If there are big derived class graphs with virtual functions in your program, there is a risk to use inattentively arguments of different types but these types actually coincide on the 32-bit system. For example, in the base class you use size_t type as an argument of a virtual function and in the derived class type unsigned. So this code will be incorrect on the 64-bit system.

But an error like this doesn’t necessarily hide in big derived class graphs and here it is one of the examples.

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

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

Let’s follow the life-cycle of the development of some applications. Imagine that firstly it was being developed for Microsoft Visual C++ 6.0 when WinHelp function in CWinApp class had the following prototype:

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

It was absolutely correct to carry out an overlap of the virtual function in CSampleApp class as it is shown in the example. Then the project was ported into Microsoft Visual C++ 2005 where the function prototype in CWinApp class had undergone some changes which consisted in the replacement of DWORD type with DWORD_PTR type. On the 32-bit system the program will work absolutely correctly for here types DWORD and DWORD_PTR coincide. Troubles will appear during the compilation of the given code for the 64-bit platform. We’ll gave two functions with the same name but different parameters and as a result the user’s code won’t be executed.

The correction consists in the use of the same types in the corresponding virtual functions.

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