pub type CPLQuadTreeForeachFunc = Option<unsafe extern "C" fn(pElt: *mut c_void, pUserData: *mut c_void) -> c_int>;
pub enum CPLQuadTreeForeachFunc { None, Some(unsafe extern "C" fn(*mut c_void, *mut c_void) -> i32), }
No value.
Some value of type T.
T