Initial constants support added

src/include/units/constant.h

@@ -0,0 +1,46 @@
+
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/quantity.h>
+
+
+namespace units {
+
+namespace detail {
+
+template<Ratio R, Exponent... Es>
+struct constant_helper {
+  using dim = downcast_dimension<typename detail::make_dimension<Es...>>;
+  using ratio = ratio_multiply<R, typename dimension_unit<dim>::ratio>;
+  using unit = downcast_unit<dim, R>;
+  static constexpr quantity<dim, unit> value{1};
+};
+
+}
+
+template<Ratio R, Exponent E, Exponent... Rest>
+inline constexpr auto constant = detail::constant_helper<R, E, Rest...>::value;
+
+}  // namespace units