serge-sans-paille
6b9a7615c8
Patch only partly generated through https://github.com/llvm/llvm-project/pull/204789, our static-analysis framework does not handle all headers as well as I would love it to do. Differential Revision: https://phabricator.services.mozilla.com/D307815
59 lines
2.8 KiB
C++
59 lines
2.8 KiB
C++
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef nsCycleCollectionContainerParticipant_h_
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#define nsCycleCollectionContainerParticipant_h_
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#include <type_traits>
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/*
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* To be able to implement ImplCycleCollectionIndexedContainer, we need to
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* handle const vs non-const. ImplCycleCollectionTrace requires that the the
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* value traced is non-const, and historically ImplCycleCollectionTraverse has
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* been declared to take either const or non-const values. This poses a problem
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* for containers, since it's not possible to define one
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* ImplCycleCollectionIndexedContainer for both the const and non-const case
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* with a templated parameter for the type contained by the container. The
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* reason for this is how overload resolution works when it sees non-generic
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* types with different constness. The standard solution for this is to use a
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* universal reference, but we can't do this only because suddenly our
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* ImplCycleCollectionIndexedContainer would be valid for any type. To make this
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* work we need a way to constrain the type and we need it to be a constraint on
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* the container type, not the type contained.
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*
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* This is pretty much the proposal for std::is_specialization_of, but with
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* names from the unofficial cycle collector namespace. We use this to be able
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* to do partial specialization to overload containers whose contents we wish to
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* have participate in cycle collection. `template <typename Container>
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* EnableCycleCollectionIf<Container, SomeContainer>` allows us to restrict an
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* overload to only happen for a type SomeContainer<T>, which we then can use to
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* make sure that an ImplCycleCollectionIndexedContainer overload is for a
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* particular container, const or not.
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*
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* Example:
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*
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* template <typename Container, typename Callback,
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* EnableCycleCollectionIf<Container, nsTHashtable> = nullptr>
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* inline void ImplCycleCollectionContainer(Container&& aField,
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* Callback&& aCallback) {
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* // Implementation goes here
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* }
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*/
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template <typename, template <typename...> typename>
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struct ImplCycleCollectionIsContainerT : std::false_type {};
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template <template <typename...> typename Container, typename... Args>
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struct ImplCycleCollectionIsContainerT<Container<Args...>, Container>
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: std::true_type {};
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template <typename T, template <typename...> typename Container>
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constexpr bool ImplCycleCollectionIsContainer =
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ImplCycleCollectionIsContainerT<std::remove_cvref_t<T>, Container>::value;
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template <typename T, template <typename...> typename Container>
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using EnableCycleCollectionIf =
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typename std::enable_if_t<ImplCycleCollectionIsContainer<T, Container>>*;
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#endif // nsCycleCollectionContainerParticipant_h_
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