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| -rw-r--r-- | mfbt/FunctionRef.h | 226 |
1 files changed, 226 insertions, 0 deletions
diff --git a/mfbt/FunctionRef.h b/mfbt/FunctionRef.h new file mode 100644 index 0000000000..12c3b2f4ea --- /dev/null +++ b/mfbt/FunctionRef.h @@ -0,0 +1,226 @@ +/* -*- 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 FunctionRef; + +template <typename Ret, typename... Params> +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; + * auto doSideEffect = [&x]{ x++; }; // state is captured reference to |x| + * FunctionRef<void()> f(doSideEffect); + */ + template < + typename Callable, + typename = detail::EnableFunctionTag<detail::MatchingFunctorTag, Callable, + Ret, Params...>, + typename std::enable_if_t<!std::is_same_v< + typename std::remove_reference_t<typename std::remove_cv_t<Callable>>, + FunctionRef>>* = nullptr> + MOZ_IMPLICIT FunctionRef(Callable& aCallable) noexcept + : mAdaptor([](const Payload& aPayload, Params... aParams) { + auto& func = *static_cast<Callable*>(aPayload.mObject); + // Unable to use std::forward here due to llvm windows bug + // https://bugs.llvm.org/show_bug.cgi?id=28299 + // + // This prevents use of move-only arguments for functors and lambdas. + // Move only arguments can be used when using function pointers + return static_cast<Ret>(func(static_cast<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 */ |