# V301. Unexpected function overloading behavior. See N argument of function 'foo' in derived class 'derived' and base class 'base'.

The analyzer found a possible error related to the changes in the overriding virtual functions behavior.

The example of the change in the virtual function behavior.

    class CWinApp {
      ...
      virtual void WinHelp(DWORD_PTR dwData, UINT nCmd);
      ...
    };
    class CSampleApp : public CWinApp {
      ...
      virtual void WinHelp(DWORD dwData, UINT nCmd);
      ...
    };

It is the common example which the developer may face while porting his application to the 64-bit architecture. Let's follow the life-cycle of the developing of some application. Suppose it was being developed for Visual Studio 6.0. at first when the function 'WinHelp' in class 'CWinApp' had the following prototype:

    virtual void WinHelp(DWORD dwData, UINT nCmd = HELP_CONTEXT);

It would be absolutely correct to implement the overlap of the virtual function in class 'CSampleApp', as it is shown in the example. Then the project was placed into Visual Studio 2005 where the prototype of the function in class 'CWinApp' underwent changes that consist in replacing 'DWORD' type with 'DWORD_PTR' type. On the 32-bit platform this program will continue to work properly for here 'DWORD' and 'DWORD_PTR' types coincide. Troubles will occur while compiling this code for the 64-bit platform. We get two functions with the same names but with different parameters the result of which is that the user's code won't be called.

The analyzer allows to find such errors the correction of which is not difficult. It is enough to change the function prototype in the successor class as follows:

    class CSampleApp : public CWinApp {
      ...
      virtual void WinHelp(DWORD_PTR dwData, UINT nCmd);
      ...
    };

Additional materials on this topic:

- 64-bit Lessons. Lesson 12. Pattern 4. Virtual functions.