feat: compare.h header added with checks against zero

Resolves #487

example/include/geographic.h

@@ -24,6 +24,7 @@
 
 #include "ranged_representation.h"
 #include <mp-units/bits/fmt_hacks.h>
+#include <mp-units/compare.h>
 #include <mp-units/format.h>
 #include <mp-units/math.h>
 #include <mp-units/quantity.h>
@@ -78,7 +79,7 @@ using longitude = mp_units::quantity_point<mp_units::si::degree, prime_meridian,
 template<class CharT, class Traits, typename T>
 std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, const latitude<T>& lat)
 {
-  if (lat > latitude<T>::zero())
+  if (is_gt_zero(lat))
     return os << lat.quantity_from_origin() << " N";
   else
     return os << -lat.quantity_from_origin() << " S";
@@ -87,7 +88,7 @@ std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>&
 template<class CharT, class Traits, typename T>
 std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, const longitude<T>& lon)
 {
-  if (lon > longitude<T>::zero())
+  if (is_gt_zero(lon))
     return os << lon.quantity_from_origin() << " E";
   else
     return os << -lon.quantity_from_origin() << " W";
@@ -137,8 +138,8 @@ struct MP_UNITS_STD_FMT::formatter<geographic::latitude<T>> :
   auto format(geographic::latitude<T> lat, FormatContext& ctx)
   {
     formatter<typename geographic::latitude<T>::quantity_type>::format(
-      lat > geographic::latitude<T>::zero() ? lat.quantity_from_origin() : -lat.quantity_from_origin(), ctx);
+      is_gt_zero(lat) ? lat.quantity_from_origin() : -lat.quantity_from_origin(), ctx);
-    MP_UNITS_STD_FMT::format_to(ctx.out(), "{}", lat > geographic::latitude<T>::zero() ? " N" : "S");
+    MP_UNITS_STD_FMT::format_to(ctx.out(), "{}", is_gt_zero(lat) ? " N" : "S");
     return ctx.out();
   }
 };
@@ -150,8 +151,8 @@ struct MP_UNITS_STD_FMT::formatter<geographic::longitude<T>> :
   auto format(geographic::longitude<T> lon, FormatContext& ctx)
   {
     formatter<typename geographic::longitude<T>::quantity_type>::format(
-      lon > geographic::longitude<T>::zero() ? lon.quantity_from_origin() : -lon.quantity_from_origin(), ctx);
+      is_gt_zero(lon) ? lon.quantity_from_origin() : -lon.quantity_from_origin(), ctx);
-    MP_UNITS_STD_FMT::format_to(ctx.out(), "{}", lon > geographic::longitude<T>::zero() ? " E" : " W");
+    MP_UNITS_STD_FMT::format_to(ctx.out(), "{}", is_gt_zero(lon) ? " E" : " W");
     return ctx.out();
   }
 };

src/utility/include/mp-units/compare.h

@@ -0,0 +1,95 @@
+// 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 <compare>
+
+namespace mp_units {
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::equality_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_eq_zero(T v)
+{
+  return v == T::zero();
+}
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::equality_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_neq_zero(T v)
+{
+  return v != T::zero();
+}
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::three_way_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_lt_zero(T v)
+{
+  return v < T::zero();
+}
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::three_way_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_gt_zero(T v)
+{
+  return v > T::zero();
+}
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::three_way_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_lteq_zero(T v)
+{
+  return v <= T::zero();
+}
+
+template<typename T>
+  requires requires {
+    {
+      T::zero()
+    } -> std::three_way_comparable_with<T>;
+  }
+[[nodiscard]] constexpr bool is_gteq_zero(T v)
+{
+  return v >= T::zero();
+}
+
+}  // namespace mp_units

test/unit_test/static/compare_test.cpp

@@ -0,0 +1,54 @@
+// 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.
+
+#include <mp-units/compare.h>
+#include <mp-units/systems/si/si.h>
+
+namespace {
+
+using namespace mp_units::si::unit_symbols;
+
+static_assert(is_eq_zero(0 * m));
+static_assert(!is_eq_zero(1 * m));
+static_assert(!is_eq_zero(-1 * m));
+
+static_assert(!is_neq_zero(0 * m));
+static_assert(is_neq_zero(1 * m));
+static_assert(is_neq_zero(-1 * m));
+
+static_assert(!is_lt_zero(0 * m));
+static_assert(!is_lt_zero(1 * m));
+static_assert(is_lt_zero(-1 * m));
+
+static_assert(!is_gt_zero(0 * m));
+static_assert(is_gt_zero(1 * m));
+static_assert(!is_gt_zero(-1 * m));
+
+static_assert(is_lteq_zero(0 * m));
+static_assert(!is_lteq_zero(1 * m));
+static_assert(is_lteq_zero(-1 * m));
+
+static_assert(is_gteq_zero(0 * m));
+static_assert(is_gteq_zero(1 * m));
+static_assert(!is_gteq_zero(-1 * m));
+
+}  // namespace