The Microsoft VC++ Virtual Property feature

A lot of legacy C++ code exists wherein member variables are directly exposed using the public or protected keywords instead of simple accessor / mutator functions. For example consider the following simple structure definition

typedef struct tagMyStruct 
{ 
   long m_lValue1; 
   ...             // Rest of the structure definition. 
} SMyStruct; 

Scattered throughout the client code that uses this structure will be code like the following:

SMyStruct       MyStruct; 
long            lTempValue; 

MyStruct.m_lValue1 = 100;       // Or any other value that is to be assigned to it. 
... 
lTempValue = MyStruct.m_lValue1; 

Now if the module that has all the above code is suddenly required to be used in a thread safe environment, you run into a problem. Because no accessor or mutator functions exist, you cannot just put a critical section (or mutex) within the definition of SMyStruct and protect m_lValue1 or any of the other public member variables.

If it is guaranteed that you will be using the Microsoft Visual C++ compiler then there is an easy solution at hand.

Just enhance the structure as follows:

typedef struct tagMyStruct 
{ 
   __declspec(property(get=GetValue1, put=PutValue1))

   long  m_lValue1; 
   ...                // Rest of the structure definition. 

   long GetValue1() 
   { 
      // Lock critical section 

      return m_lInternalValue1; 

      // Unlock critical section. 
   } 

   void PutValue1(long lValue) 
   { 
      // Lock critical section 

      m_lInternalValue = lValue; 

      // Unlock critical section 
   } 

private: 
        long m_lInternalValue1; 

        // Define critical section member variable. 
} SMyStruct; 

For code such as

MyStruct.m_lValue1 = 100

MyStruct.PutValue1(100)

For code like

lTempValue = MyStruct.m_lValue1

lTempValue = MyStruct.GetValue1()

The possibilities this opens up are many. You can even use this to add reference counting to legacy structures and classes.

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