Confused on C++ casting

Question

I have been reading a lot about C++ casting and I am starting to get confused because I have always used C style casting.

I have read that C style casting should be avoided in C++ and that reinterpret_cast is very very dangerous and should not be used whenever there is an alternative. On the contrary to not using reinterpret_cast, I have seen it used many times on MSDN in their sample code. This leads me to ask my first question, when is it ok to use reinterpret_cast?

For example:

LRESULT CALLBACK WndProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam)
{
   switch (Msg)
   {
      case WM_CREATE:
      {
          LPCREATESTRUCT lpCreateStruct = reinterpret_cast<LPCREATESTRUCT>(lParam); 
          return 0;
      }
   }

   ...
}

If that is not ok, then how would I cast the LPARAM value to a pointer using only static, dynamic, and/or const casting?

Also: If reinterpret_cast is not portable, how would I rewrite it to be portable (for good practice)

Answer 1

Using reinterpret_cast is acceptable if you know that the pointer was originally of the destination type. Any other use is taking advantage of implementation-dependent behavior, although in many cases this is necessary and useful, such as casting a pointer to a structure into a pointer to bytes so that it can be serialized.

It is considered dangerous because it does no checking, either at compile-time or at runtime. If you make a mistake, it can and will crash and burn horribly, and be difficult to debug. You are essentially telling the compiler "I know better than you what this actually is, so just compile the code and let me worry about the consequences."