Add NSString example

Important to showcase how to implement Encode for Objective-C objects that we don't know the internal representation of.

Author: madsmtm

examples/ns_string.rs

@@ -0,0 +1,25 @@
+use objc_encode::{Encode, Encoding};
+
+/// We don't know the size of NSString, so we can only hold pointers to it.
+///
+/// TODO: Use [`extern type`][rfc-1861] when that gets stabilized.
+///
+/// [rfc-1861]: https://rust-lang.github.io/rfcs/1861-extern-types.html
+#[repr(C)]
+struct NSString {
+    _priv: [u8; 0],
+}
+
+/// Implement `Encode` for references.
+///
+/// This also implements for `*mut NSString` and `Option<&mut NSString>`.
+unsafe impl<'a> Encode for &'a NSString {
+    const ENCODING: Encoding<'static> = Encoding::Object;
+}
+
+/// Implement `Encode` for mutable references.
+///
+/// This also implements for `*mut NSString` and `Option<&mut NSString>`.
+unsafe impl<'a> Encode for &'a mut NSString {
+    const ENCODING: Encoding<'static> = Encoding::Object;
+}