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// Copyright 2020 The Dawn Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef COMMON_REFBASE_H_
#define COMMON_REFBASE_H_
#include "common/Assert.h"
#include "common/Compiler.h"
#include <type_traits>
#include <utility>
// A common class for various smart-pointers acting on referenceable/releasable pointer-like
// objects. Logic for each specialization can be customized using a Traits type that looks
// like the following:
//
// struct {
// static constexpr T kNullValue = ...;
// static void Reference(T value) { ... }
// static void Release(T value) { ... }
// };
//
// RefBase supports
template <typename T, typename Traits>
class RefBase {
public:
// Default constructor and destructor.
RefBase() : mValue(Traits::kNullValue) {
}
~RefBase() {
Release(mValue);
}
// Constructors from nullptr.
constexpr RefBase(std::nullptr_t) : RefBase() {
}
RefBase<T, Traits>& operator=(std::nullptr_t) {
Set(Traits::kNullValue);
return *this;
}
// Constructors from a value T.
RefBase(T value) : mValue(value) {
Reference(value);
}
RefBase<T, Traits>& operator=(const T& value) {
Set(value);
return *this;
}
// Constructors from a RefBase<T>
RefBase(const RefBase<T, Traits>& other) : mValue(other.mValue) {
Reference(other.mValue);
}
RefBase<T, Traits>& operator=(const RefBase<T, Traits>& other) {
Set(other.mValue);
return *this;
}
RefBase(RefBase<T, Traits>&& other) {
mValue = other.Detach();
}
RefBase<T, Traits>& operator=(RefBase<T, Traits>&& other) {
if (&other != this) {
Release(mValue);
mValue = other.Detach();
}
return *this;
}
// Constructors from a RefBase<U>. Note that in the *-assignment operators this cannot be the
// same as `other` because overload resolution rules would have chosen the *-assignement
// operators defined with `other` == RefBase<T, Traits>.
template <typename U, typename UTraits, typename = typename std::is_convertible<U, T>::type>
RefBase(const RefBase<U, UTraits>& other) : mValue(other.mValue) {
Reference(other.mValue);
}
template <typename U, typename UTraits, typename = typename std::is_convertible<U, T>::type>
RefBase<T, Traits>& operator=(const RefBase<U, UTraits>& other) {
Set(other.mValue);
return *this;
}
template <typename U, typename UTraits, typename = typename std::is_convertible<U, T>::type>
RefBase(RefBase<U, UTraits>&& other) {
mValue = other.Detach();
}
template <typename U, typename UTraits, typename = typename std::is_convertible<U, T>::type>
RefBase<T, Traits>& operator=(RefBase<U, UTraits>&& other) {
Release(mValue);
mValue = other.Detach();
return *this;
}
// Comparison operators.
bool operator==(const T& other) const {
return mValue == other;
}
bool operator!=(const T& other) const {
return mValue != other;
}
const T operator->() const {
return mValue;
}
T operator->() {
return mValue;
}
// Smart pointer methods.
const T& Get() const {
return mValue;
}
T& Get() {
return mValue;
}
T Detach() DAWN_NO_DISCARD {
T value{std::move(mValue)};
mValue = Traits::kNullValue;
return value;
}
void Acquire(T value) {
Release(mValue);
mValue = value;
}
T* InitializeInto() DAWN_NO_DISCARD {
ASSERT(mValue == Traits::kNullValue);
return &mValue;
}
private:
// Friend is needed so that instances of RefBase<U> can call Reference and Release on
// RefBase<T>.
template <typename U, typename UTraits>
friend class RefBase;
static void Reference(T value) {
if (value != Traits::kNullValue) {
Traits::Reference(value);
}
}
static void Release(T value) {
if (value != Traits::kNullValue) {
Traits::Release(value);
}
}
void Set(T value) {
if (mValue != value) {
// Ensure that the new value is referenced before the old is released to prevent any
// transitive frees that may affect the new value.
Reference(value);
Release(mValue);
mValue = value;
}
}
T mValue;
};
#endif // COMMON_REFBASE_H_