diff --git a/include/ftl/finalizer.h b/include/ftl/finalizer.h
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+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <cstddef>
+
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include <ftl/function.h>
+
+namespace android::ftl {
+
+// An RAII wrapper that invokes a function object as a finalizer when destroyed.
+//
+// The function object must take no arguments, and must return void. If the function object needs
+// any context for the call, it must store it itself, for example with a lambda capture.
+//
+// The stored function object will be called once (unless canceled via the `cancel()` member
+// function) at the first of:
+//
+//   - The Finalizer instance is destroyed.
+//   - `operator()` is used to invoke the contained function.
+//   - The Finalizer instance is move-assigned a new value. The function being replaced will be
+//     invoked, and the replacement will be stored to be called later.
+//
+// The intent with this class is to keep cleanup code next to the code that requires that
+// cleanup be performed.
+//
+//   bool read_file(std::string filename) {
+//     FILE* f = fopen(filename.c_str(), "rb");
+//     if (f == nullptr) return false;
+//     const auto cleanup = ftl::Finalizer([f]() { fclose(f); });
+//     // fread(...), etc
+//     return true;
+//   }
+//
+// The `FinalFunction` template argument to Finalizer<FinalFunction> allows a polymorphic function
+// type for storing the finalization function, such as `std::function` or `ftl::Function`.
+//
+// For convenience, this header defines a few useful aliases for using those types.
+//
+//   - `FinalizerStd`, an alias for `Finalizer<std::function<void()>>`
+//   - `FinalizerFtl`, an alias for `Finalizer<ftl::Function<void()>>`
+//   - `FinalizerFtl1`, an alias for `Finalizer<ftl::Function<void(), 1>>`
+//   - `FinalizerFtl2`, an alias for `Finalizer<ftl::Function<void(), 2>>`
+//   - `FinalizerFtl3`, an alias for `Finalizer<ftl::Function<void(), 3>>`
+//
+// Clients of this header are free to define other aliases they need.
+//
+// A Finalizer that uses a polymorphic function type can be returned from a function call and/or
+// stored as member data (to be destroyed along with the containing class).
+//
+//   auto register(Observer* observer) -> ftl::FinalizerStd<void()> {
+//      const auto id = observers.add(observer);
+//      return ftl::Finalizer([id]() { observers.remove(id); });
+//   }
+//
+//   {
+//     const auto _ = register(observer);
+//     // do the things that required the registered observer.
+//   }
+//   // the observer is removed.
+//
+// Cautions:
+//
+//   1. When a Finalizer is stored as member data, you will almost certainly want that cleanup to
+//      happen first, before the rest of the other member data is destroyed. For safety you should
+//      assume that the finalization function will access that data directly or indirectly.
+//
+//      This means that Finalizers should be defined last, after all other normal member data in a
+//      class.
+//
+//          class MyClass {
+//           public:
+//            bool initialize() {
+//              ready_ = true;
+//              cleanup_ = ftl::Finalizer([this]() { ready_ = false; });
+//              return true;
+//            }
+//
+//            bool ready_ = false;
+//
+//            // Finalizers should be last so other class members can be accessed before being
+//            // destroyed.
+//            ftl::FinalizerStd<void()> cleanup_;
+//          };
+//
+//   2. Care must be taken to use `ftl::Finalizer()` when constructing locally from a lambda. If you
+//      forget to do so, you are just creating a lambda that won't be automatically invoked!
+//
+//          const auto bad = [&counter](){ ++counter; }; // Just a lambda instance
+//          const auto good = ftl::Finalizer([&counter](){ ++counter; });
+//
+template <typename FinalFunction>
+class Finalizer final {
+  // requires(std::is_invocable_r_v<void, FinalFunction>)
+  static_assert(std::is_invocable_r_v<void, FinalFunction>);
+
+ public:
+  // A default constructed Finalizer does nothing when destroyed.
+  // requires(std::is_default_constructible_v<FinalFunction>)
+  constexpr Finalizer() = default;
+
+  // Constructs a Finalizer from a function object.
+  // requires(std::is_invocable_v<F>)
+  template <typename F, typename = std::enable_if_t<std::is_invocable_v<F>>>
+  [[nodiscard]] explicit constexpr Finalizer(F&& function)
+      : Finalizer(std::forward<F>(function), false) {}
+
+  constexpr ~Finalizer() { maybe_invoke(); }
+
+  // Disallow copying.
+  Finalizer(const Finalizer& that) = delete;
+  auto operator=(const Finalizer& that) = delete;
+
+  // Move construction
+  // requires(std::is_move_constructible_v<FinalFunction>)
+  [[nodiscard]] constexpr Finalizer(Finalizer&& that)
+      : Finalizer(std::move(that.function_), std::exchange(that.canceled_, true)) {}
+
+  // Implicit conversion move construction
+  // requires(!std::is_same_v<Finalizer, Finalizer<F>>)
+  template <typename F, typename = std::enable_if_t<!std::is_same_v<Finalizer, Finalizer<F>>>>
+  // NOLINTNEXTLINE(google-explicit-constructor, cppcoreguidelines-rvalue-reference-param-not-moved)
+  [[nodiscard]] constexpr Finalizer(Finalizer<F>&& that)
+      : Finalizer(std::move(that.function_), std::exchange(that.canceled_, true)) {}
+
+  // Move assignment
+  // requires(std::is_move_assignable_v<FinalFunction>)
+  constexpr auto operator=(Finalizer&& that) -> Finalizer& {
+    maybe_invoke();
+
+    function_ = std::move(that.function_);
+    canceled_ = std::exchange(that.canceled_, true);
+
+    return *this;
+  }
+
+  // Implicit conversion move assignment
+  // requires(!std::is_same_v<Finalizer, Finalizer<F>>)
+  template <typename F, typename = std::enable_if_t<!std::is_same_v<Finalizer, Finalizer<F>>>>
+  // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
+  constexpr auto operator=(Finalizer<F>&& that) -> Finalizer& {
+    *this = Finalizer(std::move(that.function_), std::exchange(that.canceled_, true));
+    return *this;
+  }
+
+  // Cancels the final function, preventing it from being invoked.
+  constexpr void cancel() {
+    canceled_ = true;
+    maybe_nullify_function();
+  }
+
+  // Invokes the final function now, if not already invoked.
+  constexpr void operator()() { maybe_invoke(); }
+
+ private:
+  template <typename>
+  friend class Finalizer;
+
+  template <typename F, typename = std::enable_if_t<std::is_invocable_v<F>>>
+  [[nodiscard]] explicit constexpr Finalizer(F&& function, bool canceled)
+      : function_(std::forward<F>(function)), canceled_(canceled) {}
+
+  constexpr void maybe_invoke() {
+    if (!std::exchange(canceled_, true)) {
+      std::invoke(function_);
+      maybe_nullify_function();
+    }
+  }
+
+  constexpr void maybe_nullify_function() {
+    // Sets function_ to nullptr if that is supported for the backing type.
+    if constexpr (std::is_assignable_v<FinalFunction, nullptr_t>) {
+      function_ = nullptr;
+    }
+  }
+
+  FinalFunction function_;
+  bool canceled_ = true;
+};
+
+template <typename F>
+Finalizer(F&&) -> Finalizer<std::decay_t<F>>;
+
+// A standard alias for using `std::function` as the polymorphic function type.
+using FinalizerStd = Finalizer<std::function<void()>>;
+
+// Helpful aliases for using `ftl::Function` as the polymorphic function type.
+using FinalizerFtl = Finalizer<Function<void()>>;
+using FinalizerFtl1 = Finalizer<Function<void(), 1>>;
+using FinalizerFtl2 = Finalizer<Function<void(), 2>>;
+using FinalizerFtl3 = Finalizer<Function<void(), 3>>;
+
+}  // namespace android::ftl
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