How do I send floats in window messages

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 开发者_JS百科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?

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

You could use memcpy:

#include <memory.h>
int main() {
    float f = 1.0;
    int n;
    memcpy( &n, &f, sizeof(f) );
}

I don't think there is an elegant solution to this, but whatever you do, I'd wrap it in a function to make it obvious what I was up to:

int FloatToInt( float f ) {
    int n;
    assert( sizeof(n) == sizeof(f) );
    memcpy( &n, &f, sizeof(f) );
    return n;
}

I would prefer the union. It is the clearest and easiest to understand. It also probably has the least amount of undefined behavior.

I'd go the simple way of taking a pointer to your float, casting it to a pointer to UINT, and then dereferencing the resulting pointer:

WPARAM wParam = *((UINT*)(&f));

To convert it back you do the opposite. Doing the cast at pointer-level makes sure the compiler won't try any "conversion" behind your back.

float f = *((float*)(&wParam))