3 files changed, 96 insertions, 0 deletions
diff --git a/nihil.guard/CMakeLists.txt b/nihil.guard/CMakeLists.txt
new file mode 100644
index 0000000..bba4284
--- /dev/null
+++ b/nihil.guard/CMakeLists.txt
@@ -0,0 +1,23 @@
+# This source code is released into the public domain.
+
+add_library(nihil.guard STATIC)
+target_sources(nihil.guard
+	PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
+	guard.ccm
+)
+
+if(NIHIL_TESTS)
+	enable_testing()
+
+	add_executable(nihil.guard.test test.cc)
+	target_link_libraries(nihil.guard.test PRIVATE
+		nihil.guard
+		Catch2::Catch2WithMain
+	)
+
+	find_package(Catch2 REQUIRED)
+
+	include(CTest)
+	include(Catch)
+	catch_discover_tests(nihil.guard.test)
+endif()
diff --git a/nihil.guard/guard.ccm b/nihil.guard/guard.ccm
new file mode 100644
index 0000000..7b6cf66
--- /dev/null
+++ b/nihil.guard/guard.ccm
@@ -0,0 +1,53 @@
+/*
+ * This source code is released into the public domain.
+ */
+
+module;
+
+#include <concepts>
+#include <functional>
+#include <optional>
+#include <utility>
+
+export module nihil.guard;
+
+namespace nihil {
+
+/*
+ * guard: invoke a callable when this object is destroyed; this is similar to
+ * scope_exit from the library fundamentals TS, which LLVM doesn't implement.
+ */
+export template<std::invocable F>
+struct guard final {
+	// Initialise the guard with a callable we will invoke later.
+	guard(F func) : m_func(std::move(func)) {}
+
+	/*
+	 * We are being destroyed, so call the callable.
+	 * If the callable throws, std::terminate() will be called.
+	 */
+	~guard()
+	{
+		if (m_func)
+			std::invoke(*m_func);
+	}
+
+	// Release the guard.  This turns the destructor into a no-op.
+	auto release(this guard &self) noexcept -> void
+	{
+		self.m_func.reset();
+	}
+
+	// Not default-constructible, movable or copyable.
+	guard() = delete;
+	guard(guard const &) = delete;
+	guard(guard &&) noexcept = delete;
+	auto operator=(this guard &, guard const &) -> guard & = delete;
+	auto operator=(this guard &, guard &&) noexcept -> guard & = delete;
+
+private:
+	// The callable to be invoked when we are destroyed.
+	std::optional<F> m_func;
+};
+
+} // namespace nihil
diff --git a/nihil.guard/test.cc b/nihil.guard/test.cc
new file mode 100644
index 0000000..11f7d37
--- /dev/null
+++ b/nihil.guard/test.cc
@@ -0,0 +1,20 @@
+/*
+ * This source code is released into the public domain.
+ */
+
+#include <catch2/catch_test_macros.hpp>
+
+import nihil.guard;
+
+using namespace std::literals;
+
+TEST_CASE("guard: basic", "[guard]") {
+	int n = 0;
+
+	{
+		auto guard = nihil::guard([&] { n = 1; });
+		REQUIRE(n == 0);
+	}
+
+	REQUIRE(n == 1);
+}