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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A generic callable type that can be initialized from any compatible callable,
+ * suitable for use as a function argument for the duration of the function
+ * call (and no longer).
+ */
+
+#ifndef mozilla_FunctionRef_h
+#define mozilla_FunctionRef_h
+
+#include "mozilla/OperatorNewExtensions.h" // mozilla::NotNull, ::operator new
+
+#include <cstddef> // std::nullptr_t
+#include <type_traits> // std::{declval,integral_constant}, std::is_{convertible,same,void}_v, std::{enable_if,remove_reference,remove_cv}_t
+#include <utility> // std::forward
+
+// This concept and its implementation are substantially inspired by foonathan's
+// prior art:
+//
+// https://foonathan.net/2017/01/function-ref-implementation/
+// https://github.com/foonathan/type_safe/blob/2017851053f8dd268372f1612865792c5c621570/include/type_safe/reference.hpp
+
+namespace mozilla {
+
+namespace detail {
+
+// Template helper to determine if |Returned| is a return type compatible with
+// |Required|: if the former converts to the latter, or if |Required| is |void|
+// and nothing is returned.
+template <typename Returned, typename Required>
+using CompatibleReturnType =
+ std::integral_constant<bool, std::is_void_v<Required> ||
+ std::is_convertible_v<Returned, Required>>;
+
+// Template helper to check if |Func| called with |Params| arguments returns
+// a type compatible with |Ret|.
+template <typename Func, typename Ret, typename... Params>
+using EnableMatchingFunction = std::enable_if_t<
+ CompatibleReturnType<
+ decltype(std::declval<Func&>()(std::declval<Params>()...)), Ret>::value,
+ int>;
+
+struct MatchingFunctionPointerTag {};
+struct MatchingFunctorTag {};
+struct InvalidFunctorTag {};
+
+// Template helper to determine the proper way to store |Callable|: as function
+// pointer, as pointer to object, or unstorable.
+template <typename Callable, typename Ret, typename... Params>
+struct GetCallableTag {
+ // Match the case where |Callable| is a compatible function pointer or
+ // converts to one. (|+obj| invokes such a conversion.)
+ template <typename T>
+ static MatchingFunctionPointerTag test(
+ int, T& obj, EnableMatchingFunction<decltype(+obj), Ret, Params...> = 0);
+
+ // Match the case where |Callable| is callable but can't be converted to a
+ // function pointer. (|short| is a worse match for 0 than |int|, causing the
+ // function pointer match to be preferred if both apply.)
+ template <typename T>
+ static MatchingFunctorTag test(short, T& obj,
+ EnableMatchingFunction<T, Ret, Params...> = 0);
+
+ // Match all remaining cases. (Any other match is preferred to an ellipsis
+ // match.)
+ static InvalidFunctorTag test(...);
+
+ using Type = decltype(test(0, std::declval<Callable&>()));
+};
+
+// If the callable is |nullptr|, |std::declval<std::nullptr_t&>()| will be an
+// error. Provide a specialization for |nullptr| that will fail substitution.
+template <typename Ret, typename... Params>
+struct GetCallableTag<std::nullptr_t, Ret, Params...> {};
+
+template <typename Result, typename Callable, typename Ret, typename... Params>
+using EnableFunctionTag = std::enable_if_t<
+ std::is_same_v<typename GetCallableTag<Callable, Ret, Params...>::Type,
+ Result>,
+ int>;
+
+} // namespace detail
+
+/**
+ * An efficient, type-erasing, non-owning reference to a callable. It is
+ * intended for use as the type of a function parameter that is not used after
+ * the function in question returns.
+ *
+ * This class does not own the callable, so in general it is unsafe to store a
+ * FunctionRef.
+ */
+template <typename Fn>
+class MOZ_TEMPORARY_CLASS FunctionRef;
+
+template <typename Ret, typename... Params>
+class MOZ_TEMPORARY_CLASS FunctionRef<Ret(Params...)> {
+ union Payload;
+
+ // |FunctionRef| stores an adaptor function pointer, determined by the
+ // arguments passed to the constructor. That adaptor will perform the steps
+ // needed to invoke the callable passed at construction time.
+ using Adaptor = Ret (*)(const Payload& aPayload, Params... aParams);
+
+ // If |FunctionRef|'s callable can be stored as a function pointer, that
+ // function pointer is stored after being cast to this *different* function
+ // pointer type. |mAdaptor| then casts back to the original type to call it.
+ // ([expr.reinterpret.cast]p6 guarantees that A->B->A function pointer casts
+ // produce the original function pointer value.) An outlandish signature is
+ // used to emphasize that the exact function pointer type doesn't matter.
+ using FuncPtr = Payload***** (*)(Payload*****);
+
+ /**
+ * An adaptor function (used by this class's function call operator) that
+ * invokes the callable in |mPayload|, forwarding arguments and converting
+ * return type as needed.
+ */
+ const Adaptor mAdaptor;
+
+ /** Storage for the wrapped callable value. */
+ union Payload {
+ // This arm is used if |FunctionRef| is passed a compatible function pointer
+ // or a lambda/callable that converts to a compatible function pointer.
+ FuncPtr mFuncPtr;
+
+ // This arm is used if |FunctionRef| is passed some other callable or
+ // |nullptr|.
+ void* mObject;
+ } mPayload;
+
+ template <typename RealFuncPtr>
+ static Ret CallFunctionPointer(const Payload& aPayload,
+ Params... aParams) noexcept {
+ auto func = reinterpret_cast<RealFuncPtr>(aPayload.mFuncPtr);
+ return static_cast<Ret>(func(std::forward<Params>(aParams)...));
+ }
+
+ template <typename Ret2, typename... Params2>
+ FunctionRef(detail::MatchingFunctionPointerTag, Ret2 (*aFuncPtr)(Params2...))
+ : mAdaptor(&CallFunctionPointer<Ret2 (*)(Params2...)>) {
+ ::new (KnownNotNull, &mPayload.mFuncPtr)
+ FuncPtr(reinterpret_cast<FuncPtr>(aFuncPtr));
+ }
+
+ public:
+ /**
+ * Construct a |FunctionRef| that's like a null function pointer that can't be
+ * called.
+ */
+ MOZ_IMPLICIT FunctionRef(std::nullptr_t) noexcept : mAdaptor(nullptr) {
+ // This is technically unnecessary, but it seems best to always initialize
+ // a union arm.
+ ::new (KnownNotNull, &mPayload.mObject) void*(nullptr);
+ }
+
+ FunctionRef() : FunctionRef(nullptr) {}
+
+ /**
+ * Constructs a |FunctionRef| from an object callable with |Params| arguments,
+ * that returns a type convertible to |Ret|, where the callable isn't
+ * convertible to function pointer (often because it contains some internal
+ * state). For example:
+ *
+ * int x = 5;
+ * DoSomething([&x] { x++; });
+ */
+ template <typename Callable,
+ typename = detail::EnableFunctionTag<detail::MatchingFunctorTag,
+ Callable, Ret, Params...>,
+ typename std::enable_if_t<!std::is_same_v<
+ std::remove_cv_t<std::remove_reference_t<Callable>>,
+ FunctionRef>>* = nullptr>
+ MOZ_IMPLICIT FunctionRef(Callable&& aCallable MOZ_LIFETIME_BOUND) noexcept
+ : mAdaptor([](const Payload& aPayload, Params... aParams) {
+ auto& func = *static_cast<std::remove_reference_t<Callable>*>(aPayload.mObject);
+ return static_cast<Ret>(func(std::forward<Params>(aParams)...));
+ }) {
+ ::new (KnownNotNull, &mPayload.mObject) void*(&aCallable);
+ }
+
+ /**
+ * Constructs a |FunctionRef| from an value callable with |Params| arguments,
+ * that returns a type convertible to |Ret|, where the callable is stateless
+ * and is (or is convertible to) a function pointer. For example:
+ *
+ * // Exact match
+ * double twice(double d) { return d * 2; }
+ * FunctionRef<double(double)> func1(&twice);
+ *
+ * // Compatible match
+ * float thrice(long double d) { return static_cast<float>(d) * 3; }
+ * FunctionRef<double(double)> func2(&thrice);
+ *
+ * // Non-generic lambdas that don't capture anything have a conversion
+ * // function to the appropriate function pointer type.
+ * FunctionRef<int(double)> f([](long double){ return 'c'; });
+ */
+ template <typename Callable,
+ typename = detail::EnableFunctionTag<
+ detail::MatchingFunctionPointerTag, Callable, Ret, Params...>>
+ MOZ_IMPLICIT FunctionRef(const Callable& aCallable) noexcept
+ : FunctionRef(detail::MatchingFunctionPointerTag{}, +aCallable) {}
+
+ /** Call the callable stored in this with the given arguments. */
+ Ret operator()(Params... params) const {
+ return mAdaptor(mPayload, std::forward<Params>(params)...);
+ }
+
+ /** Return true iff this wasn't created from |nullptr|. */
+ explicit operator bool() const noexcept { return mAdaptor != nullptr; }
+};
+
+} /* namespace mozilla */
+
+#endif /* mozilla_FunctionRef_h */