c++: add finally RAII utility

This mirrors the `finally` utility in Guidelines Support Library for
ensuring that a cleanup function is always called when the utility
object leaves scope, even when unwinding the stack from an exception.
This is to aid in transitioning malloc/free pairs in function bodies to
be more exception-safe.

src/cxxutil.hpp

@@ -0,0 +1,37 @@
+#ifndef __SRB2_CXXUTIL_HPP__
+#define __SRB2_CXXUTIL_HPP__
+
+#include <type_traits>
+#include <utility>
+
+namespace srb2 {
+
+template <class F>
+class Finally {
+public:
+	explicit Finally(const F& f) noexcept : f_(f) {}
+	explicit Finally(F&& f) noexcept : f_(f) {}
+
+	Finally(Finally&& from) noexcept : f_(std::move(from.f_)), call_(std::exchange(from.call_, false)) {}
+
+	Finally(const Finally& from) = delete;
+	void operator=(const Finally& from) = delete;
+	void operator=(Finally&& from) = delete;
+
+	~Finally() noexcept {
+		f_();
+	}
+
+private:
+	F f_;
+	bool call_ = true;
+};
+
+template <class F>
+Finally<std::decay_t<F>> finally(F&& f) noexcept {
+	return Finally {std::forward<F>(f)};
+}
+
+}
+
+#endif // __SRB2_CXXUTIL_HPP__