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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 class for non-null strong pointers to reference-counted objects. */
#ifndef mozilla_OwningNonNull_h
#define mozilla_OwningNonNull_h
#include "nsCOMPtr.h"
#include "nsCycleCollectionNoteChild.h"
namespace mozilla {
// OwningNonNull<T> is similar to a RefPtr<T>, which is not null after initial
// initialization. It has a restricted interface compared to RefPtr, with some
// additional operations defined. The main use is in DOM bindings. Use it
// outside DOM bindings only if you can ensure it never escapes without being
// properly initialized, and you don't need to move it. Otherwise, use a
// RefPtr<T> instead.
//
// Compared to a plain RefPtr<T>, in particular
// - it is copyable but not movable
// - it can be constructed and assigned from T& and is convertible to T&
// implicitly
// - it cannot be cleared by the user once initialized, though it can be
// re-assigned a new (non-null) value
// - it is not convertible to bool, but there is an explicit isInitialized
// member function
//
// Beware that there are two cases where an OwningNonNull<T> actually is nullptr
// - it was default-constructed and not yet initialized
// - it was cleared during CC unlinking.
// All attempts to use it in an invalid state will trigger an assertion in debug
// builds.
//
// The original intent of OwningNonNull<T> was to implement a class with the
// same auto-conversion and annotation semantics as mozilla::dom::NonNull<T>
// (i.e. never null once you have properly initialized it, auto-converts to T&),
// but that holds a strong reference to the object involved. This was designed
// for use in DOM bindings and in particular for storing what WebIDL represents
// as InterfaceName (as opposed to `InterfaceName?`) in various containers
// (dictionaries, sequences). DOM bindings never allow a default-constructed
// uninitialized OwningNonNull to escape. RefPtr could have been used for this
// use case, just like we could have used T* instead of NonNull<T>, but it
// seemed desirable to explicitly annotate the non-null nature of the things
// involved to eliminate pointless null-checks, which otherwise tend to
// proliferate.
template <class T>
class MOZ_IS_SMARTPTR_TO_REFCOUNTED OwningNonNull {
public:
using element_type = T;
OwningNonNull() = default;
MOZ_IMPLICIT OwningNonNull(T& aValue) { init(&aValue); }
template <class U>
MOZ_IMPLICIT OwningNonNull(already_AddRefed<U>&& aValue) {
init(aValue);
}
template <class U>
MOZ_IMPLICIT OwningNonNull(RefPtr<U>&& aValue) {
init(std::move(aValue));
}
template <class U>
MOZ_IMPLICIT OwningNonNull(const OwningNonNull<U>& aValue) {
init(aValue);
}
// This is no worse than get() in terms of const handling.
operator T&() const { return ref(); }
operator T*() const { return get(); }
// Conversion to bool is always true, so delete to catch errors
explicit operator bool() const = delete;
T* operator->() const { return get(); }
T& operator*() const { return ref(); }
OwningNonNull<T>& operator=(T* aValue) {
init(aValue);
return *this;
}
OwningNonNull<T>& operator=(T& aValue) {
init(&aValue);
return *this;
}
template <class U>
OwningNonNull<T>& operator=(already_AddRefed<U>&& aValue) {
init(aValue);
return *this;
}
template <class U>
OwningNonNull<T>& operator=(RefPtr<U>&& aValue) {
init(std::move(aValue));
return *this;
}
template <class U>
OwningNonNull<T>& operator=(const OwningNonNull<U>& aValue) {
init(aValue);
return *this;
}
// Don't allow assigning nullptr, it makes no sense
void operator=(decltype(nullptr)) = delete;
T& ref() const {
MOZ_ASSERT(mInited);
MOZ_ASSERT(mPtr, "OwningNonNull<T> was set to null");
return *mPtr;
}
// Make us work with smart pointer helpers that expect a get().
T* get() const {
MOZ_ASSERT(mInited);
MOZ_ASSERT(mPtr, "OwningNonNull<T> was set to null");
return mPtr;
}
template <typename U>
void swap(U& aOther) {
mPtr.swap(aOther);
#ifdef DEBUG
mInited = mPtr;
#endif
}
// We have some consumers who want to check whether we're inited in non-debug
// builds as well. Luckily, we have the invariant that we're inited precisely
// when mPtr is non-null.
bool isInitialized() const {
MOZ_ASSERT(!!mPtr == mInited, "mInited out of sync with mPtr?");
return mPtr;
}
private:
void unlinkForCC() {
#ifdef DEBUG
mInited = false;
#endif
mPtr = nullptr;
}
// Allow ImplCycleCollectionUnlink to call unlinkForCC().
template <typename U>
friend void ImplCycleCollectionUnlink(OwningNonNull<U>& aField);
protected:
template <typename U>
void init(U&& aValue) {
mPtr = std::move(aValue);
MOZ_ASSERT(mPtr);
#ifdef DEBUG
mInited = true;
#endif
}
RefPtr<T> mPtr;
#ifdef DEBUG
bool mInited = false;
#endif
};
template <typename T>
inline void ImplCycleCollectionUnlink(OwningNonNull<T>& aField) {
aField.unlinkForCC();
}
template <typename T>
inline void ImplCycleCollectionTraverse(
nsCycleCollectionTraversalCallback& aCallback, OwningNonNull<T>& aField,
const char* aName, uint32_t aFlags = 0) {
CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
}
} // namespace mozilla
// Declared in nsCOMPtr.h
template <class T>
template <class U>
nsCOMPtr<T>::nsCOMPtr(const mozilla::OwningNonNull<U>& aOther)
: nsCOMPtr(aOther.get()) {}
template <class T>
template <class U>
nsCOMPtr<T>& nsCOMPtr<T>::operator=(const mozilla::OwningNonNull<U>& aOther) {
return operator=(aOther.get());
}
// Declared in mozilla/RefPtr.h
template <class T>
template <class U>
RefPtr<T>::RefPtr(const mozilla::OwningNonNull<U>& aOther)
: RefPtr(aOther.get()) {}
template <class T>
template <class U>
RefPtr<T>& RefPtr<T>::operator=(const mozilla::OwningNonNull<U>& aOther) {
return operator=(aOther.get());
}
#endif // mozilla_OwningNonNull_h
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