Function to cast event types without copying

[wolfiestyle]

I use this in my code, and I think it would be a great addition to the library:

fn cast_event<T: From<xlib::XEvent>>(ev: &xlib::XEvent) -> &T
{
    unsafe { mem::transmute(ev) }
}

it should be safe because the size of XEvent is >= the size of T (tested elsewhere). Using From<XEvent> does an unnecessary copy and this provides an alternative.

[ghost]

This could be unsafe if another crate implements From<XEvent> for a type not defined in x11-rs. Another way to accomplish the same thing could be to add a function for each type such as:

impl XEvent {
    pub fn as_x_client_message_event (&self) -> &XClientMessageEvent {
        unsafe { mem::transmute(self) }
    }
}

And vice versa:

impl XClientMessageEvent {
    pub fn as_x_event (&self) -> &XEvent {
        unsafe { mem::transmute(self) }
    }
}

A local macro can be used to make these much less verbose.

I recall reading somewhere that it is generally preferred to pass structs by value if they are copyable, but I wonder if the Xlib structures are big enough (I believe XEvent is 128 bytes) for the impact to be significant. Either way, it doesn't hurt to have this option available.

[ghost]

On second thought, this would only work when converting from XEvent to the other types. The reason is that these structs each have different sizes, which would cause undefined behavior after converting the reference to a larger struct and trying to access an element near the end. We could implement AsRef to get a reference to a concrete event type from XEvent, but not the other way around.

[wolfiestyle]

On C this is handled by using an union. What about converting it into a Rust enum?

[ghost]

With the changes in 4a0c484, you should be able to use XEvent like this:

let mut xevent: XEvent = mem::zeroed();

{
    let mut xclient: &mut XClientMessageEvent = xevent.as_mut();
    xclient.type_ = xlib::ClientMessage;
    xclient.message_type = wm_protocols;
    xclient.format = 32;
    xclient.data.set_long(0, wm_delete_window);
}

xlib::XSendEvent(display, window, xlib::False, 0, &mut xevent);

[wolfiestyle]

A pretty nice solution. But XGenericEventCookie was left out.

[ghost]

22dff4d and the changes are published :D

[wolfiestyle]

Thanks!

BTW I think the union RFC was approved some time ago, so we won't need this type of stuff in the future.