For my first project I wanted to create a terminal implementation of Monopoly. I have created a Card, Player, and App structs. Originally my plan was to create an array inside the App struct holding a list of cards which I could then randomly select and run an execute() method on, which would push a log to the logs field of the App. What I thought would be best was this:
pub struct Card {
pub group: u8,
pub name: String,
pub id: u8,
}
impl Card {
pub fn new(group: u8, name: String, id: u8) -> Card {
Card { group, name, id }
}
pub fn execute(self, app: &mut App) {
match self.id {
1 => {
app.players[app.current_player].index = 24;
app.push_log("this works".to_string(), "LOG: ".to_string());
}
_ => {}
}
}
}
pub struct Player<'a> {
pub name: &'a str,
pub money: u64,
pub index: u8,
pub piece: char,
pub properties: Vec<u8>,
pub goojf: u8, // get out of jail freeh
}
impl<'a> Player<'a> {
pub fn new(name: &'a str, piece: char) -> Player<'a> {
Player {
name,
money: 1500,
index: 0,
piece,
properties: Vec::new(),
goojf: 0,
}
}
}
pub struct App<'a> {
pub cards: [Card; 1],
pub logs: Vec<(String, String)>,
pub players: Vec<Player<'a>>,
pub current_player: usize,
}
impl<'a> App<'a> {
pub fn new() -> App<'a> {
App {
cards: [Card::new(
11,
"Take a ride on the Penn. Railroad!".to_string(),
0,
)],
logs: vec![(
String::from("You've begun a game!"),
String::from("BEGIN!:"),
)],
players: vec![Player::new("Joe", '#')],
current_player: 0,
}
}
pub fn draw_card(&mut self) {
if self.players[self.current_player].index == 2
|| self.players[self.current_player].index == 17
|| self.players[self.current_player].index == 33
{
self.cards[0].execute(self);
}
}
pub fn push_log(&mut self, message: String, status: String) {
self.logs.push((message, status));
}
}
fn main() {}
However this code throws the following error:
error[E0507]: cannot move out of `self.cards[_]` which is behind a mutable reference
--> src/main.rs:76:13
|
76 | self.cards[0].execute(self);
| ^^^^^^^^^^^^^ move occurs because `self.cards[_]` has type `Card`, which does not implement the `Copy` trait
I managed to fix this error by simply declaring the array of cards in the method itself, however, this seem to be pretty brute and not at all efficient, especially since other methods in my program depend on cards. How could I just refer to a single array of cards for all of my methods implemented in App or elsewhere?
As others have pointed out, Card::execute() needs to take &self instead of self, but then you run into the borrow checker issue, which I'll spend the rest of this answer discussing.
It may seem odd, but Rust is actually protecting you here. The borrow checker does not look into functions to see what they do, so it has no idea that Card::execute() won't do something to invalidate the referenced passed as the first argument. For example, if App::cards was a vector instead of an array, it could clear the vector.
Something that could actually practically happen here to cause undefined behavior would be if Card::execute() took a string slice from self.name and then cleared the card's name attribute through the mutable reference to app. None of these actions would be prohibited, and you'd be left with an invalid reference to a string slice. This is why the borrow checker isn't letting you make this method call, and this is exactly the kind of accident that Rust is designed to prevent.
There's a few ways around this. One option is to only pass the pieces of the App value needed to complete the task. You can borrow different parts of the same value. The problem here is that the reborrow of self overlaps with the borrow self.cards[0]. Passing each field separately isn't very ergonomic though, as in this case you'll wind up having to pass a reference to pretty much everything else on App.
It looks like the Card values don't actually contain any game state, and are used as data for the game engine. If this is the case, then the cards can live outside of App, like so:
pub struct App<'a> {
// Changed to an unowned slice.
pub cards: &'a [Card],
pub logs: Vec<(String, String)>,
pub players: Vec<Player<'a>>,
pub current_player: usize,
}
impl<'a> App<'a> {
pub fn new(cards: &'a [Card]) -> App<'a> {
App {
cards,
// ...
Then in your main() you can initialize the data and borrow it:
fn main() {
let cards: [Card; 1] = [Card::new(
11,
"Take a ride on the Penn. Railroad!".to_string(),
0,
)];
let app = App::new(&cards);
}
This solves the compilation problem. I'd suggest making other changes, as well:
App should probably be renamed GameState or something to emphasize that this struct should contain only mutable game state, and no immutable reference data.
Player's name field should probably be an owned String instead of an unowned &str, otherwise some other entity in the program will need to own a string slice for the duration of the program so that Player can borrow it.
Does a card need to be able to mutate the cards in self? If you know that execute will not need access to cards, the easiest solution is to split App into further structs so you can better limit the access of the function.
Unless there is another layer to your program that interacts with App as a singular object, requiring everything be in a single struct to perform operations will likely only constrain your code. If possible it is better to split it into its core components so you can be more selective when sharing references and avoid needing to make so many fields pub.
Here is a rough example:
pub struct GameState<'a> {
pub players: Vec<Player<'a>>,
pub current_player: usize,
}
/// You may want to look into using https://crates.io/crates/log instead to make logging easier.
pub struct Logs {
entries: Vec<(String, String)>,
}
impl Logs {
/// Use ToOwned so you can use both both &str and String
pub fn push<S: ToOwned<String>>(&mut self, message: S, status: S) {
self.entries.push((message.to_owned(), status.to_owned()));
}
}
pub struct Card {
group: u8,
name: String,
id: u8,
}
impl Card {
pub fn new(group: u8, name: String, id: u8) -> Self {
Card { group, name, id }
}
/// Use &self because there is no reason we need are required to consume the card
pub fn execute(&self, state: &mut GameState, logs: &mut Logs) {
if self.id == 1{
state.players[state.current_player].index = 24;
logs.push("this works", "LOG");
}
}
}
Also as a side note, execute consumes a card when called since it does not take a reference. Since Card does not implement Copy, that would require it be removed from cards so it can be moved.
Misc Tips and Code Review
Using IDs
It looks like you frequently use IDs to distinguish between items. However, I think your code will look cleaner and be easier to write if you used more human readable types. For example, do cards need an ID? Usually it is preferable to define your struct based on how the data is used. Last time I played monopoly, I don't remember picking up a card and referring to the card ID to determine what to do. I would instead recommend defining a Card by how it is used in the came. If I am remembering correctly, each card contains a short message telling the player what to do. Technically a card could consist of just the message, but you can instead make your code a bit cleaner by separating out the action to an enum so actions are not hard coded to the text on the card.
pub struct Card {
text: String,
action: CardAction,
}
// Note: enums which can also hold data, are more commonly referred to as
// "tagged unions" in computer science. It can be a pain to search for them if
// you don't know what they are called.
pub enum CardAction {
ProceedToGo,
GoToJail,
GainOrLoseMoney(i64),
// etc.
}
On a similar note, it looks like you are trying to be memory conscious by using the smallest type required for a given value. I would recommend against this thinking. If a value of 253u8 is equally as invalid as 324234i32, then the smaller type is not doing anything to help you. You might as well use i32/u32 or i64/u64 since most systems will have an easier time operating on these types. The same thing goes for indices and using other integer types instead of usize since choosing to use another type will only give you more work converting it to and from a usize.
Sharing Owned References
Depending on your design philosophy you might want to store a reference to a struct in multiple places. This can be done using a reference counter Rc<T>. Here are some quick examples. Note that these can not be shared between threads.
let property: Rc<Property> = Rc::new(Property::new(/* etc */));
// Holds an owned reference to the same property as property that can be accessed immutably
let ref_to_property: Rc<Property> = property.clone();
// Or if you want interior mutability you can use Rc<RefCell<T>> instead.
let mutable_property = Rc::new(RefCell::new(Property::new(/* etc */)));
To make the question more clear, i am trying to find a better way for the below task, say i have a Config struct which will be initialised with the config values provided by user, but all the configs are optional, in the case that user didn't provide a value, then it will be initialised with some default values.
pub struct Config {
pub a: Option<u128>,
pub b: Option<Duration>
// ...
}
impl Config {
pub fn new() -> Self {
// ...
Self {
// ...
}
}
}
My confusion is that even i know for sure that each field of the Config instance i get from the Config::new() function has a solid value Some(x), but still in the later program everywhere when i need a value from this config instance, i need to call unwrap, is there a better way to avoid these ubiquitous unwrap？
Or is there a better design i should go with?
I am still learning rust, sorry if it is some stupid questions.
In general: One way to go is to have two structs. One with all things Options, and another with all of them plain values, and a function to create an Option of the latter from the former.
struct ConfigPre {
pub a: Option<u128>,
pub b: Option<Duration>,
}
struct Config {
pub a: u128,
pub b: Duration,
}
impl Config {
fn new(from: ConfigPre) -> Option<Self> {
Some(Self { a: from.a?, b: from.b? })
}
}
This is tedious to type out, and you can take it one step further by using builders and have this code + some convenience setters autogenerated. Try e.g. typed-builder.
In your specific case: If you set all the values of Config to Some in new, why do they need to be Options in the first place?
For writing a very large program, I see no way to alleviate having to write the same code for each struct that uses a certain shared behaviour.
For example, Dog may "bark":
struct Dog {
is_barking: bool,
....
}
impl Dog {
pub fn bark(self) {
self.is_barking = true;
emit_sound("b");
emit_sound("a");
emit_sound("r");
emit_sound("k");
self.is_barking = false;
}
....
}
And many breeds of this dog may exist:
struct Poodle {
unique_poodle_val: &str
}
impl Poodle {
pub fn unique_behaviour(self) {
self.some_behaviour();
}
}
struct Rottweiler {
unique_rottweiler_val: u32
}
impl Rottweiler{
pub fn unique_behaviour(self) {
self.some_behaviour();
}
}
The issue is that Rust seems incapable of this in my current knowledge, but it needs to be done and I need a workaround for:
Allowing Poodle and Rottweiler to bark using the exact same behavior which the breeds should not need to regard.
Allowing this to be possible without recoding bark() in every breed module, which is programming hell as it leads to repetitious code and every module has to implement bark().
Traits are the inverse and cannot access the struct, so default-trait implements do not work. Rust does not support OOP-like inheritance and nor is it desired here.
Therefore, I ask:
How would it be possible to implement bark() without rewriting bark() in each module, since Poodle and Rottweiler bark exactly the same way after all?
Please provide an example of how to solve this issue in Rust code, idiomatic and slightly hacky solutions are welcome but please state which they are as I am still learning Rust.
Thank you.
Edit: The boolean is not a thread thing, rather it's a example of setting some state before doing something, i.e. emit_sound is within this state. Any code we put into bark() has the same issue. It's the access to the struct variables that is impossible with say, traits.
You've put the finger on something that Rust doesn't do nicely today: concisely add to a struct a behavior based on both internal data and a function.
The most typical way of solving it would probably be to isolate Barking in a struct owned by all dogs (when in doubt, prefer composition over inheritance):
pub struct Barking {
is_barking: bool,
}
impl Barking {
pub fn do_it(&mut self) {
self.is_barking = true; // <- this makes no sense in rust, due to the ownership model
println!("bark");
println!("a");
println!("r");
println!("k");
self.is_barking = false;
}
}
struct Poodle {
unique_poodle_val: String,
barking: Barking,
}
impl Poodle {
pub fn unique_behaviour(self) {
println!("some_behaviour");
}
pub fn bark(&mut self) {
self.barking.do_it();
}
}
struct Rottweiler {
unique_rottweiler_val: u32,
barking: Barking,
}
impl Rottweiler{
pub fn unique_behaviour(self) {
println!("unique behavior");
}
pub fn bark(&mut self) {
// maybe decide to bite instead
self.barking.do_it();
}
}
In some cases it can make sense to define a Barking trait, with a common implementation and declaring some functions to deal with the state:
pub trait Barking {
fn bark(&mut self) {
self.set_barking(true);
println!("bark");
println!("a");
println!("r");
println!("k");
self.set_barking(false);
}
fn set_barking(&mut self, b: bool);
}
struct Poodle {
unique_poodle_val: String,
is_barking: bool,
}
impl Poodle {
pub fn unique_behaviour(self) {
println!("some_behaviour");
}
}
impl Barking for Poodle {
fn set_barking(&mut self, b: bool) {
self.is_barking = b;
}
}
Beware that this half-OOP approach often ends up too much complex and less maintainable (like inheritance in OOP languages).
I'm working on a pdf generation application. I have a struct (Document) that owns all pages. Whenever i want to add a page i also want give the caller a borrowed value of the page back, so the document still is the owner of the struct. Like returning a pointer in C.
pub struct Document {
pages: Vec<Page>,
totalPages: i32,
_secret: ()
}
pub fn add_page(&mut self, dimension: PageDimension) -> &mut Page {
let newPage = Page::new(self.pages.len(), dimension);
self.pages.push(newPage);
newPage
}
Can anyone explain how to achieve what i want.
This can be done with the Vec::last_mut() method. You'll have to unwrap the option that it returns, but that should never panic since you just pushed a new element in.
Here's a Minimal Reproducible Example. You should provide one of those yourself if you want faster/better answers. Without one, it's hard to know what exactly is intended. Check out this page too for some Rust-specific advice for MREs.
pub struct PageDimension;
pub struct Page(usize, PageDimension);
impl Page {
fn new(page_number: usize, dimension: PageDimension) -> Self {
Self(page_number, dimension)
}
}
pub struct Document {
pages: Vec<Page>,
totalPages: i32,
_secret: (),
}
impl Document {
pub fn add_page(&mut self, dimension: PageDimension) -> &mut Page {
let newPage = Page::new(self.pages.len(), dimension);
self.pages.push(newPage);
self.pages.last_mut().unwrap()
}
}
(playground)
Just one last thing. To conform with the community naming conventions, totalPages and newPage should have snake_case names, like total_pages and new_page. The compiler will warn about this.
Suppose I have a rust trait that contains a function that does not take a &self parameter. Is there a way for me to call this function based on a generic type parameter of the concrete type that implements that trait? For example, in the get_type_id function below, how do I successfully call the type_id() function for the CustomType trait?
pub trait TypeTrait {
fn type_id() -> u16;
}
pub struct CustomType {
// fields...
}
impl TypeTrait for CustomType {
fn type_id() -> u16 { 0 }
}
pub fn get_type_id<T : TypeTrait>() {
// how?
}
Thanks!
As Aatch mentioned, this isn't currently possible. A workaround is to use a dummy parameter to specify the type of Self:
pub trait TypeTrait {
fn type_id(_: Option<Self>) -> u16;
}
pub struct CustomType {
// fields...
}
impl TypeTrait for CustomType {
fn type_id(_: Option<CustomType>) -> u16 { 0 }
}
pub fn get_type_id<T : TypeTrait>() {
let type_id = TypeTrait::type_id(None::<T>);
}
Unfortunately, this isn't currently possible. It used to be, based on a implementation detail, however that was removed in favor of eventually implementing a proper way of doing this.
When it is eventually implemented, it may end up looking something like this: TypeTrait::<for T>::type_id(), however there is, currently, no syntax set in stone.
This is a known case and one that is fully intended to be supported, it is just unfortunate that it currently is not possible.
The full discussion about this topic (called associated methods) is here: https://github.com/mozilla/rust/issues/6894 and here: https://github.com/mozilla/rust/issues/8888