Feature: is_zst (#60)

README.md

@@ -93,6 +93,9 @@ Like `std::variant` but specifically optimized for usage with two types/variants
 The internal representation is a `union` of the two types plus a 1 byte (3 state) discriminant.
 Additionally, `visit` does not involve any virtual function calls.
 
+### `type_traits.hpp`
+Things that are missing in the standard library `<type_traits>` header.
+
 ### Further Examples
 
 Compilable code examples can be found in [examples](./examples). The example build requires the cmake

include/dice/template-library/type_traits.hpp

@@ -0,0 +1,36 @@
+#ifndef DICE_TEMPLATELIBRARY_ZST_HPP
+#define DICE_TEMPLATELIBRARY_ZST_HPP
+
+#include <type_traits>
+
+namespace dice::template_library {
+	namespace detail_zst {
+		template<typename T>
+		struct zst_checker {
+			uint64_t pad;
+			[[no_unique_address]] T value;
+
+			static constexpr bool check() noexcept {
+				return sizeof(zst_checker) == sizeof(uint64_t);
+			}
+		};
+	} // namespace detail_zst
+
+	/**
+	 * Determine if the provided type is a ZST (zero-sized type).
+	 * I.e. a type that can benefit from EBO (empty-base-optimization) or [[no_unique_address]].
+	 * Note: this is not the same as comparing sizeof(T) to 0, types never have a size of 0
+ 	 */
+	template<typename T>
+	struct is_zst : std::bool_constant<detail_zst::zst_checker<T>::check()> {
+	};
+
+	template<>
+	struct is_zst<void> : std::false_type {};
+
+	template<typename T>
+	inline constexpr bool is_zst_v = is_zst<T>::value;
+
+} // namespace dice::template_library
+
+#endif // DICE_TEMPLATELIBRARY_ZST_HPP

tests/CMakeLists.txt

@@ -60,3 +60,6 @@ custom_add_test(tests_channel)
 
 add_executable(tests_variant2 tests_variant2.cpp)
 custom_add_test(tests_variant2)
+
+add_executable(tests_type_traits tests_type_traits.cpp)
+custom_add_test(tests_type_traits)

tests/tests_type_traits.cpp

@@ -0,0 +1,23 @@
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include <doctest/doctest.h>
+
+#include <dice/template-library/type_traits.hpp>
+
+TEST_SUITE("type_traits") {
+
+	TEST_CASE("is_zst") {
+		struct zst {};
+		struct non_zst {
+			int x;
+		};
+
+		static_assert(dice::template_library::is_zst<zst>::value);
+		static_assert(dice::template_library::is_zst_v<zst>);
+
+		static_assert(!dice::template_library::is_zst<non_zst>::value);
+		static_assert(!dice::template_library::is_zst_v<non_zst>);
+
+		static_assert(!dice::template_library::is_zst<void>::value);
+		static_assert(!dice::template_library::is_zst_v<void>);
+	}
+}