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The Microsoft VC++ Virtual Property feature

, 6 Jan 2000
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Using the __declspec(property) method to enhance legacy code
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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 

        long m_lInternalValue1; 

        // Define critical section member variable. 
} SMyStruct; 
That's all.

For code such as

MyStruct.m_lValue1 = 100
the compiler will automatically substitute

For code like

lTempValue = MyStruct.m_lValue1
the compiler will substitute
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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About the Author

Jeremiah Talkar

United States United States
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Comments and Discussions

GeneralAnybody help pls Pinmemberbeatselva14-Dec-04 20:02 
Generalmore info on __declspec(property) PinmemberTemplMetaProg3-Jul-02 10:56 
GeneralInfinte Array PinmemberFahad22-Jul-01 15:16 
GeneralRe: Infinte Array PinmemberTemplMetaProg3-Jul-02 10:54 
GeneralRe: Infinte Array Pinmemberdhumalswati23-Aug-03 0:08 
GeneralThere is a second way to do it PinsussMh2x24-Aug-00 14:42 
GeneralRe: There is a second way to do it PinmemberAli Demir8-Jan-01 22:41 
GeneralRe: There is a second way to do it PinmemberDmitry Sychov27-Aug-01 2:13 
GeneralA set of questions about this code -> PinsussAsaf Karass15-Jul-00 2:14 
GeneralRe: A set of questions about this code -> PinsussJeremiah S. Talkar17-Jul-00 4:57 

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