started ch15.2

Deref Trait.md

@@ -1 +1,223 @@
 # Treating Smart Pointers Like Regular References with the `Deref` Trait
+Implmenting the `Deref` trait allows you to customize the behavior of the *dereference operator* `*`.
+
+This is not the multiplication or glob operator
+
+Implementing `Deref` in a way such that a smart pointer can be treated like a regular reference.
+
+You can wirte code that operators on references and use that code with smart pointers as well.
+
+Now lets look at the dereference operator works with regualr references.
+
+Next we will try to define a custom type that behaves like `Box<T>`.
+
+Then we will see why the dereference operator doesnt work like a reference on our newly behaves lik `Box<T>`.
+
+We will explore how implementing the `Deref` trait makes it possible for smart pointers to work in a way that is simialr to references.
+
+Finally we will lokk at Rust's *deref coercion* feature and how it let us work with either references or smart pointers.
+
+Note: The big difference between the `MyBox<T>` type, the custom type we are about to build and the real `Box<T>`.
+
+Our version will not store its data on the heap.
+
+We are focusing on the `Deref` trait, so where the data is actualy stored is less improtant than the pointer-like behavior.
+
+## Following the Pointer to the Value
+A regular reference is a type of pointer.
+
+One way to think about a pointer is, an arrow to a value stored somewhere else.
+
+In this example we create a reference to an `i32` value.
+
+We then use the dereference operator to follwo the reference to the value.
+```rust
+fn main() {
+    let x = 5;
+    let y = &x;
+
+    assert_eq!(5, x);
+    assert_eq!(5, *y);
+}
+```
+
+The variable `x` holds a `5` (`i32`) value.
+
+We then set `y` equal to a refernce to `x`.
+
+We assert that `x` is equal to `5`.
+
+If we want to make an assertion about the value in `y`, we have touse `*y` to follow the reference to the value it's pointing to.
+
+Hence the need for a *dereference*.
+
+Once we dereference `y` we havce access to the integer value `y` is pointing to so that we can compare with `5`
+
+If we tired to use `assert_eq!(5, t);` instead we woul get the compiler error
+```
+$ cargo run
+   Compiling deref-example v0.1.0 (file:///projects/deref-example)
+error[E0277]: can't compare `{integer}` with `&{integer}`
+ --> src/main.rs:6:5
+  |
+6 |     assert_eq!(5, y);
+  |     ^^^^^^^^^^^^^^^^ no implementation for `{integer} == &{integer}`
+  |
+  = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`
+  = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider dereferencing here
+ --> file:///home/.rustup/toolchains/1.82/lib/rustlib/src/rust/library/core/src/macros/mod.rs:46:35
+  |
+46|                 if !(*left_val == **right_val) {
+  |                                   +
+
+For more information about this error, try `rustc --explain E0277`.
+error: could not compile `deref-example` (bin "deref-example") due to 1 previous error
+```
+Comparing a number to a reference is not allowed because they are differnect types.
+
+We must ue the dereference operator to follow the reference to the value they are pointing at.
+
+## Using `Box<T>` Like Reference
+We can rewrite the code from the example to use a `Box<T>` instead of a reference.
+
+Here dereference operator using on the `Box<T>` functions the same way as the dereference operator used on the reference (from before).
+
+```rust
+fn main() {
+    let x = 5;
+    let y = Box::new(x);
+
+    assert_eq!(5, x);
+    assert_eq!(5, *y);
+}
+```
+
+The main difference between the two eamples is that we ext `y` to be an instance of a `Box<T>` pointing to a copied value of `x` rather than a reference pointing to the value of `x`
+
+In the last assertion we can use the dereference operator to following the pointer of the `Box<T>` in the way as we deferenced `y` when it was a reference.
+
+## Defining Our Own Smart Pointer
+Here we will bouild a smart pointer simialr to the `Box<T>` type provided by the std library.
+
+This is in order to explain fully how smart pointers behave differently from references by defualt.
+
+Then we will look into how to add the ability to use the dereference operator.
+
+The `Box<T>` type is defined as a tuple struct with one element.
+
+In this example we defined `MyBox<T>` type in the same way.
+
+We will aslo define a `new` function to match the `new` function defined in `Box<T>`
+```rust
+struct MyBox<T>(T);
+
+impl<T> MyBox<T> {
+    fn new(x: T) -> MyBox<T> {
+        MyBox(x)
+    }
+}
+```
+
+Here we defined a struct named `MyBox` and declarce a generic paramter `T` beucause we want our type to hold values of any type.
+
+The `MyBox` type is a tuple struct with one element of type `T`.
+
+The `MyBox::new` function takes one parameter of type `T` and returns a `MyBox` instance that holds the value passed in.
+
+If we try adding the `main` function from above to our preivous examples.
+
+We will change it to use `MyBox<T>` type we defined instead of `Box<T>`.
+
+The code will not compie because Rust doesn't know how to dereference `MyBox`
+```rust
+fn main() {
+    let x = 5;
+    let y = MyBox::new(x);
+
+    assert_eq!(5, x);
+    assert_eq!(5, *y);
+}
+```
+Here is the resulting compilation error
+```
+$ cargo run
+   Compiling deref-example v0.1.0 (file:///projects/deref-example)
+error[E0614]: type `MyBox<{integer}>` cannot be dereferenced
+  --> src/main.rs:14:19
+   |
+14 |     assert_eq!(5, *y);
+   |                   ^^
+
+For more information about this error, try `rustc --explain E0614`.
+error: could not compile `deref-example` (bin "deref-example") due to 1 previous error
+```
+The `MyBox<T>` type cant be dereferenced because we havn't implemented the ability to on our type.
+
+To enable dereferencing with the `*` operator, we need to implemented the `Deref` trait.
+
+## Treating a Type Like a Reference by Implementing the `Deref` Trait
+As discussed before to implement a trait we need to provide implementations for the trait's required methods.
+
+Here the `Deref` trait, provided by the std library, requires us to implement one method named `deref` that borrows `self` and returns a refernce to the inner data.
+
+Here is th implementation of the `Deref` to add to the definition of `MyBox`
+```rust
+use std::ops::Deref;
+
+impl<T> Deref for MyBox<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+```
+
+The `type Target = T;` syntax defines an associated type for the `Deref` trait to use.
+
+Associated types are a slightly different way of declaring a generic paramter, we will cover them later in ch20.
+
+We fill in the body of the `deref` method with `&self.0` so `deref` returns a reference to the vbalue we want  to access with the `*` operator.
+
+Recall that using a "Tuple Struct without a Named Filed to Create Different Types" section.
+
+We use `.0` accesses the first value in a tuple struct.
+
+The `main` function from the example before will now compile and the assertions will pass now with thuis implementation.
+
+Without the `Deref` trait, the compiler can only dereference `&` references.
+
+The `deref` method gives the compiler the ability to take a value of any type that implements `Deref` and call the `deref` method to get a `&` reference that it knows how to deference.
+
+When we write `*y`.
+
+Behind the scenes Rust actually ran this code
+```rust
+*(y.deref())
+```
+Rust substitues the `*` operator with a call to `deref` method.
+
+Then a plain derefence so we dont have to think about whether or not we need to call the `deref` method.
+
+This Rust feature lets us write code that functions identically regardless of if we have a regular reference or a type that implements `Deref`
+
+The reason that the `deref` method reutnrs a reference to a value, and that the plain dereferncee outside the parentheses in `*(y.deref())` is still necessary, is to do with the ownership system.
+
+If the `deref` method retuned the value directly instead of a refernece to the value, the value would be moved out of `self`.
+
+We dont want to take ownership of the inner value insid `MyBox<T>` in this case or in most cases where we use the dereference oerator.
+
+Note: the `*` operator is replaced with a call to the `deref` method and then a call to trhe `*` operator just once.
+
+Each time we use a `*` in our code.
+
+Due to the substituation of the `*` operator doesn't recurse infinitely we end up with data of type `i32`.
+
+This then matches the `5` in `assert_eq!`
+
+## Implicit Deref Coercions iwth Functions and Methods
+*Deref coercion* converts a reference to a type that implements the `Deref` trait into a reference to another type.
+
+For example a deref coercion can convert `&String` to `&str` because `String` implemented the `Deref` trait such that it returns `&str`.
+

guessing_game/target/.rustc_info.json

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hello_cargo/target/.rustc_info.json

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