24. Override and final specifiers should become your new friends

The fragment is taken from the MFC library. The error is detected by the following PVS-Studio diagnostic: V301 Unexpected function overloading behavior. See first argument of function 'WinHelpW' in derived class 'CFrameWndEx' and base class 'CWnd'.

class CWnd : public CCmdTarget {
  ....
  virtual void WinHelp(DWORD_PTR dwData,
                       UINT nCmd = HELP_CONTEXT);
  ....
};
class CFrameWnd : public CWnd {
  ....
};
class CFrameWndEx : public CFrameWnd {
  ....
  virtual void WinHelp(DWORD dwData,
                       UINT nCmd = HELP_CONTEXT);
  ....
};

Explanation

When you override a virtual function it's quite easy to make an error in the signature and to define a new function, which won't be in any way connected with the function in the base class. There can be various errors in this case.

1. Another type is used in the parameter of the overridden function.
2. The overridden function has a different number of parameters, this can be especially crucial when there are many parameters.
3. The overridden function differs in const modifier.
4. The base class function is not a virtual one. It was assumed that the function in the derived class would override it in the base class, but in reality it hides it.

The same error can occur during the change of types or parameter quantity in the existing code, when the programmer changed the virtual function signature in almost the entire hierarchy, but forgot to do it in some derived class.

This error can appear particularly often during the porting process to the 64-bit platform when replacing the DWORD type with DWORD_PTR, LONG with LONG_PTR and so on. Details. This is exactly our case.

Even in the case of such an error the 32-bit system will work correctly, as both DWORD and DWORD_PTR are synonyms of unsigned long; but in 64-bit version there will be an error because DWORD_PTR is a synonym of unsigned __int64 there.

Correct code

class CFrameWndEx : public CFrameWnd {
  ....
  virtual void WinHelp(DWORD_PTR dwData,
                       UINT nCmd = HELP_CONTEXT) override;
  ....
};

Recommendation

Now we have a way to protect ourselves from the error we described above. Two new specifiers were added in C++11:

• Override - to indicate that the method is overriding a virtual method in a base class
• Final - to indicate that derived classes do not need to override this virtual method.

We are interested in the override specifier. This is an indication for the compiler to check if the virtual function is really overriding the base class function, and to issue an error if it isn't.

If override was used when determining the function WinHelp in the CFrameWndEx class, we would have an error of compilation on a 64-bit version of an application. Thus the error could have been prevented at an early stage.

Always use the override specifier (or final), when overriding virtual functions. More details about override and final can be seen here:

• Cppreference.com. override specifier (since C++11)
• Cppreference.com. final specifier (since C++11)
• Wikipedia.org. Explicit overrides and final.
• Stackoverflow.com. 'override' in c++11.