From d9d9d9d3b74ec9ce3a584301f64758cc4ae55c24 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 21 Jun 2023 11:29:22 +0200
Subject: [PATCH] feat(example): `storage_tank` example extended with custom
 quantity types

---
 example/CMakeLists.txt   |  2 +-
 example/storage_tank.cpp | 18 ++++++++++++------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/example/CMakeLists.txt b/example/CMakeLists.txt
index 5741fc67..a06b518a 100644
--- a/example/CMakeLists.txt
+++ b/example/CMakeLists.txt
@@ -53,7 +53,7 @@ add_example(glide_computer mp-units::core-fmt mp-units::international mp-units::
 add_example(hello_units mp-units::core-fmt mp-units::core-io mp-units::si mp-units::usc)
 add_example(measurement mp-units::core-io mp-units::si)
 add_example(si_constants mp-units::core-fmt mp-units::si)
-add_example(storage_tank mp-units::core-fmt mp-units::si)
+add_example(storage_tank mp-units::core-fmt mp-units::si mp-units::utility)
 add_example(
     strong_angular_quantities mp-units::core-fmt mp-units::core-io mp-units::si mp-units::isq_angle mp-units::utility
 )
diff --git a/example/storage_tank.cpp b/example/storage_tank.cpp
index 0acddd2d..f8f4d9ee 100644
--- a/example/storage_tank.cpp
+++ b/example/storage_tank.cpp
@@ -21,6 +21,7 @@
 // SOFTWARE.
 
 #include <mp-units/format.h>
+#include <mp-units/math.h>
 #include <mp-units/systems/isq/mechanics.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/constants.h>
@@ -28,6 +29,7 @@
 #include <mp-units/systems/si/units.h>
 #include <cassert>
 #include <iostream>
+#include <numbers>
 #include <utility>
 
 template<class T>
@@ -39,6 +41,13 @@ namespace {
 using namespace mp_units;
 using namespace mp_units::si::unit_symbols;
 
+// add a custom quantity type of kind isq::length
+QUANTITY_SPEC(horizontal_length, isq::length);
+
+// add a custom derived quantity type of kind isq::area
+// with a constrained quantity equation
+QUANTITY_SPEC(horizontal_area, isq::area, horizontal_length* isq::width);
+
 inline constexpr auto g = 1 * si::standard_gravity;
 inline constexpr auto air_density = isq::mass_density(1.225 * (kg / m3));
 
@@ -80,14 +89,14 @@ public:
 class CylindricalStorageTank : public StorageTank {
 public:
   constexpr CylindricalStorageTank(const quantity<isq::radius[m]>& radius, const quantity<isq::height[m]>& height) :
-      StorageTank(std::numbers::pi * radius * radius, height)
+      StorageTank(quantity_cast<horizontal_area>(std::numbers::pi * pow<2>(radius)), height)
   {
   }
 };
 
 class RectangularStorageTank : public StorageTank {
 public:
-  constexpr RectangularStorageTank(const quantity<isq::length[m]>& length, const quantity<isq::width[m]>& width,
+  constexpr RectangularStorageTank(const quantity<horizontal_length[m]>& length, const quantity<isq::width[m]>& width,
                                    const quantity<isq::height[m]>& height) :
       StorageTank(length * width, height)
   {
@@ -99,11 +108,8 @@ public:
 
 int main()
 {
-  using namespace mp_units;
-  using namespace mp_units::si::unit_symbols;
-
   const auto height = isq::height(200 * mm);
-  auto tank = RectangularStorageTank(isq::length(1'000 * mm), isq::width(500 * mm), height);
+  auto tank = RectangularStorageTank(horizontal_length(1'000 * mm), isq::width(500 * mm), height);
   tank.set_contents_density(1'000 * isq::mass_density[kg / m3]);
 
   const auto fill_time = 200 * s;       // time since starting fill