Structs and Related Concepts

Rust struct

• Seen a bunch of these already

• Basic idea is the same as structures in any language

• Rust structs are fancy, and an essential building block

• Most of the standard API is built out of struct, enum, trait

Basics

• Basic decl syntax is straightforward

    struct Point {
        x: i64,
        y: i64,
    }
  

• There is no default constructor: can create with again-obvious syntax

    let p = Point{ x: 5, y: -2 };
  

Fancy Features

• Common case of tag-name = varname is special

    let x = 5;
    let p = Point{ x, y: -2 };
  

• Can have empty structures: these are inhabited by only one value

  https://play.rust-lang.org/?gist=c4f53c4e833a2e8b9f624440c9e3b7fe&version=stable

• "Tuple structs" have anonymous fields

    struct Point(int64, int64);
  
    let p = Point(3, 2);
    let x = p.0;
  

• Single-element tuple structs are often used for "newtyping"

    struct Temperature(int64);
  

Ownership and Lifetimes

• Ownership is a bit complicated with structs

  https://play.rust-lang.org/?gist=e74541b3fc609dfce49ad2aff13ba11a&version=stable

• A struct owns all of its fields individually

• Moving a field out of a struct makes it "partially moved"

• References stored in structs need to outlast the struct

• Normally need to explicitly specify reference lifetime

impl

• Mechanism for getting that fancy OO-style syntax

• Implementor defines whether self argument is by move, reference or mut reference

  https://play.rust-lang.org/?gist=ad54d7fbac5010d57a787357aa5d96e2&version=stable

• Allows chaining of operators, which can be syntactically nice

Deriving

• Mechanism for getting traits defined automatically for your struct

• Uses attribute syntax

• Common kind of thing to write

    #[derive(Debug, Clone, Copy, PartialEq, PartialOrd)]
    struct Temperature(i64);
  

  https://play.rust-lang.org/?gist=bca2d7bc4b303ff3d5331b0bc476aeef&version=stable

Pattern Matching

• Not specific to structs / enums

• Mechanism for deconstructing a composite thing

  https://play.rust-lang.org/?gist=27543ca8dcac3e9062949db610b97190&version=nightly

• We've seen some of this before.

• Two kinds: "refutable" and "irrefutable"

  • let requires irrefutable pattern
  • match allows refutable pattern, but all possible patterns must be matched

Match

• The match statement is the canonical pattern matching tool

  https://play.rust-lang.org/?gist=c18cf7385e82e70eeb75fbdd5ff5dfc5&version=stable

• Note the existence of .., ..., | (not shown, only at top level), @

• First match is chosen

• Compiler checks that every possible value will be matched, and that later matches are not subsumed by earlier

Pattern Variable Modifiers

• Rules for move/borrow are the same for pattern variables

• For convenience, you can modify a pattern variable with ref or ref mut to have it borrowed instead of moved

Misc

• Struct layout concepts are in the book: not too important

• Need to master structs: they are used everywhere