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      1 // Copyright 2017 PDFium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #ifndef CORE_FXCRT_UNOWNED_PTR_H_
      6 #define CORE_FXCRT_UNOWNED_PTR_H_
      7 
      8 #include <functional>
      9 #include <memory>
     10 #include <type_traits>
     11 #include <utility>
     12 
     13 // UnownedPtr is a smart pointer class that behaves very much like a
     14 // standard C-style pointer. The advantages of using it over raw
     15 // pointers are:
     16 //
     17 // 1. It documents the nature of the pointer with no need to add a comment
     18 //    explaining that is it // Not owned. Additionally, an attempt to delete
     19 //    an unowned ptr will fail to compile rather than silently succeeding,
     20 //    since it is a class and not a raw pointer.
     21 //
     22 // 2. When built for a memory tool like ASAN, the class provides a destructor
     23 //    which checks that the object being pointed to is still alive.
     24 //
     25 // Hence, when using UnownedPtr, no dangling pointers are ever permitted,
     26 // even if they are not de-referenced after becoming dangling. The style of
     27 // programming required is that the lifetime an object containing an
     28 // UnownedPtr must be strictly less than the object to which it points.
     29 //
     30 // The same checks are also performed at assignment time to prove that the
     31 // old value was not a dangling pointer, either.
     32 //
     33 // The array indexing operation [] is not supported on an unowned ptr,
     34 // because an unowned ptr expresses a one to one relationship with some
     35 // other heap object.
     36 
     37 namespace fxcrt {
     38 
     39 template <class T>
     40 class UnownedPtr {
     41  public:
     42   UnownedPtr() = default;
     43   UnownedPtr(const UnownedPtr& that) : UnownedPtr(that.Get()) {}
     44 
     45   template <typename U>
     46   explicit UnownedPtr(U* pObj) : m_pObj(pObj) {}
     47 
     48   // Deliberately implicit to allow returning nullptrs.
     49   // NOLINTNEXTLINE(runtime/explicit)
     50   UnownedPtr(std::nullptr_t ptr) {}
     51 
     52   ~UnownedPtr() { ProbeForLowSeverityLifetimeIssue(); }
     53 
     54   UnownedPtr& operator=(T* that) {
     55     ProbeForLowSeverityLifetimeIssue();
     56     m_pObj = that;
     57     return *this;
     58   }
     59 
     60   UnownedPtr& operator=(const UnownedPtr& that) {
     61     ProbeForLowSeverityLifetimeIssue();
     62     if (*this != that)
     63       m_pObj = that.Get();
     64     return *this;
     65   }
     66 
     67   bool operator==(const UnownedPtr& that) const { return Get() == that.Get(); }
     68   bool operator!=(const UnownedPtr& that) const { return !(*this == that); }
     69   bool operator<(const UnownedPtr& that) const {
     70     return std::less<T*>()(Get(), that.Get());
     71   }
     72 
     73   template <typename U>
     74   bool operator==(const U* that) const {
     75     return Get() == that;
     76   }
     77 
     78   template <typename U>
     79   bool operator!=(const U* that) const {
     80     return !(*this == that);
     81   }
     82 
     83   T* Get() const { return m_pObj; }
     84 
     85   T* Release() {
     86     ProbeForLowSeverityLifetimeIssue();
     87     T* pTemp = nullptr;
     88     std::swap(pTemp, m_pObj);
     89     return pTemp;
     90   }
     91 
     92   explicit operator bool() const { return !!m_pObj; }
     93   T& operator*() const { return *m_pObj; }
     94   T* operator->() const { return m_pObj; }
     95 
     96  private:
     97   inline void ProbeForLowSeverityLifetimeIssue() {
     98 #if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
     99     if (m_pObj)
    100       reinterpret_cast<const volatile uint8_t*>(m_pObj)[0];
    101 #endif
    102   }
    103 
    104   T* m_pObj = nullptr;
    105 };
    106 
    107 template <typename T, typename U>
    108 inline bool operator==(const U* lhs, const UnownedPtr<T>& rhs) {
    109   return rhs == lhs;
    110 }
    111 
    112 template <typename T, typename U>
    113 inline bool operator!=(const U* lhs, const UnownedPtr<T>& rhs) {
    114   return rhs != lhs;
    115 }
    116 
    117 }  // namespace fxcrt
    118 
    119 using fxcrt::UnownedPtr;
    120 
    121 #endif  // CORE_FXCRT_UNOWNED_PTR_H_
    122