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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/. */
#ifndef nsCycleCollectionContainerParticipant_h__
#define nsCycleCollectionContainerParticipant_h__
#include <type_traits>
/*
* To be able to implement ImplCycleCollectionIndexedContainer, we need to
* handle const vs non-const. ImplCycleCollectionTrace requires that the the
* value traced is non-const, and historically ImplCycleCollectionTraverse has
* been declared to take either const or non-const values. This poses a problem
* for containers, since it's not possible to define one
* ImplCycleCollectionIndexedContainer for both the const and non-const case
* with a templated parameter for the type contained by the container. The
* reason for this is how overload resolution works when it sees non-generic
* types with different constness. The standard solution for this is to use a
* universal reference, but we can't do this only because suddenly our
* ImplCycleCollectionIndexedContainer would be valid for any type. To make this
* work we need a way to constrain the type and we need it to be a constraint on
* the container type, not the type contained.
*
* This is pretty much the proposal for std::is_specialization_of, but with
* names from the unofficial cycle collector namespace. We use this to be able
* to do partial specialization to overload containers whose contents we wish to
* have participate in cycle collection. `template <typename Container>
* EnableCycleCollectionIf<Container, SomeContainer>` allows us to restrict an
* overload to only happen for a type SomeContainer<T>, which we then can use to
* make sure that an ImplCycleCollectionIndexedContainer overload is for a
* particular container, const or not.
*
* Example:
*
* template <typename Container, typename Callback,
* EnableCycleCollectionIf<Container, nsTHashtable> = nullptr>
* inline void ImplCycleCollectionContainer(Container&& aField,
* Callback&& aCallback) {
* // Implementation goes here
* }
*/
template <typename, template <typename...> typename>
struct ImplCycleCollectionIsContainerT : std::false_type {};
template <template <typename...> typename Container, typename... Args>
struct ImplCycleCollectionIsContainerT<Container<Args...>, Container>
: std::true_type {};
template <typename T, template <typename...> typename Container>
constexpr bool ImplCycleCollectionIsContainer = ImplCycleCollectionIsContainerT<
std::remove_cv_t<std::remove_reference_t<T>>, Container>::value;
template <typename T, template <typename...> typename Container>
using EnableCycleCollectionIf =
typename std::enable_if_t<ImplCycleCollectionIsContainer<T, Container>>*;
#endif // nsCycleCollectionContainerParticipant_h__
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