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 **/

utest/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SELF_EXE utest.unit)
 set(SELF_SRCS
     unit_utest_main.cpp  #mpl_unit.test.cpp
     Quantity.test.cpp
+    quantity.test.cpp
     bpu.test.cpp
     basis_unit.test.cpp
     natural_unit.test.cpp

utest/quantity.test.cpp

@@ -0,0 +1,30 @@
+/* @file quantity.test.cpp */
+
+#include "xo/unit/quantity.hpp"
+#include "xo/indentlog/scope.hpp"
+#include <catch2/catch.hpp>
+
+namespace xo {
+    namespace qty {
+        TEST_CASE("quantity", "[quantity]") {
+            constexpr bool c_debug_flag = true;
+
+            // can get bits from /dev/random by uncommenting the 2nd line below
+            //uint64_t seed = xxx;
+            //rng::Seed<xoshio256ss> seed;
+
+            //auto rng = xo::rng::xoshiro256ss(seed);
+
+            scope log(XO_DEBUG2(c_debug_flag, "TEST_CASE.quantity"));
+            //log && log("(A)", xtag("foo", foo));
+
+            constexpr auto g = qty::grams(1.0);
+
+            static_assert(g.scale() == 1.0);
+
+            log && log(xtag("g.abbrev", g.abbrev()));
+        } /*TEST_CASE(quantity)*/
+    } /*namespace qty*/
+} /*namespace xo*/
+
+/* end quantity.test.cpp */