Add new example for Rc.

src/SUMMARY.md

@@ -175,6 +175,7 @@
     - [HashMap](std/hash.md)
         - [Alternate/custom key types](std/hash/alt_key_types.md)
         - [HashSet](std/hash/hashset.md)
+    - [`Rc`](std/rc.md)
 
 - [Std misc](std_misc.md)
     - [Threads](std_misc/threads.md)

src/std/rc.md

@@ -0,0 +1,54 @@
+# `Rc`
+
+When multiple ownership is needed, `Rc`(Reference Counting) can be used. `Rc` keeps track of the number of the references which means the number of owners of the value wrapped inside an `Rc`. 
+
+Reference count of an `Rc` increases by 1 whenever an `Rc` is cloned, and decreases by 1 whenever one cloned `Rc` is dropped out of the scope. When an `Rc`'s reference count becomes zero, which means there are no owners remained, both the `Rc` and the value are all dropped. 
+
+Cloning an `Rc` never do a deep copy. Cloning creates just another pointer to the wrapped value, and increments the count.
+
+```rust,editable
+use std::rc::Rc;
+
+fn main() {
+    let rc_examples = "Rc examples".to_string();
+    {
+        println!("--- rc_a is created ---");
+
+        let rc_a: Rc<String> = Rc::new(rc_examples);
+        println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
+
+        {
+            println!("--- rc_a is cloned to rc_b ---");
+
+            let rc_b: Rc<String> = Rc::clone(&rc_a);
+            println!("Reference Count of rc_b: {}", Rc::strong_count(&rc_b));
+            println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
+
+            // Two `Rc`s are equal if their inner values are equal
+            println!("rc_a and rc_b are equal: {}", rc_a.eq(&rc_b));
+
+            // We can use methods of a value directly
+            println!("Length of the value inside rc_a: {}", rc_a.len());
+            println!("Value of rc_b: {}", rc_b);
+
+            println!("--- rc_b is dropped out of scope ---");
+        }
+
+        println!("Reference Count of rc_a: {}", Rc::strong_count(&rc_a));
+
+        println!("--- rc_a is dropped out of scope ---");
+    }
+
+    // Error! `rc_examples` already moved into `rc_a`
+    // And when `rc_a` is dropped, `rc_examples` is dropped together
+    // println!("rc_examples: {}", rc_examples);
+    // TODO ^ Try uncommenting this line
+}
+```
+
+### See also
+
+[std::rc][1] and [Arc][2].
+
+[1]: https://doc.rust-lang.org/std/rc/index.html
+[2]: https://doc.rust-lang.org/std/sync/struct.Arc.html