4 files changed, 60 insertions, 0 deletions
diff --git a/nihil.util/CMakeLists.txt b/nihil.util/CMakeLists.txt
index 2fb5111..5d6be7c 100644
--- a/nihil.util/CMakeLists.txt
+++ b/nihil.util/CMakeLists.txt
@@ -14,6 +14,7 @@ target_sources(nihil.util
 	capture_stream.ccm
 	construct.ccm
 	ctype.ccm
+	guard.ccm
 	parse_size.ccm
 	next_word.ccm
 	save_errno.ccm
@@ -27,6 +28,7 @@ if(NIHIL_TESTS)
 	add_executable(nihil.util.test
 		capture_stream.test.cc
 		ctype.test.cc
+		guard.test.cc
 		parse_size.test.cc
 		next_word.test.cc
 		skipws.test.cc
diff --git a/nihil.util/guard.ccm b/nihil.util/guard.ccm
new file mode 100644
index 0000000..77394fc
--- /dev/null
+++ b/nihil.util/guard.ccm
@@ -0,0 +1,41 @@
+// This source code is released into the public domain.
+export module nihil.util:guard;
+
+import nihil.std;
+
+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.
+	explicit 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.util/guard.test.cc b/nihil.util/guard.test.cc
new file mode 100644
index 0000000..2c3ac2f
--- /dev/null
+++ b/nihil.util/guard.test.cc
@@ -0,0 +1,16 @@
+// This source code is released into the public domain.
+
+#include <catch2/catch_test_macros.hpp>
+
+import nihil.util;
+
+TEST_CASE("guard: basic", "[guard]") {
+	int n = 0;
+
+	{
+		auto guard = nihil::guard([&] { n = 1; });
+		REQUIRE(n == 0);
+	}
+
+	REQUIRE(n == 1);
+}
diff --git a/nihil.util/nihil.util.ccm b/nihil.util/nihil.util.ccm
index 132978e..1911ced 100644
--- a/nihil.util/nihil.util.ccm
+++ b/nihil.util/nihil.util.ccm
@@ -4,6 +4,7 @@ export module nihil.util;
 export import :capture_stream;
 export import :construct;
 export import :ctype;
+export import :guard;
 export import :parse_size;
 export import :next_word;
 export import :save_errno;