Question

What is the best way to send a float in a windows message using c++ casting operators?

The reason I ask is that the approach which first occurred to me did not work. For the record I’m using the standard win32 function to send messages:

PostWindowMessage(UINT nMsg, WPARAM wParam, LPARAM lParam)

What does not work:

• Using static_cast<WPARAM>() does not work since WPARAM is typedef’ed to UINT_PTR and will do a numeric conversion from float to int, effectively truncating the value of the float.
• Using reinterpret_cast<WPARAM>() does not work since it is meant for use with pointers and fails with a compilation error.

I can think of two workarounds at the moment:

• Using reinterpret_cast in conjunction with the address of operator:

  float f = 42.0f;
  ::PostWindowMessage(WM_SOME_MESSAGE, *reinterpret_cast<WPARAM*>(&f), 0);
  

• Using an union:

  union { WPARAM wParam, float f };
  f = 42.0f;
  ::PostWindowMessage(WM_SOME_MESSAGE, wParam, 0);
  

Which of these are preffered? Are there any other more elegant way of accomplishing this?

Answer 1

Use reinterpret_cast< WPARAM &>(f). This cast is not restricted to pointers, it also works with references.