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diff --git a/src/doc/book/nostarch/appendix_c.md b/src/doc/book/nostarch/appendix_c.md new file mode 100644 index 000000000..53131eb5f --- /dev/null +++ b/src/doc/book/nostarch/appendix_c.md @@ -0,0 +1,184 @@ +<!-- DO NOT EDIT THIS FILE. + +This file is periodically generated from the content in the `/src/` +directory, so all fixes need to be made in `/src/`. +--> + +[TOC] + +## Appendix C: Derivable Traits + +In various places in the book, we’ve discussed the `derive` attribute, which +you can apply to a struct or enum definition. The `derive` attribute generates +code that will implement a trait with its own default implementation on the +type you’ve annotated with the `derive` syntax. + +In this appendix, we provide a reference of all the traits in the standard +library that you can use 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 + +If you want different behavior from that provided by the `derive` attribute, +consult the standard library documentation for each trait for details on how to +manually implement them. + +The traits listed here are the only ones defined by the standard library that +can be implemented on your types using `derive`. Other traits defined in the +standard library don’t have sensible default behavior, so it’s up to you to +implement them in the way that makes sense for what you’re trying to accomplish. + +An example of a trait that can’t be derived is `Display`, which handles +formatting for end users. You should always consider 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, so +it can’t provide appropriate default behavior for you. + +The list of derivable traits provided in this appendix is not comprehensive: +libraries can implement `derive` for their own traits, making the list of +traits you can use `derive` with truly open ended. Implementing `derive` +involves using a procedural macro, which is covered in “Macros” on page XX. + +## Debug for Programmer Output + +The `Debug` trait enables debug formatting in format strings, which you +indicate by adding `:?` within `{}` placeholders. + +The `Debug` trait allows you to print instances of a type for debugging +purposes, so you and other programmers using your type can inspect an instance +at a particular point in a program’s execution. + +The `Debug` trait is required, for example, in the use of the `assert_eq!` +macro. This macro prints the values of instances given as arguments if the +equality assertion fails so programmers can see why the two instances weren’t +equal. + +## PartialEq and Eq for Equality Comparisons + +The `PartialEq` trait allows you to compare instances of a type to check for +equality and enables use of the `==` and `!=` operators. + +Deriving `PartialEq` implements the `eq` method. When `PartialEq` is derived on +structs, two instances are equal only if *all* fields are equal, and the +instances are 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. + +The `PartialEq` trait is required, for example, with the use of the +`assert_eq!` macro, which needs to be able to compare two instances of a type +for equality. + +The `Eq` trait has no methods. Its purpose is to signal 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`, although not all types that +implement `PartialEq` can implement `Eq`. One example of this is floating-point +number types: the implementation of floating-point numbers states that two +instances of the not-a-number (`NaN`) value are not equal to each other. + +An example of when `Eq` is required is for keys in a `HashMap<K, V>` so that +the `HashMap<K, V>` can tell whether two keys are the same. + +## PartialOrd and Ord for Ordering Comparisons + +The `PartialOrd` trait allows you to compare instances of a type for sorting +purposes. A type that implements `PartialOrd` can be used with the `<`, `>`, +`<=`, and `>=` operators. You can only apply the `PartialOrd` trait to types +that also implement `PartialEq`. + +Deriving `PartialOrd` implements the `partial_cmp` method, which returns an +`Option<Ordering>` that will be `None` when the values given don’t produce an +ordering. An example of a value that doesn’t produce an ordering, even though +most values of that type can be compared, is the not-a-number (`NaN`) floating +point value. Calling `partial_cmp` with any floating-point number and the `NaN` +floating-point value will return `None`. + +When derived on structs, `PartialOrd` compares two instances 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 considered less than the variants listed later. + +The `PartialOrd` trait is required, for example, for the `gen_range` method +from the `rand` crate that generates a random value in the range specified by a +range expression. + +The `Ord` trait allows you to know that for any two values of the annotated +type, a valid ordering will exist. The `Ord` trait implements the `cmp` method, +which returns an `Ordering` rather than an `Option<Ordering>` because a valid +ordering will always be possible. You can only apply the `Ord` trait 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 allows you to explicitly create a deep copy of a value, and +the duplication process might involve running arbitrary code and copying heap +data. See “Variables and Data Interacting with Clone” on page XX for more +information on `Clone`. + +Deriving `Clone` implements the `clone` method, which when implemented for the +whole type, calls `clone` on each of the parts of the type. This means all 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. The slice doesn’t own the type instances it contains, but the vector +returned from `to_vec` will need to own its instances, so `to_vec` calls +`clone` on each item. Thus the type stored in the slice must implement `Clone`. + +The `Copy` trait allows you to duplicate a value by only copying bits stored on +the stack; no arbitrary code is necessary. See “Stack-Only Data: Copy” on page +XX for more information on `Copy`. + +The `Copy` trait doesn’t define any methods to prevent programmers from +overloading those methods and violating the assumption that no arbitrary code +is being run. That way, all programmers can assume that copying a value will be +very fast. + +You can derive `Copy` on any type whose parts all implement `Copy`. A type that +implements `Copy` must also implement `Clone` because a type that implements +`Copy` has a trivial implementation of `Clone` that performs the same task as +`Copy`. + +The `Copy` trait is rarely required; types that implement `Copy` have +optimizations available, meaning you don’t have to call `clone`, which makes +the code more concise. + +Everything possible with `Copy` you can also accomplish 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 allows you to take an instance of a type of arbitrary size and +map that instance to a value of fixed size using a hash function. Deriving +`Hash` implements the `hash` method. The derived implementation of the `hash` +method combines the result of calling `hash` on each of the parts of the type, +meaning all fields or values must also implement `Hash` to derive `Hash`. + +An example of when `Hash` is required is in storing keys in a `HashMap<K, V>` +to store data efficiently. + +## Default for Default Values + +The `Default` trait allows you to create a default value for a type. Deriving +`Default` implements the `default` function. The derived implementation of the +`default` function calls the `default` function on each part of the type, +meaning all fields or values in the type must also implement `Default` to +derive `Default`. + +The `Default::default` function is commonly used in combination with the struct +update syntax discussed in “Creating Instances from Other Instances with Struct +Update Syntax” on page XX. You can customize a few fields of a struct and then +set and use a default value for the rest of the fields by using +`..Default::default()`. + +The `Default` trait is required when you use the method `unwrap_or_default` on +`Option<T>` instances, for example. If the `Option<T>` is `None`, the method +`unwrap_or_default` will return the result of `Default::default` for the type +`T` stored in the `Option<T>`. + |