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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 */
* 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:
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<
decltype(std::declval<Func&>()(std::declval<Params>()...)), Ret>::value,
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,
} // 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)
* 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<
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>*>(
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 */