xo-unit: compile-time constexpr unit stalking horse [wip]

include/xo/unit/quantity.hpp

@@ -0,0 +1,53 @@
+/** @file quantity.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "natural_unit.hpp"
+#include "scaled_unit.hpp"
+
+namespace xo {
+    namespace qty {
+        /** @class quantity
+         *  @brief represent a scalar quantity with associated units.
+         *
+         *  Enforce dimensional consistency at compile time.
+         *  sizeof(quantity) == sizeof(Repr).
+         **/
+        template <
+            typename Repr = double,
+            typename Int = std::int64_t,
+            natural_unit<Int> NaturalUnit = natural_unit<Int>(),
+            typename Int2x = detail::width2x<Int>
+            >
+        class quantity {
+        public:
+            using repr_type = Repr;
+            using unit_type = natural_unit<Int>;
+            using ratio_int_type = Int;
+            using ratio_int2x_type = Int2x;
+
+        public:
+            constexpr quantity() : scale_{0} {}
+            explicit constexpr quantity(Repr scale) : scale_{scale} {}
+
+            constexpr const repr_type & scale() const { return scale_; }
+            constexpr const unit_type & unit() const { return s_unit; }
+
+            constexpr nu_abbrev_type abbrev() const { return s_unit.abbrev(); }
+
+        public: /* need public members so that a quantity instance can be a non-type template parameter (is a structural type) */
+            static constexpr natural_unit<Int> s_unit = NaturalUnit;
+
+            Repr scale_ = Repr{};
+        };
+
+        namespace qty {
+            inline constexpr auto grams(double x) { return quantity<double, std::int64_t, nu::gram>(x); }
+        }
+    } /*namespace qty*/
+} /*namespace xo*/
+
+/** end quantity.hpp **/