Include Arc chapter.

Author: Justo Hernández Romera

src/SUMMARY.md

@@ -182,6 +182,7 @@
         - [Alternate/custom key types](std/hash/alt_key_types.md)
         - [HashSet](std/hash/hashset.md)
     - [`Rc`](std/rc.md)
+    - [`Arc`](std/arc.md)
 
 - [Std misc](std_misc.md)
     - [Threads](std_misc/threads.md)

src/std/arc.md

@@ -0,0 +1,27 @@
+# Arc
+
+When shared ownership between threads is needed, `Arc`(Atomic Reference Counted) can be used. This struct, via the `Clone` implementation can create a reference pointer for the location of a value in the memory heap while increasing the reference counter. As it shares ownership between threads, when the last reference pointer to a value is out of scope, the variable is dropped.
+
+```rust,editable
+
+fn main() {
+use std::sync::Arc;
+use std::thread;
+
+// This variable declaration is where it's value is specified.
+let apple = Arc::new("the same apple");
+
+for _ in 0..10 {
+    // Here there is no value specification as it is a pointer to a reference
+    // in the memory heap.
+    let apple = Arc::clone(&apple);
+
+    thread::spawn(move || {
+        // As Arc was used, threads can be spawned using the value allocated
+        // in the Arc variable pointer's location.
+        println!("{:?}", apple);
+    });
+}
+}
+
+```

src/std/rc.md

@@ -48,7 +48,8 @@ fn main() {
 
 ### See also:
 
-[std::rc][1] and [Arc][2].
+[std::rc][1] and [std::sync::arc][2].
 
 [1]: https://doc.rust-lang.org/std/rc/index.html
+
 [2]: https://doc.rust-lang.org/std/sync/struct.Arc.html