ch4.2 wip

ch4/refs_and_borrow/src/main.rs

@@ -5,21 +5,67 @@ fn main() {
     // greet(m1, m2);
     // let s = format!("{} {}", m1, m2);
 
-    // This is suboptimal
+    // This is suboptimal due to verboseness of solution
     let m1 = String::from("Hello");
     let m2 = String::from("World");
     let (m1_again, m2_again) = greet_ret(m1, m2);
     let s = format!("{} {}", m1_again, m2_again);
     println!("{s}");
 
-    // stopped here:
-    // https://rust-book.cs.brown.edu/ch04-02-references-and-borrowing.html#references-are-non-owning-pointers
+    // references make a convenient usage of ownership
+    // Can now use mm1 and mm2 later after the call to greet_ref
+    // References are non-owning pointers.
+    let mm1 = String::from("Hello");
+    let mm2 = String::from("World");
+    greet_ref(&mm1, &mm2);
+    let ss = format!("{} {}", mm1, mm2);
+    println!("{ss}");
+
+    // -----------------------------------------------------------------------------
+    // Dereferencing a pointer accesses it data
+
+    // Rust often implicitly inserts references and dereferences throughout
+    // the code; ergo, won't see * often in Rust code
+    // This can be seen in str::len
+
+    let mut x: Box<i32> = Box::new(1);
+    let a: i32 = *x; // reads heap value, so a stores 1 not following x
+    *x += 1; // *x points to value on heap, now adds 1 to x
+    println!("{}", *x);
+
+    let r1: &Box<i32> = &x; // r1 points to x on the stack
+    let b: i32 = **r1; // two dereferences get the heap value
+    println!("{}", b);
+
+    let r2: &i32 = &*x; // r2 points to heap value directly. Making a new reference to the deferenced i32
+    let c: i32 = *r2; // one dereference needed to read it
+
+    // -----------------------------------------------------------------------------
+    // Rust avoids simultaneous aliasing and mutation
+    //
+    // Aliasing: accessing same data through different variables
+    // aliasing on its own okay. Combining with mutation allows "rug pulling" from other variables
+
+    // Invalid code:
+    // let mut v: Vec<i32> = vec![1, 2, 3];
+    // let num: &i32 = &v[2];
+    // v.push(4);
+    // println!("3rd element is {}", *num); // undefined behavior here
+
+    // Data in Rust should never be aliased and mutated at the same time
+
+    // Stopped here:
+    // https://rust-book.cs.brown.edu/ch04-02-references-and-borrowing.html#references-change-permissions-on-places
 }
 
 fn greet(g1: String, g2: String) {
     println!("{} {}!", g1, g2);
 }
 
+fn greet_ref(g1: &String, g2: &String) {
+    println!("{} {}!", g1, g2);
+}
+
 fn greet_ret(g1: String, g2: String) -> (String, String) {
     println!("{} {}!", g1, g2);
     (g1, g2)