Add scale-gated unary minus to temperature (enables negative literals)

CMakeLists.txt

@@ -7,7 +7,7 @@ if(ESP_PLATFORM)
 endif()
 
 project(thermo
-    VERSION 2.0.0
+    VERSION 2.1.0
     DESCRIPTION "Type-safe temperature handling library modeled after std::chrono"
     HOMEPAGE_URL "https://github.com/cleishm/thermo-cpp"
     LANGUAGES CXX

README.md

@@ -143,12 +143,23 @@ using namespace thermo_literals;
 293_k     // kelvin(293)
 68_f      // fahrenheit(68)
 
+// Negative absolute temperatures (unary minus on the literal)
+-123_dc   // decicelsius(-123) = -12.3°C
+-40_f     // fahrenheit(-40)
+// -25_k  // ill-formed: negating a Kelvin temperature falls below absolute zero
+
 // Temperature deltas
 5_Δc      // delta_celsius(5)
 5000_Δmc  // delta_millicelsius(5000)
 9_Δf      // delta_fahrenheit(9) = delta_celsius(5)
 ```
 
+Unary minus is available on relative scales (Celsius, Fahrenheit) so that
+negative literals like `-123_dc` work — the sign is applied to the value the
+literal produces, i.e. `-(123_dc)`. It is deliberately *not* available on the
+absolute Kelvin scale, where a negated temperature would fall below absolute
+zero; `-25_k` is a compile error. Deltas are negatable on every scale.
+
 ## Conversion Rules
 
 Conversions follow the same philosophy as `std::chrono`:

docs/Doxyfile

@@ -6,7 +6,7 @@
 
 DOXYFILE_ENCODING      = UTF-8
 PROJECT_NAME           = "thermo"
-PROJECT_NUMBER         = "2.0.0"
+PROJECT_NUMBER         = "2.1.0"
 PROJECT_BRIEF          = "Type-safe temperature handling library modeled after std::chrono"
 PROJECT_LOGO           =
 OUTPUT_DIRECTORY       = .

idf_component.yml

@@ -1,4 +1,4 @@
-version: "2.0.0"
+version: "2.1.0"
 description: "Type-safe temperature handling library modeled after std::chrono"
 url: "https://github.com/cleishm/thermo-cpp"
 repository: "https://github.com/cleishm/thermo-cpp.git"

include/thermo/thermo.hpp

@@ -585,6 +585,17 @@ struct fahrenheit_scale {
     using degree = std::ratio<5, 9>;
     static constexpr const char* suffix = "°F";
 };
+
+/**
+ * @brief True when a scale's zero point is not absolute zero.
+ *
+ * Negating an absolute temperature only makes sense on a "relative" scale
+ * (e.g. Celsius, Fahrenheit), where the zero point sits above absolute zero.
+ * On an absolute scale (Kelvin, offset == 0) a negated temperature always
+ * falls below absolute zero, so unary minus is disabled there.
+ */
+template<typename Scale>
+inline constexpr bool _is_relative_scale = std::ratio_not_equal_v<typename Scale::offset, std::ratio<0>>;
 /** @endcond */
 
 template<typename Scale, typename Delta = delta<int64_t>>
@@ -729,6 +740,19 @@ class temperature {
 
     constexpr temperature operator--(int) { return temperature(_d--); }
 
+    /**
+     * @brief Negates the temperature (e.g. to write @c -123_dc as @c -(123_dc)).
+     *
+     * Only available on relative scales such as Celsius and Fahrenheit; on an
+     * absolute scale (Kelvin) negation would fall below absolute zero and is a
+     * compile error. See @ref _is_relative_scale.
+     */
+    constexpr temperature operator-() const
+        requires _is_relative_scale<Scale>
+    {
+        return temperature(-_d);
+    }
+
     template<typename Rep2, typename Precision2>
     constexpr temperature& operator+=(const delta<Rep2, Precision2>& d) {
         _d += d;

tests/thermo_test.cpp

@@ -333,6 +333,36 @@ TEST_CASE("delta literals", "[thermo][literals]") {
     REQUIRE((9000_Δmf).count() == 9000);
 }
 
+// =============================================================================
+// Negation (enables negative literals, e.g. -123_dc == -(123_dc))
+// =============================================================================
+
+template<typename T>
+concept has_unary_minus = requires(T t) { -t; };
+
+// Available on relative scales (zero above absolute zero)...
+static_assert(has_unary_minus<celsius>);
+static_assert(has_unary_minus<decicelsius>);
+static_assert(has_unary_minus<fahrenheit>);
+// ...but not on the absolute scale, where a negated value is below absolute zero.
+static_assert(!has_unary_minus<kelvin>);
+static_assert(!has_unary_minus<decikelvin>);
+static_assert(!has_unary_minus<millikelvin>);
+
+// Compile-time tests
+static_assert((-123_dc).count() == -123);
+static_assert((-40_f).count() == -40);
+static_assert((-20_c).count() == -20);
+static_assert(-(-5_c) == 5_c);
+
+// Runtime tests
+TEST_CASE("temperature negation", "[thermo][temperature]") {
+    REQUIRE((-123_dc).count() == -123);
+    REQUIRE((-40_f).count() == -40);
+    REQUIRE((-20_c).count() == -20);
+    REQUIRE((-(-5_c)).count() == 5);
+}
+
 // =============================================================================
 // String formatting
 // =============================================================================

vcpkg-port/vcpkg.json

@@ -1,6 +1,6 @@
 {
   "name": "cleishm-thermo-cpp",
-  "version": "2.0.0",
+  "version": "2.1.0",
   "description": "Type-safe temperature handling library modeled after std::chrono",
   "homepage": "https://github.com/cleishm/thermo-cpp",
   "license": "MIT",