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53 lines
1.6 KiB
ReStructuredText
53 lines
1.6 KiB
ReStructuredText
.. namespace:: units
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quantity_kind
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=============
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The ``quantity_kind`` class template provides a similar interface to ``quantity``.
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Kinds
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-----
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The first template parameter of ``quantity_kind`` is a ``Kind``.
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``Kind`` s, by themselves:
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* Wrap a ``Dimension``, and
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* opt into the downcasting facility.
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If a dimensional analysis operator on a quantity of kind *A*
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can result on a quantity of kind *B*,
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*A* and *B* are *related*.
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The library provides two ``Kind`` bases, ``kind`` and ``derived_kind``.
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``kind`` is an entry point, and there's only one per set of related kinds.
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``derived_kind`` is used to explicitly name other related kinds.
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Unnamed, they look like ``detail::_kind_base<the-entry-kind, some_dimension>``.
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Unnamed kinds, like unnamed units and dimensions,
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allows (intermediate) results in formulas without having to name them.
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Quantity kinds
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--------------
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``quantity_kind`` wraps a ``quantity`` and layers over its dimensional analysis.
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While all properties of the ``quantity`` apply transparently,
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results are always ``quantity_kind`` s related to the kind(s) of the argument(s).
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One of the arguments to the dimensional analysis operators can also be ``Quantity``.
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The way back to ``quantity`` is through the ``.common()`` observer.
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Intra-kind operations
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---------------------
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``quantity_kind`` takes care of operators between related quantity kinds.
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Intra-kind operations can be opted into as follows::
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struct width : kind<width, dim_length> {};
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struct height : kind<height, dim_length> {};
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size2d operator+(QuantityKindOf<width> auto, QuantityKindOf<height> auto);
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