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      1 // Copyright 2016 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 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
      6 
      7 #ifndef CORE_FXCRT_CFX_SHARED_COPY_ON_WRITE_H_
      8 #define CORE_FXCRT_CFX_SHARED_COPY_ON_WRITE_H_
      9 
     10 #include "core/fxcrt/cfx_retain_ptr.h"
     11 #include "core/fxcrt/fx_system.h"
     12 
     13 // A shared object with Copy on Write semantics that makes it appear as
     14 // if each one were independent.
     15 template <class ObjClass>
     16 class CFX_SharedCopyOnWrite {
     17  public:
     18   CFX_SharedCopyOnWrite() {}
     19   CFX_SharedCopyOnWrite(const CFX_SharedCopyOnWrite& other)
     20       : m_pObject(other.m_pObject) {}
     21   ~CFX_SharedCopyOnWrite() {}
     22 
     23   template <typename... Args>
     24   ObjClass* Emplace(Args... params) {
     25     m_pObject.Reset(new CountedObj(params...));
     26     return m_pObject.Get();
     27   }
     28 
     29   CFX_SharedCopyOnWrite& operator=(const CFX_SharedCopyOnWrite& that) {
     30     if (*this != that)
     31       m_pObject = that.m_pObject;
     32     return *this;
     33   }
     34 
     35   void SetNull() { m_pObject.Reset(); }
     36   const ObjClass* GetObject() const { return m_pObject.Get(); }
     37 
     38   template <typename... Args>
     39   ObjClass* GetPrivateCopy(Args... params) {
     40     if (!m_pObject)
     41       return Emplace(params...);
     42     if (!m_pObject->HasOneRef())
     43       m_pObject.Reset(new CountedObj(*m_pObject));
     44     return m_pObject.Get();
     45   }
     46 
     47   bool operator==(const CFX_SharedCopyOnWrite& that) const {
     48     return m_pObject == that.m_pObject;
     49   }
     50   bool operator!=(const CFX_SharedCopyOnWrite& that) const {
     51     return !(*this == that);
     52   }
     53   explicit operator bool() const { return !!m_pObject; }
     54 
     55  private:
     56   class CountedObj : public ObjClass {
     57    public:
     58     template <typename... Args>
     59     // NOLINTNEXTLINE(runtime/explicit)
     60     CountedObj(Args... params) : ObjClass(params...), m_RefCount(0) {}
     61 
     62     CountedObj(const CountedObj& src) : ObjClass(src), m_RefCount(0) {}
     63     ~CountedObj() { m_RefCount = 0; }
     64 
     65     bool HasOneRef() const { return m_RefCount == 1; }
     66     void Retain() { m_RefCount++; }
     67     void Release() {
     68       ASSERT(m_RefCount);
     69       if (--m_RefCount == 0)
     70         delete this;
     71     }
     72 
     73    private:
     74     // To ensure ref counts do not overflow, consider the worst possible case:
     75     // the entire address space contains nothing but pointers to this object.
     76     // Since the count increments with each new pointer, the largest value is
     77     // the number of pointers that can fit into the address space. The size of
     78     // the address space itself is a good upper bound on it.
     79     intptr_t m_RefCount;
     80   };
     81 
     82   CFX_RetainPtr<CountedObj> m_pObject;
     83 };
     84 
     85 #endif  // CORE_FXCRT_CFX_SHARED_COPY_ON_WRITE_H_
     86