docs: project documentation updated with the latest unit-related changes
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Mateusz Pusz committed
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@@ -22,7 +22,7 @@ Defining a New Unit
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My working desk is of ``180 cm x 60 cm`` which gives an area of ``0.3 m²``. I would like to
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make it a unit of area for my project::
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struct desk : named_scaled_unit<desk, "desk", no_prefix, ratio(3, 10), si::square_metre> {};
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struct desk : named_scaled_unit<desk, "desk", ratio(3, 10), si::square_metre> {};
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With the above I can define a quantity with the area of ``2 desks``::
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@@ -56,28 +56,22 @@ In case I would like to check how much area ``6 desks`` take in SI units::
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Enabling a Unit for Prefixing
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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In case I decide it is reasonable to express my desks with SI prefixes the only thing I have
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to change in the above code is to replace `no_prefix` with `isq::si::prefix`::
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struct desk : named_scaled_unit<desk, "desk", si::prefix, ratio(3, 10), si::square_metre> {};
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Now I can define a new unit named ``kilodesk``::
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In case I decide it is reasonable to express my desks with SI prefixes, I can define
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a new unit named ``kilodesk``::
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struct kilodesk : prefixed_unit<kilodesk, si::kilo, desk> {};
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static_assert(3_d * 1000 == si::area<kilodesk>(3));
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But maybe SI prefixes are not good for me. Maybe I always pack ``6`` desks into one package
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for shipment and ``40`` such packages fit into my lorry. To express this with prefixes a new
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prefix family and prefixes are needed::
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for shipment and ``40`` such packages fit into my lorry. To express this new prefix definitions
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are needed::
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struct shipping_prefix : prefix_family {};
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struct package : prefix<package, shipping_prefix, "pkg", ratio(6)> {};
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struct lorry : prefix<lorry, shipping_prefix, "lorry", ratio(6 * 40)> {};
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struct package : prefix<package, "pkg", ratio(6)> {};
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struct lorry : prefix<lorry, "lorry", ratio(6 * 40)> {};
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Now we can use it for our unit::
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struct desk : named_scaled_unit<desk, "desk", shipping_prefix, ratio(3, 10), si::square_metre> {};
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struct desk : named_scaled_unit<desk, "desk", ratio(3, 10), si::square_metre> {};
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struct packagedesk : prefixed_unit<packagedesk, package, desk> {};
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struct lorrydesk : prefixed_unit<lorrydesk, lorry, desk> {};
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@@ -111,14 +105,14 @@ rate of wood during production I need to define a new derived dimension together
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coherent unit::
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// coherent unit must apply to the system rules (in this case SI)
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struct square_metre_per_second : unit<square_metre_per_second> {};
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struct square_metre_per_second : derived_unit<square_metre_per_second> {};
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// new derived dimensions
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struct dim_desk_rate : derived_dimension<dim_desk_rate, square_metre_per_second,
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exponent<si::dim_area, 1>, exponent<si::dim_time, -1>> {};
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// our unit of interest for a new derived dimension
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struct desk_per_hour : derived_unit<desk_per_hour, dim_desk_rate, desk, si::hour> {};
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struct desk_per_hour : derived_scaled_unit<desk_per_hour, dim_desk_rate, desk, si::hour> {};
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// a quantity of our dimension
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template<UnitOf<dim_desk_rate> U, Representation Rep = double>
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@@ -147,7 +141,7 @@ a customer's office I would need a unit called ``person_per_desk`` of a new deri
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dimension. However, our library does not know what a ``person`` is. For this I need to
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define a new base dimension, its units, quantity helper, concept, and UDLs::
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struct person : named_unit<person, "person", no_prefix> {};
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struct person : named_unit<person, "person"> {};
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struct dim_people : base_dimension<"people", person> {};
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template<UnitOf<dim_people> U, Representation Rep = double>
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@@ -162,12 +156,12 @@ define a new base dimension, its units, quantity helper, concept, and UDLs::
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With the above we can now define a new derived dimension::
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struct person_per_square_metre : unit<person_per_square_metre> {};
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struct person_per_square_metre : derived_unit<person_per_square_metre> {};
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struct dim_occupancy_rate : derived_dimension<dim_occupancy_rate, person_per_square_metre,
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exponent<dim_people, 1>,
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exponent<si::dim_area, -1>> {};
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struct person_per_desk : derived_unit<person_per_desk, dim_occupancy_rate, person, desk> {};
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struct person_per_desk : derived_scaled_unit<person_per_desk, dim_occupancy_rate, person, desk> {};
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template<UnitOf<dim_occupancy_rate> U, Representation Rep = double>
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using occupancy_rate = quantity<dim_occupancy_rate, U, Rep>;
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@@ -211,8 +205,8 @@ Such units do not share their references with base units of other systems:
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namespace fps {
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struct foot : named_unit<foot, "ft", no_prefix> {};
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struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio(3), foot> {};
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struct foot : named_unit<foot, "ft"> {};
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struct yard : named_scaled_unit<yard, "yd", ratio(3), foot> {};
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struct dim_length : base_dimension<"L", foot> {};
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@@ -231,7 +225,7 @@ different systems:
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namespace si {
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struct metre : named_unit<metre, "m", units::isq::si::prefix> {};
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struct metre : named_unit<metre, "m"> {};
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struct kilometre : prefixed_unit<kilometre, units::isq::si::kilo, metre> {};
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struct dim_length : base_dimension<"L", metre> {};
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@@ -241,8 +235,8 @@ different systems:
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namespace fps {
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struct foot : named_scaled_unit<foot, "ft", no_prefix, ratio(3'048, 1'000, -1), metre> {};
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struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio(3), foot> {};
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struct foot : named_scaled_unit<foot, "ft", ratio(3'048, 1'000, -1), metre> {};
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struct yard : named_scaled_unit<yard, "yd", ratio(3), foot> {};
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struct dim_length : base_dimension<"L", foot> {};
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@@ -300,7 +294,7 @@ the library always keeps the ratio relative to the primary reference unit
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which in this case is ``si::metre``. This results in much bigger ratios
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and in case of some units may result with a problem of limited resolution
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of ``std::int64_t`` used to store numerator, denominator, and exponent
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values of ratio. For example the ``si::fps::qubic_foot`` already has the
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values of ratio. For example the ``si::fps::cubic_foot`` already has the
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ratio of ``ratio{55306341, 1953125, -3}``. In case of more complicated
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conversion ratio we can overflow `ratio` and get a compile-time error.
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In such a situation the standalone system may be a better choice here.
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