refactor: conversion_factor example moved to the main examples directory

example/conversion_factor.cpp

@@ -0,0 +1,60 @@
+/*
+ Copyright (c) 2003-2020 Andy Little.
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see http://www.gnu.org/licenses./
+*/
+
+#include <units/format.h>
+#include <units/isq/si/length.h>
+#include <iostream>
+#include <type_traits>
+
+/*
+  get conversion factor from one dimensionally equivalent
+  quantity type to another
+*/
+
+namespace {
+
+template<units::Quantity Target, units::Quantity Source>
+  requires units::equivalent<typename Source::dimension, typename Target::dimension>
+inline constexpr std::common_type_t<typename Target::rep, typename Source::rep> conversion_factor(Target, Source)
+{
+  // get quantities looking like inputs but with Q::rep that doesn't have narrowing conversion
+  typedef std::common_type_t<typename Target::rep, typename Source::rep> rep;
+  typedef units::quantity<typename Source::dimension, typename Source::unit, rep> source;
+  typedef units::quantity<typename Target::dimension, typename Target::unit, rep> target;
+  return target{source{1}}.number();
+}
+
+}  // namespace
+
+int main()
+{
+  using namespace units::isq::si;
+
+  std::cout << "conversion factor in mp-units...\n\n";
+
+  constexpr length<metre> lengthA(2.0);
+  constexpr length<millimetre> lengthB = lengthA;
+
+  std::cout << fmt::format("lengthA( {} ) and lengthB( {} )\n", lengthA, lengthB)
+            << "represent the same length in different units.\n\n";
+
+  std::cout << fmt::format("therefore ratio lengthA / lengthB == {}\n\n", lengthA / lengthB);
+
+  std::cout << fmt::format("conversion factor from lengthA::unit of {:%q} to lengthB::unit of {:%q}:\n\n", lengthA, lengthB)
+            << fmt::format("lengthB.number( {} ) == lengthA.number( {} ) * conversion_factor( {} )\n",
+                           lengthB.number(), lengthA.number(), conversion_factor(lengthB, lengthA));
+}