forked from mpusz/mp-units
Mateusz Pusz
9527b39005
We had some fun exploring the STD UDLs for potential collisions, we have learnt our lesson and know how to proceed. Now is high time to start behaving and obeying C++ rules.
135 lines
5.4 KiB
ReStructuredText
135 lines
5.4 KiB
ReStructuredText
.. namespace:: units
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FAQ
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===
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.. contents:: Questions:
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:local:
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General
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-------
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Why dimensions depend on units and not vice versa?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Most of the libraries define units in terms of dimensions and this was also an
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initial approach for this library. However, it turns out that for such a design
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it is hard to provide support for all the required scenarios.
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The first of them is to support multiple unit systems (like SI, CGS, ...) where
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each of can have a different base unit for the same dimension. Base quantity of
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dimension length in SI should use ``m`` to print the unit symbol to the text
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output, while the same dimension for CGS should use ``cm``. Also, it helps in
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conversions among those systems.
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The second one is to support natural units where more than one dimension can be
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measured with the same unit (i.e. ``GeV``). Also if someone will decide to
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implement a systems where SI quantities of the same kind are expressed as
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different dimensions (i.e. height, width, and depth) all of them will just be
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measured in meters.
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Why a dimensionless quantity is not just an fundamental arithmetic type?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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In the initial design of this library the resulting type of the division of
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two quantities was their common representation type::
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static_assert(std::is_same_v<decltype(10_q_km / 5_q_km), std::int64_t>);
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The reasoning behind it was to not provide a false impression of a strong `quantity` type
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for something that looks and feels like a regular number. Also all of the mathematic
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and trigonometric functions were working fine out of the box with such representation
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types, so we did not have to rewrite ``sin()``, ``cos()``, ``exp()``, and others.
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However, the feedback we got from the production usage was that such an approach
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is really bad for generic programming. It is really hard to handle the result of
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division (or multiplication) of two quantities as it might be either a `quantity`
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or a fundamental type. If we want to raise such a result to some power we have to
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either use ``units::pow`` or ``std::pow`` depending on the resulting type. Those
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are only a few from many similar issues related to such an approach.
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This is why it was decided to go with the current approach.
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.. seealso::
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More information on the current design can be found in :ref:`Dimensionless quantities`
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chapter.
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Why do we spell ``metre`` instead of ``meter``?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Well, this is how [ISO-80000]_ defines it (British English spelling by default).
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User Defined Literals (UDLs)
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----------------------------
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Why all UDLs are prefixed with ``_q_`` instead of just using unit symbol?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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..note ..
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Every ``_q_*`` UDL will be replaced by the ``q_*`` literal when/if **mp-units**
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will become a part of the C++ Standard Library.
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Usage of only unit symbols in UDLs would be a preferred approach (less to type,
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easier to understand and maintain). However, while increasing the coverage for
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the library we learned that there are a lot unit symbols that conflict with
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built-in types or numeric extensions. A few of those are: ``F`` (farad),
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``J`` (joule), ``W`` (watt), ``K`` (kelvin), ``d`` (day),
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``l`` or ``L`` (litre), ``erg``, ``ergps``. For a while for those we used ``_``
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prefix to make the library work at all, but at some point we had to unify the
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naming and we came up with ``_q_`` prefix which results in a creation of
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quantity of a provided unit.
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Text formatting
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---------------
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Why Unicode quantity symbols are used by default instead of ASCII-only characters?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Both C++ and :term:`SI` ([ISO-80000]_) are standardized by the
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:abbr:`ISO (International Organization for Standardization)`. :term:`SI` standard
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specifies Unicode symbols as the official unit names for some quantities (i.e. ``Ω``
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symbol for the resistance quantity). As **mp-units** library
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is being proposed for standardization as a part of the C++ Standard Library we have
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to obey the rules and be consistent with ISO specifications.
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.. seealso::
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We do understand engineering reality and constraints and that is why the library
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has the option of :ref:`ASCII-only quantity symbols`.
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Compilation Errors
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------------------
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error: reference to ‘time’ is ambiguous
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Unfortunately, if `using-directives <https://en.cppreference.com/w/cpp/language/namespace#Using-directives>`_
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(i.e. ``using namespace units::physical::si``) are being used, `units::physical::si::time` will
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collide with C `time <https://en.cppreference.com/w/c/chrono/time>`_ function. In such a case the library's
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`time` function needs to be prefixed with at least one (or all) namespace names.
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.. rubric:: Footnotes:
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.. [ISO-80000] ISO 80000 or IEC 80000 is an international standard promulgated jointly
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by the :abbr:`ISO (International Organization for Standardization)` and the
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:abbr:`IEC (International Electrotechnical Commission)`. The standard introduces the
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:term:`International System of Quantities`. It is a style guide for the use of
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physical quantities and units of measurement, formulas involving them, and their
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corresponding units, in scientific and educational documents for worldwide use.
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Read more on `Wikipedia <ISO/IEC 80000 https://en.wikipedia.org/wiki/ISO/IEC_80000>`_.
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