the libraries define units in terms of dimensions and this was also an
|
2020-03-19 10:49:09 +01:00
|
|
|
|
initial approach for this library. However, it turns out that for such a design
|
2020-03-09 18:55:41 +01:00
|
|
|
|
it is hard to provide support for all the required scenarios.
|
|
|
|
|
|
|
|
|
|
|
|
The first of them is to support multiple unit systems (like SI, CGS, ...) where
|
|
|
|
|
|
each of can have a different base unit for the same dimension. Base quantity of
|
2020-03-19 10:49:09 +01:00
|
|
|
|
dimension length in SI should use ``m`` to print the unit symbol to the text
|
|
|
|
|
|
output, while the same dimension for CGS should use ``cm``. Also, it helps in
|
|
|
|
|
|
conversions among those systems.
|
2020-03-09 18:55:41 +01:00
|
|
|
|
|
|
|
|
|
|
The second one is to support natural units where more than one dimension can be
|
|
|
|
|
|
measured with the same unit (i.e. ``GeV``). Also if someone will decide to
|
|
|
|
|
|
implement a systems where SI quantities of the same kind are expressed as
|
|
|
|
|
|
different dimensions (i.e. height, width, and depth) all of them will just be
|
|
|
|
|
|
measured in meters.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Why do we spell ``metre`` instead of ``meter``?
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
2020-03-19 10:49:09 +01:00
|
|
|
|
Well, this is how [ISO-80000]_ defines it (British English spelling by default).
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Text formatting
|
|
|
|
|
|
---------------
|
|
|
|
|
|
|
|
|
|
|
|
Why Unicode quantity symbols are used by default instead of ASCII-only characters?
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
|
|
|
|
|
Both C++ and :term:`SI` ([ISO-80000]_) are standardized by the
|
|
|
|
|
|
:abbr:`ISO (International Organization for Standardization)`. :term:`SI` standard
|
|
|
|
|
|
specifies Unicode symbols as the official unit names for some quantities (i.e. ``Ω``
|
|
|
|
|
|
symbol for the resistance quantity). As **mp-units** library
|
|
|
|
|
|
is being proposed for standardization as a part of the C++ Standard Library we have
|
|
|
|
|
|
to obey the rules and be consistent with ISO specifications.
|
|
|
|
|
|
|
|
|
|
|
|
.. seealso::
|
|
|
|
|
|
|
|
|
|
|
|
We do understand engineering reality and constraints and that is why the library
|
|
|
|
|
|
has the option of :ref:`ASCII-only quantity symbols`.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
.. rubric:: Footnotes:
|
|
|
|
|
|
|
|
|
|
|
|
.. [ISO-80000] ISO 80000 or IEC 80000 is an international standard promulgated jointly
|
|
|
|
|
|
by the :abbr:`ISO (International Organization for Standardization)` and the
|
|
|
|
|
|
:abbr:`IEC (International Electrotechnical Commission)`. The standard introduces the
|
|
|
|
|
|
:term:`International System of Quantities`. It is a style guide for the use of
|
|
|
|
|
|
physical quantities and units of measurement, formulas involving them, and their
|
|
|
|
|
|
corresponding units, in scientific and educational documents for worldwide use.
|
2020-05-09 13:12:35 +02:00
|
|
|
|
Read more on `Wikipedia <ISO/IEC 80000 https://en.wikipedia.org/wiki/ISO/IEC_80000>`_.
|
|
|
|
|
|
|
|
|
|
|
|
Compilation Errors
|
|
|
|
|
|
------------------
|
|
|
|
|
|
|
|
|
|
|
|
error: reference to ‘time’ is ambiguous
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
|
|
|
|
|
Unfortunately, if `using-directives <https://en.cppreference.com/w/cpp/language/namespace#Using-directives>`_
|
|
|
|
|
|
(i.e. :expr:`using namespace units::physical::si`) are being used, `units::physical::si::time` will
|
|
|
|
|
|
collide with C `time <https://en.cppreference.com/w/c/chrono/time>`_ function. In such a case the library's
|
|
|
|
|
|
`time` function needs to be prefixed with at least one (or all) namespace names.
|
|
|
|
|
|
|
|
|
|
|
|
error: template argument 1 is invalid
|
|
|
|
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
|
|
|
|
|
|
Again, usage of :expr:`using namespace units`
|
|
|
|
|
|
`using-directive <https://en.cppreference.com/w/cpp/language/namespace#Using-directives>`_ may result in
|
|
|
|
|
|
the collision between `units::exp` class template and C `exp <https://en.cppreference.com/w/c/numeric/math/exp>`_
|
|
|
|
|
|
function. In such a case the library's `exp` class template needs to be prefixed with `units` namespace name.
|
