added phantom example

Author: BartMassey

README.md

@@ -25,3 +25,4 @@ in a controlled repository now.
 * `addy.rs`: trait def/impl example
 * `bno-salad.rs`: trait salad example from Blandy and Orendorff
 * `custom_sum.rs`: sum example with custom `Sum` type
+* `phantom.rs`: phantom type example

phantom.rs

@@ -0,0 +1,36 @@
+use std::marker::PhantomData;
+
+#[derive(Debug)]
+struct Hash<T> {
+    h: u128,
+    p: PhantomData<T>,
+}
+
+unsafe fn hasher(start: *const u8, nbytes: usize) -> u128 {
+    let mut result: u128 = 0;
+    for i in 0..nbytes {
+        let b = *start.offset(i as isize);
+        result += b as u128;
+    }
+    result
+}
+
+impl<T> Hash<T> {
+    fn hash(val: T) -> Hash<T> {
+        let h = unsafe { hasher(
+            std::ptr::addr_of!(val) as *const u8,
+            std::mem::size_of::<T>(),
+        )};
+        Hash { h, p: PhantomData }
+    }
+}
+
+fn main() {
+    let h1 = Hash::hash((0i32,));
+    println!("{:?}", h1);
+    let h2 = Hash::hash((0u32,));
+    println!("{:?}", h2);
+    // Can't even compare these, since they are of
+    // different type.
+    // println!("{}", h1 == h2);
+}