forked from mpusz/mp-units
44 lines
1.3 KiB
ReStructuredText
44 lines
1.3 KiB
ReStructuredText
.. namespace:: units
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Quantity Points
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===============
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A quantity point is an absolute quantity with respect to zero
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(which represents some origin) and is represented in the library with a
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`quantity_point` class template.
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.. _quantity-point-construction:
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Construction
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------------
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To create the quantity point object from a `quantity` we just have to pass
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the value to the `quantity_point` class template explicit constructor::
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quantity_point<si::dim_length, si::kilometre, double> d(123_q_km);
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.. note::
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As the constructor is explicit, the quantity object can be created from
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a quantity only via
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`direct initialization <https://en.cppreference.com/w/cpp/language/direct_initialization>`_.
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This is why the code below using
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`copy initialization <https://en.cppreference.com/w/cpp/language/copy_initialization>`_
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**does not compile**::
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quantity_point<si::dim_length, si::kilometre, double> d = 123_q_km; // ERROR
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Differences to quantity
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-----------------------
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Unlike `quantity`, the library provides:
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- no helper aliases for quantity points, such as ``length_point``,
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- no UDLs for quantity points,
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- no dimension-specific concepts, such as ``LengthPoint``
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(there's the dimension-agnostic `QuantityPoint`),
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- a more limited set of operations on quantity points
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(see the :ref:`Conversions and Casting` chapter)
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