Add notes on Deref Trait

Author: Brian Obot

src/bin/41_utility_traits.rs

@@ -1,3 +1,13 @@
+#![allow(dead_code, unused_variables)]
+
+// you can think of utility trait as a bunch of traits in the standrd library that has alot of impact on the way Rust is wrriten
+// mastering them would make one write more idiomatic code and build crates public interfaces better
+
+// broading speaking, there are categorized into following sets
+// - Language extension traits: like the operator overloading traits, Drop traits, Deref, DerefMut and conversion trait
+// - Marker traits: traits used to expression contraits on generic types that can be caputured in any other trivial way, Size and Copy
+// - Public Vocabulary Trait: they are not magival to the compiler but using them mirrors convetional solutions for common problems, Default, AsRed, AsMut, Borrow and BorrowMut
+
 fn main() {
     // Traits, Description
     // Drop: Clean up code rust runs automatically when a value is dropped
@@ -14,26 +24,109 @@ fn main() {
     // Drop
     // you can customize how run process dropping of types by implementing Drop for your type
 
+    let mut names = vec![1, 2, 3, 4, 5];
+    println!("Capacity: {}", names.capacity());
+    println!("Length: {}", names.len());
+
+    names.truncate(4);
+    println!("After Truncation");
+    println!("Capacity: {}", names.capacity());
+    println!("Length: {}", names.len());
+
     #[derive(Debug)]
     struct Person {
         name: String,
     }
 
     impl Drop for Person {
         fn drop(&mut self) {
-            println!("Dropping Person")
+            println!("Dropping Person: Name = {}", self.name);
+            // because the drop method on a value is called, before the value is actually drop
+            // the value is still fully initialized and all it's field can still be used
         }
     }
 
     let brian = Person {
         name: "Brian".to_string(),
     };
+    println!("✅ Start of Section on Drop Trait");
     println!("brian: {brian:?}");
     println!("person name: {:?}", brian.name);
     // rust calls the drop trait of a type if it implement the Drop trait
     // and then after that drops the value, so the value is not dropped in the drop
     // method of the Drop trait but some other logic can happen there
+    //
+    {
+        let mut _person = Person {
+            name: "Albert".to_string(),
+        };
+        println!("Before Assignment");
+
+        _person = Person {
+            name: "James".to_string(),
+        };
+        println!("At the end of the block");
+    }
+
+    println!("✅ End of Section on Drop Trait");
 
     // if a variable value is moved into another variable, when the variable goes out of scope
     // rust will not try to drop it, since it does not contain any value
+    // if a type implement Drop it can not implement Copy and vice versa
+
+    // Sized: is a type whose values have the same size in memory
+    // all sized types implement the Sized marker trait which has no methods or associated types
+    // you can not implement it yourself, the only need of the Size trait is as a trait bound for types that implement it
+    //
+    // rust can not store unsized values in variables or pass them around
+    // that why you must interact with them through pointers, like & and Box
+    // a pointer to a unsize value is always a fat pointer, a pointer to the item in the head and the length
+    let information = "Some Information that might be important for the next learner";
+
+    // all generics types are implicitly Sized
+    // when you write fn gen<T>() {} rust treats T as fn gen<T: Size>() {}
+    //
+    // Clone: Clone must be infalliable
+    // Copy: these are the exception to the moving values rule of rust, since the types that implement this
+    // are simple and do not have resources, the are simply Copied, a Copy is an subtrait of the Clone trait
+
+    trait Copy: Clone {} // it is a marker trait
+    // types that own resources like heap buffers or operating system handles can not implement copy
+    // likewise any type that implement the Drop trait can not implement Copy
+    //
+    // Deref and DerefMut: this specifies how Rust handles the dereferencing operators like * and .
+    // let b = Box<Complex> implements Deref and DerefMut, so *b refers to Complex, and b.re refers to the real part of Complex
+
+    #[allow(dead_code)]
+    trait Deref {
+        type Target: ?Sized;
+
+        fn deref(&self) -> &Self::Target;
+    }
+
+    #[allow(dead_code)]
+    trait DerefMut: Deref {
+        fn deref_mut(&mut self) -> &mut Self::Target;
+    }
+
+    // Target should be something that Self Contains, owns or refer to
+    let underlying_value = 5;
+    let door = &underlying_value;
+    let opened_door = *door;
+
+    assert_eq!(5, underlying_value);
+    assert_eq!(5, opened_door);
+
+    struct MyBox<T>(T);
+
+    impl<T> std::ops::Deref for MyBox<T> {
+        type Target = T;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+
+    let my_box = MyBox(10);
+    assert_eq!(10, *my_box); // *(my_box.deref())
 }