added more strings and todos

src/string.rs

@@ -1,3 +1,4 @@
+use std::rc::Rc;
 
 // when a String is passed to this function a move occurs:
 // BUT only the stack allocation (ptr, len, cap) is copied over
@@ -10,6 +11,8 @@ fn takes_string(_s: String) {
 
 }
 
+// You can do this, but you probably want &str
+// UNLESS you want `&mut String`
 #[allow(dead_code)]
 fn takes_string_ref(_s: &String) {
 
@@ -42,6 +45,12 @@ fn _get_static_str() -> &'static str {
     "hello"
 }
 
+// The #[cfg(test)] annotation on the tests module tells Rust to compile
+// and run the test code only when you run cargo test, not when you run cargo build
+#[cfg(test)]
+mod tests {
+    use super::*; // need access to functions above
+
     #[test]
     fn test_string() {
         let s = String::new(); // no heap allocation until we add characters (capacity > 0)
@@ -78,6 +87,33 @@ fn test_string() {
         let _s5 = String::from("literal is copied to heap"); // data copied to heap
         let _s6 = String::from(s_lit); //underlying data copied to heap
 
-    // todo: Cow
+        // Box<str> is owned, heap allocated, fixed size, immutable
-}
+        // where as `&str` is borrowed view, linked to source, immutable
+        // both have (ptr, len) -> underlying mem
+        let _s7 : Box<str> = "hello".into();
 
+        // Rc<str> (single-threaded reference counting)
+        // HEAP memory layout:
+        // * counter (strong)
+        // * counter (weak) 
+        // * len
+        // * data
+        // STACK memory:
+        // * ptr
+        // * len
+        //
+        // Why are there two lengths?
+        let rc_str : Rc<str> = Rc::from("rc_str");
+        // let rc_str2 : Rc<str> = rc_str; // this moves
+        let rc_str2 : Rc<str> = rc_str.clone(); // copy ptr, increment counter
+        println!("printing {}", rc_str);
+        println!("printing {}", rc_str);
+        println!("printing {}", rc_str2);
+
+        // todo: Arc<str>
+        // todo: Cow
+        // todo: byte strings: &[u8] and Vec<u8>
+        // todo: Os & platform strings: OsStr & OsString, Path & PathBuf
+        // todo: ffi strings: CString & CStr
+    }
+}