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