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// Copyright 2017 PDFium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef CORE_FXCRT_UNOWNED_PTR_H_
#define CORE_FXCRT_UNOWNED_PTR_H_
#include <functional>
#include <memory>
#include <type_traits>
#include <utility>
// UnownedPtr is a smart pointer class that behaves very much like a
// standard C-style pointer. The advantages of using it over raw
// pointers are:
//
// 1. It documents the nature of the pointer with no need to add a comment
// explaining that is it // Not owned. Additionally, an attempt to delete
// an unowned ptr will fail to compile rather than silently succeeding,
// since it is a class and not a raw pointer.
//
// 2. When built for a memory tool like ASAN, the class provides a destructor
// which checks that the object being pointed to is still alive.
//
// Hence, when using UnownedPtr, no dangling pointers are ever permitted,
// even if they are not de-referenced after becoming dangling. The style of
// programming required is that the lifetime an object containing an
// UnownedPtr must be strictly less than the object to which it points.
//
// The same checks are also performed at assignment time to prove that the
// old value was not a dangling pointer, either.
//
// The array indexing operation [] is not supported on an unowned ptr,
// because an unowned ptr expresses a one to one relationship with some
// other heap object. Use pdfium::span<> for the cases where indexing
// into an unowned array is desired, which performs the same checks.
namespace fxcrt {
template <class T>
class UnownedPtr {
public:
constexpr UnownedPtr() noexcept = default;
constexpr UnownedPtr(const UnownedPtr& that) noexcept = default;
template <typename U>
explicit constexpr UnownedPtr(U* pObj) noexcept : m_pObj(pObj) {}
// Deliberately implicit to allow returning nullptrs.
// NOLINTNEXTLINE(runtime/explicit)
constexpr UnownedPtr(std::nullptr_t ptr) noexcept {}
~UnownedPtr() { ProbeForLowSeverityLifetimeIssue(); }
UnownedPtr& operator=(T* that) noexcept {
ProbeForLowSeverityLifetimeIssue();
m_pObj = that;
return *this;
}
UnownedPtr& operator=(const UnownedPtr& that) noexcept {
ProbeForLowSeverityLifetimeIssue();
if (*this != that)
m_pObj = that.Get();
return *this;
}
bool operator==(const UnownedPtr& that) const { return Get() == that.Get(); }
bool operator!=(const UnownedPtr& that) const { return !(*this == that); }
bool operator<(const UnownedPtr& that) const {
return std::less<T*>()(Get(), that.Get());
}
template <typename U>
bool operator==(const U* that) const {
return Get() == that;
}
template <typename U>
bool operator!=(const U* that) const {
return !(*this == that);
}
T* Get() const noexcept { return m_pObj; }
T* Release() {
ProbeForLowSeverityLifetimeIssue();
T* pTemp = nullptr;
std::swap(pTemp, m_pObj);
return pTemp;
}
explicit operator bool() const { return !!m_pObj; }
T& operator*() const { return *m_pObj; }
T* operator->() const { return m_pObj; }
private:
inline void ProbeForLowSeverityLifetimeIssue() {
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
if (m_pObj)
reinterpret_cast<const volatile uint8_t*>(m_pObj)[0];
#endif
}
T* m_pObj = nullptr;
};
template <typename T, typename U>
inline bool operator==(const U* lhs, const UnownedPtr<T>& rhs) {
return rhs == lhs;
}
template <typename T, typename U>
inline bool operator!=(const U* lhs, const UnownedPtr<T>& rhs) {
return rhs != lhs;
}
} // namespace fxcrt
using fxcrt::UnownedPtr;
#endif // CORE_FXCRT_UNOWNED_PTR_H_
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