C - Derivable Traits - C - Derivable Traits - The Rust Programming Language

# C - Derivable Traits

In various places in the book, we discussed the `derive` attribute that is applied to a struct or enum. This attribute generates code that implements a trait on the annotated type with a default implementation. In this example, the `#[derive(Debug)]` attribute implements the `Debug` trait for the `Point` struct:

    # #![allow(unused_variables)]
    #fn main() {
    #[derive(Debug)]
    struct Point {
        x: i32,
        y: i32,
    }
    #}

The code that the compiler generates for the implementation of `Debug` is similar to this code:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point {
    #     x: i32,
    #     y: i32,
    # }
    #
    impl ::std::fmt::Debug for Point {
        fn fmt(&self, __arg_0: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
            match *self {
                Point { x: ref __self_0_0, y: ref __self_0_1 } => {
                    let mut builder = __arg_0.debug_struct("Point");
                    let _ = builder.field("x", &&(*__self_0_0));
                    let _ = builder.field("y", &&(*__self_0_1));
                    builder.finish()
                }
            }
        }
    }
    #}

The generated code implements sensible default behavior for the `Debug` trait’s `fmt` function: a `match` expression destructures a `Point` instance into its field values. Then it builds up a string containing the struct’s name and each field’s name and value. This means we’re able to use debug formatting on a `Point` instance to see what value each field has.

The generated code isn’t particularly easy to read because it’s only for the compiler to consume, rather than for programmers to read! The `derive` attribute and the default implementation of `Debug` has saved us all of the work of writing this code for every struct or enum that we want to be able to print using debug formatting.

The `derive` attribute has default implementations for the following traits provided by the standard library. If you want different behavior than what the `derive` attribute provides, consult the standard library documentation for each trait for the details needed for manual implementation of the traits.

## Standard Library Traits that Can Be Derived

The following sections list all of the traits in the standard library that can be used with `derive`. Each section covers:

- What operators and methods deriving this trait will enable
- What the implementation of the trait provided by `derive` does
- What implementing the trait signifies about the type
- The conditions in which you’re allowed or not allowed to implement the trait
- Examples of operations that require the trait

### `Debug` for Programmer Output

The `Debug` trait enables debug formatting in format strings, indicated by adding `:?` within `{}` placeholders.

The `Debug` trait signifies that instances of a type may be printed by programmers in order to debug their programs by inspecting an instance of a type at a particular point in a program’s execution.

An example of when `Debug` is required is the `assert_eq!` macro, which prints the values of the instances given as arguments if the equality assertion fails so that programmers can see why the two instances weren’t equal.

### `PartialEq` and `Eq` for Equality Comparisons

The `PartialEq` trait signifies that instances of a type can be compared to each other for equality, and enables use of the `==` and `!=` operators.

Deriving `PartialEq` implements the `eq` method. When derived on structs, two instances are equal if all fields are equal, and not equal if any fields are not equal. When derived on enums, each variant is equal to itself and not equal to the other variants.

An example of when `PartialEq` is required is the `assert_eq!` macro, which needs to be able to compare two instances of a type for equality.

The `Eq` trait doesn’t have any methods. It only signals that for every value of the annotated type, the value is equal to itself. The `Eq` trait can only be applied to types that also implement `PartialEq`. An example of types that implements `PartialEq` but that cannot implement `Eq` are floating point number types: the implementation of floating point numbers says that two instances of the not-a-number value, `NaN`, are not equal to each other.

An example of when `Eq` is required is for keys in a `HashMap` so that the `HashMap` can tell whether two keys are the same.

### `PartialOrd` and `Ord` for Ordering Comparisons

The `PartialOrd` trait signifies that instances of a type can be compared to each other to see which is larger than the other for sorting purposes. A type that implements `PartialOrd` may be used with the `<`, `>`, `<=`, and `>=` operators. The `PartialOrd` trait can only be applied to types that also implement `PartialEq`.

Deriving `PartialOrd` implements the `partial_cmp` method, which returns an `Option<Ordering>` that may be `None` if comparing the given values does not produce an ordering. When derived on structs, two instances of the struct are compared by comparing the value in each field in the order in which the fields appear in the struct definition. When derived on enums, variants of the enum declared earlier in the enum definition are greater than the variants listed later.

An example of when `PartialOrd` is required is the `gen_range` method in the `rand` crate that generates a random value in the range specified by a low value and a high value.

The `Ord` trait signifies that for any two value of the annotated type, a valid ordering exists. The `Ord` trait implements the `cmp` method, which returns an `Ordering` rather than an `Option<Ordering>` because a valid ordering will always be possible. The `Ord` trait can only be applied to types that also implement `PartialOrd` and `Eq` (and `Eq` requires `PartialEq`). When derived on structs and enums, `cmp` behaves the same way as the derived implementation for `partial_cmp` does with `PartialOrd`.

An example of when `Ord` is required is when storing values in a `BTreeSet<T>`, a data structure that stores data based on the sort order of the values.

### `Clone` and `Copy` for Duplicating Values

The `Clone` trait signifies there is a way to explicitly create a duplicate of a value, and the duplication process might involve running arbitrary code. Deriving `Clone` implements the `clone` method. When derived, the implementation of `clone` for the whole type calls `clone` on each of the parts of the type, so all of the fields or values in the type must also implement `Clone` to derive `Clone`.

An example of when `Clone` is required is when calling the `to_vec` method on a slice containing instances of some type. The slice doesn’t own the instances but the vector returned from `to_vec` will need to own its instances, so the implementation of `to_vec` calls `clone` on each item. Thus, the type stored in the slice must implement `Clone`.

The `Copy` trait signifies that a value can be duplicated by only copying bits; no other code is necessary. The `Copy` trait does not define any methods to prevent programmers from overloading those methods violating the assumption that no arbitrary code is being run. You can derive `Copy` on any type whose parts all implement `Copy`. The `Copy` trait can only be applied to types that also implement `Clone`, as a type that implements `Copy` has a trivial implementation of `Clone`, doing the same thing as `Copy`.

`Copy` is rarely required; when types implement `Copy`, there are optimizations that can be applied and the code becomes nicer because you don’t have to call `clone`. Everything possible with `Copy` can also be accomplished with `Clone`, but the code might be slower or have to use `clone` in places.

### `Hash` for Mapping a Value to a Value of Fixed Size

The `Hash` trait signifies there is a way to take an instance of a type that takes up an arbitrary amount of size and map that instance to a value of fixed size by using a hash function. Deriving `Hash` implements the `hash` method. When derived, the implementation of `hash` for the whole type combines the result of calling `hash` on each of the parts of the type, so all of the fields or values in the type must also implement `Hash` to derive `Hash`.

An example of when `Hash` is required is for keys in a `HashMap` so that the `HashMap` can store data efficiently.

### `Default` for Default Values

The `Default` trait signifies there is a way to create a default value for a type. Deriving `Default` implements the `default` method. When derived, the implementation of `Default` for the whole type calls the `default` method on each of the parts of the type, so all of the fields or values in the type must also implement `Default` to derive `Default.`

A common use of `Default::default` is in combination with the struct update syntax discussed in the “Creating Instances From Other Instances With Struct Update Syntax” section in Chapter 5. You can customize a few fields of a struct and then use the default values for the rest by using `..Default::default()`.

An example of when `Default` is required is the `unwrap_or_default` method on `Option<T>` instances. If the `Option<T>` is `None`, the `unwrap_or_default` method will return the result of `Default::default` for the type `T` stored in the `Option<T>`.

## Standard Library Traits that Can’t Be Derived

The rest of the traits defined in the standard library can’t be implemented on your types using `derive`. These traits don’t have a sensible default behavior they could have, so you are required to implement them in the way that makes sense for what you are trying to accomplish with your code.

An example of a trait that can’t be derived is `Display`, which handles formatting of a type for end users of your programs. You should put thought into the appropriate way to display a type to an end user: what parts of the type should an end user be allowed to see? What parts would they find relevant? What format of the data would be most relevant to them? The Rust compiler doesn’t have this insight into your application, so you must provide it.

## Making Custom Traits Derivable

The above list is not comprehensive, however: libraries can implement `derive` for their own types! In this way, the list of traits you can use `derive` with is truly open-ended. Implementing `derive` involves using a procedural macro, which is covered in the next appendix, “Macros.”