Instead of a Little Program today, I'm going to answer a Little Question. This is a common beginner question, but I figure I'll just spell it out right here for posterity.

First of all, you probably noticed that you can't do this:

class CountWindows
{
public:
  int CountThem();

private:
  BOOL CALLBACK WndEnumProc(HWND hwnd, LPARAM lParam);

  int m_count;
};

BOOL CountWindows::WndEnumProc(HWND hwnd, LPARAM lParam)
{
   m_count++;
   return TRUE;
}

int CountWindows::CountThem()
{
  m_count = 0;
  EnumWindows(WndEnumProc, 0); // compiler error here
  return m_count;
}

That's because the WNDENUMPROC is declared as a so-called free function, but member functions are not free. Neither are function objects (also known as functors) so you can't use a boost::function as a window procedure either. The reason is that member functions and functors need to have a hidden this parameter, but free functions do not have a hidden this parameter.

On the other hand, static methods are free functions. They can get away with it because they don't have a hidden this parameter either.

Win32 has a general principle that callback functions have a special parameter where you can pass any information you like (known as context or reference data), and that same value is passed back to your callback function so it knows what's going on. In practice, most people will pass a pointer to a class or structure.

In other words, the reference data parameter makes explicit what C++ hides (the this parameter).

class CountWindows
{
public:
  int CountThem();

private:
  static BOOL CALLBACK StaticWndEnumProc(HWND hwnd, LPARAM lParam);

  int m_count;
};

BOOL CountWindows::StaticWndEnumProc(HWND hwnd, LPARAM lParam)
{
   CountWindows *pThis = reinterpret_cast<CountWindows *>(lParam);
   pThis->m_count++;
   return TRUE;
}

int CountWindows::CountThem()
{
  m_count = 0;
  EnumWindows(StaticWndEnumProc, reinterpret_cast<LPARAM>(this));
  return m_count;
}

What we did was pass our this parameter explicitly as the reference data to the Enum­Windows function, and then in the callback, cast the reference data back to this so that we can use it to access our member variables.

If the Wnd­Enum­Proc is long, then it can get tedious typing pThis-> in front of everything, so a common follow-up technique is to make the static member function a wrapper that calls a normal member function.

class CountWindows
{
public:
  int CountThem();

private:
  static BOOL CALLBACK StaticWndEnumProc(HWND hwnd, LPARAM lParam);
  BOOL WndEnumProc(HWND hwnd);

  int m_count;
};

BOOL CountWindows::StaticWndEnumProc(HWND hwnd, LPARAM lParam)
{
   CountWindows *pThis = reinterpret_cast<CountWindows* >(lParam);
   return pThis->WndEnumProc(hwnd);
}

BOOL CountWindows::WndEnumProc(HWND hwnd)
{
    m_count++;
    return TRUE;
}

int CountWindows::CountThem()
{
  m_count = 0;
  EnumWindows(StaticWndEnumProc, reinterpret_cast<LPARAM>(this));
  return m_count;
}

Observe that by putting all the real work inside the traditional member function Count­Windows::Wnd­Enum­Proc, we avoid having to type pThis-> in front of everything.

This principle of using reference data to pass context through a callback is very common in Windows programming. We'll see a few more examples in the future, but I'm not going to jam all the beginner articles in a row because that would bore my regular readers.

Historical note: The term reference data was used in 16-bit Windows, but the Windows NT folks preferred to use the term context. You can tell which team introduced a particular callback function by seeing what they call that extra parameter.