add reference and borrowing

en/src/ownership/borrowing.md

@@ -1 +1,166 @@
 # Reference and Borrowing
+
+### Reference
+🌟
+```rust,editable
+
+fn main() {
+   let x = 5;
+   // fill the blank
+   let p = __;
+
+   println!("the memory address of x is {:p}", p); // output: 0x16fa3ac84
+}
+```
+
+🌟
+```rust,editable
+
+fn main() {
+    let x = 5;
+    let y = &x;
+
+    // modify this line only
+    assert_eq!(5, y);
+}
+```
+
+🌟
+```rust,editable
+
+// fix error
+fn main() {
+    let mut s = String::from("hello, ");
+
+    borrow_object(s)
+}
+
+fn borrow_object(s: &String) {}
+```
+
+🌟
+```rust,editable
+
+// fix error
+fn main() {
+    let mut s = String::from("hello, ");
+
+    borrow_object(&s)
+}
+
+fn borrow_object(s: &mut String) {}
+```
+
+🌟🌟
+```rust,editable
+
+fn main() {
+    let mut s = String::from("hello, ");
+
+    // fill the blank to make it work
+    let p = __;
+    
+    p.push_str("world");
+}
+```
+
+#### ref
+`ref` can be used to take references to a value, similar to `&`.
+
+🌟🌟🌟
+```rust,editable
+
+fn main() {
+    let c = '中';
+
+    let r1 = &c;
+    // fill the blank，dont change other code
+    let __ r2 = c;
+
+    assert_eq!(*r1, *r2);
+    
+    // check the equality of the two address strings
+    assert_eq!(get_addr(r1),get_addr(r2));
+}
+
+// get memory address string
+fn get_addr(r: &char) -> String {
+    format!("{:p}", r)
+}
+```
+
+### Borrowing rules
+🌟
+```rust,editable
+
+// remove something to make it work
+// don't remove a whole line !
+fn main() {
+    let mut s = String::from("hello");
+
+    let r1 = &mut s;
+    let r2 = &mut s;
+
+    println!("{}, {}", r1, r2);
+}
+```
+
+#### Mutablity
+🌟 Error: Borrow a immutable object as mutable
+```rust,editable
+
+fn main() {
+    //fix error by modifying this line
+    let  s = String::from("hello, ");
+
+    borrow_object(&mut s)
+}
+
+fn borrow_object(s: &mut String) {}
+```
+
+🌟🌟 Ok: Borrow a mutable object as immutable
+```rust,editable
+
+// this code has no errors!
+fn main() {
+    let mut s = String::from("hello, ");
+
+    borrow_object(&s);
+    
+    s.push_str("world");
+}
+
+fn borrow_object(s: &String) {}
+```
+
+### NLL
+🌟🌟
+```rust,editable
+
+// comment one line to make it work
+fn main() {
+    let mut s = String::from("hello, ");
+
+    let r1 = &mut s;
+    r1.push_str("world");
+    let r2 = &mut s;
+    r2.push_str("!");
+    
+    println!("{}",r1);
+}
+```
+
+🌟🌟🌟
+```rust,editable
+
+fn main() {
+    let mut s = String::from("hello, ");
+
+    let r1 = &mut s;
+    let r2 = &mut s;
+
+    // add one line below to make a compiler error: cannot borrow `s` as mutable more than once at a time
+    // you can't use r1 and r2 at the same time
+}
+```