107 lines
3.2 KiB
C++
107 lines
3.2 KiB
C++
// Copyright 2016 The PDFium Authors
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef CORE_FXCRT_MAYBE_OWNED_H_
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#define CORE_FXCRT_MAYBE_OWNED_H_
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#include <memory>
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#include <utility>
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#include "core/fxcrt/unowned_ptr.h"
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#include "third_party/abseil-cpp/absl/types/variant.h"
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namespace fxcrt {
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// A template that can hold either owned or unowned references, and cleans up
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// appropriately. Possibly the most pernicious anti-pattern imaginable, but
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// it crops up throughout the codebase due to a desire to avoid copying-in
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// objects or data.
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template <typename T, typename D = std::default_delete<T>>
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class MaybeOwned {
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public:
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using OwnedType = std::unique_ptr<T, D>;
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using UnownedType = UnownedPtr<T>;
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MaybeOwned() = default;
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explicit MaybeOwned(T* ptr) : ptr_(UnownedType(ptr)) {}
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explicit MaybeOwned(const UnownedType& ptr) : ptr_(ptr) {}
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explicit MaybeOwned(OwnedType ptr) : ptr_(std::move(ptr)) {}
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MaybeOwned(const MaybeOwned& that) = delete;
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MaybeOwned(MaybeOwned&& that) noexcept = default;
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MaybeOwned& operator=(const MaybeOwned& that) = delete;
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MaybeOwned& operator=(MaybeOwned&& that) noexcept = default;
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~MaybeOwned() = default;
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void Reset(T* ptr = nullptr) { ptr_ = UnownedType(ptr); }
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void Reset(OwnedType ptr) { ptr_ = std::move(ptr); }
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bool IsOwned() const { return absl::holds_alternative<OwnedType>(ptr_); }
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// Helpful for untangling a collection of intertwined MaybeOwned<>.
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void ResetIfUnowned() {
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if (!IsOwned())
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Reset();
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}
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T* Get() const& {
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return absl::visit([](const auto& obj) { return obj.get(); }, ptr_);
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}
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T* Get() && {
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auto local_variable_preventing_move_elision = std::move(ptr_);
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return absl::visit([](const auto& obj) { return obj.get(); },
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local_variable_preventing_move_elision);
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}
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// Downgrades to unowned, caller takes ownership.
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OwnedType Release() {
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auto result = std::move(absl::get<OwnedType>(ptr_));
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ptr_ = UnownedType(result.get());
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return result;
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}
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// Downgrades to empty, caller takes ownership.
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OwnedType ReleaseAndClear() {
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auto result = std::move(absl::get<OwnedType>(ptr_));
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ptr_ = UnownedType();
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return result;
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}
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MaybeOwned& operator=(T* ptr) {
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Reset(ptr);
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return *this;
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}
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MaybeOwned& operator=(const UnownedType& ptr) {
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Reset(ptr);
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return *this;
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}
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MaybeOwned& operator=(OwnedType ptr) {
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Reset(std::move(ptr));
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return *this;
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}
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bool operator==(const MaybeOwned& that) const { return Get() == that.Get(); }
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bool operator==(const OwnedType& ptr) const { return Get() == ptr.get(); }
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bool operator==(T* ptr) const { return Get() == ptr; }
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bool operator!=(const MaybeOwned& that) const { return !(*this == that); }
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bool operator!=(const OwnedType ptr) const { return !(*this == ptr); }
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bool operator!=(T* ptr) const { return !(*this == ptr); }
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explicit operator bool() const { return !!Get(); }
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T& operator*() const { return *Get(); }
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T* operator->() const { return Get(); }
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private:
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absl::variant<UnownedType, OwnedType> ptr_;
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};
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} // namespace fxcrt
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using fxcrt::MaybeOwned;
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#endif // CORE_FXCRT_MAYBE_OWNED_H_
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