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< title > Rc - Easy Rust< / title >
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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "Chapter_0.html" > < strong aria-hidden = "true" > 1.< / strong > Updates< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_1.html" > < strong aria-hidden = "true" > 2.< / strong > Introduction< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_2.html" > < strong aria-hidden = "true" > 3.< / strong > Who am I?< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_3.html" > < strong aria-hidden = "true" > 4.< / strong > Writing Rust in Easy English< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_4.html" > < strong aria-hidden = "true" > 5.< / strong > Rust Playground< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_5.html" > < strong aria-hidden = "true" > 6.< / strong > 🚧 and ⚠️< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_6.html" > < strong aria-hidden = "true" > 7.< / strong > Comments< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_7.html" > < strong aria-hidden = "true" > 8.< / strong > Types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_8.html" > < strong aria-hidden = "true" > 9.< / strong > Type inference< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_9.html" > < strong aria-hidden = "true" > 10.< / strong > Printing 'hello, world!'< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_10.html" > < strong aria-hidden = "true" > 11.< / strong > Display and debug< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_11.html" > < strong aria-hidden = "true" > 12.< / strong > Mutability (changing)< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_12.html" > < strong aria-hidden = "true" > 13.< / strong > The stack, the heap, and pointers< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_13.html" > < strong aria-hidden = "true" > 14.< / strong > More about printing< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_14.html" > < strong aria-hidden = "true" > 15.< / strong > Strings< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_15.html" > < strong aria-hidden = "true" > 16.< / strong > const and static< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_16.html" > < strong aria-hidden = "true" > 17.< / strong > More on references< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_17.html" > < strong aria-hidden = "true" > 18.< / strong > Mutable references< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_18.html" > < strong aria-hidden = "true" > 19.< / strong > Giving references to functions< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_19.html" > < strong aria-hidden = "true" > 20.< / strong > Copy types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_20.html" > < strong aria-hidden = "true" > 21.< / strong > Collection types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_21.html" > < strong aria-hidden = "true" > 22.< / strong > Vectors< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_22.html" > < strong aria-hidden = "true" > 23.< / strong > Tuples< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_23.html" > < strong aria-hidden = "true" > 24.< / strong > Control flow< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_24.html" > < strong aria-hidden = "true" > 25.< / strong > Structs< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_25.html" > < strong aria-hidden = "true" > 26.< / strong > Enums< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_26.html" > < strong aria-hidden = "true" > 27.< / strong > Loops< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_27.html" > < strong aria-hidden = "true" > 28.< / strong > Implementing structs and enums< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_28.html" > < strong aria-hidden = "true" > 29.< / strong > Destructuring< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_29.html" > < strong aria-hidden = "true" > 30.< / strong > References and the dot operator< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_30.html" > < strong aria-hidden = "true" > 31.< / strong > Generics< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_31.html" > < strong aria-hidden = "true" > 32.< / strong > Option and Result< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_32.html" > < strong aria-h
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< h2 id = "rc" > < a class = "header" href = "#rc" > Rc< / a > < / h2 >
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< p > Rc means " reference counter" . You know that in Rust, every variable can only have one owner. That is why this doesn't work:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn takes_a_string(input: String) {
println!(" It is: {}" , input)
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}
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fn also_takes_a_string(input: String) {
println!(" It is: {}" , input)
}
fn main() {
let user_name = String::from(" User MacUserson" );
takes_a_string(user_name);
also_takes_a_string(user_name); // ⚠️
}
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< / code > < / pre > < / pre >
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< p > After < code > takes_a_string< / code > takes < code > user_name< / code > , you can't use it anymore. Here that is no problem: you can just give it < code > user_name.clone()< / code > . But sometimes a variable is part of a struct, and maybe you can't clone the struct. Or maybe the < code > String< / code > is really long and you don't want to clone it. These are some reasons for < code > Rc< / code > , which lets you have more than one owner. An < code > Rc< / code > is like a good office worker: < code > Rc< / code > writes down who has ownership, and how many. Then once the number of owners goes down to 0, the variable can disappear.< / p >
< p > Here's how you use an < code > Rc< / code > . First imagine two structs: one called < code > City< / code > , and another called < code > CityData< / code > . < code > City< / code > has information for one city, and < code > CityData< / code > puts all the cities together in < code > Vec< / code > s.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > #[derive(Debug)]
struct City {
name: String,
population: u32,
city_history: String,
}
#[derive(Debug)]
struct CityData {
names: Vec< String> ,
histories: Vec< String> ,
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}
fn main() {
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let calgary = City {
name: " Calgary" .to_string(),
population: 1_200_000,
// Pretend that this string is very very long
city_history: " Calgary began as a fort called Fort Calgary that..." .to_string(),
};
let canada_cities = CityData {
names: vec![calgary.name], // This is using calgary.name, which is short
histories: vec![calgary.city_history], // But this String is very long
};
println!(" Calgary's history is: {}" , calgary.city_history); // ⚠️
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}
< / code > < / pre > < / pre >
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< p > Of course, it doesn't work because < code > canada_cities< / code > now owns the data and < code > calgary< / code > doesn't. It says:< / p >
< pre > < code class = "language-text" > error[E0382]: borrow of moved value: `calgary.city_history`
--> src\main.rs:27:42
|
24 | histories: vec![calgary.city_history], // But this String is very long
| -------------------- value moved here
...
27 | println!(" Calgary's history is: {}" , calgary.city_history); // ⚠️
| ^^^^^^^^^^^^^^^^^^^^ value borrowed here after move
|
= note: move occurs because `calgary.city_history` has type `std::string::String`, which does not implement the `Copy` trait
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< / code > < / pre >
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< p > We can clone the name: < code > names: vec![calgary.name.clone()]< / code > but we don't want to clone the < code > city_history< / code > , which is long. So we can use an < code > Rc< / code > .< / p >
< p > Add the < code > use< / code > declaration:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::rc::Rc;
fn main() {}
< / code > < / pre > < / pre >
< p > Then put < code > Rc< / code > around < code > String< / code > .< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::rc::Rc;
#[derive(Debug)]
struct City {
name: String,
population: u32,
city_history: Rc< String> ,
}
#[derive(Debug)]
struct CityData {
names: Vec< String> ,
histories: Vec< Rc< String> > ,
}
fn main() {}
< / code > < / pre > < / pre >
< p > To add a new reference, you have to < code > clone< / code > the < code > Rc< / code > . But hold on, didn't we want to avoid using < code > .clone()< / code > ? Not exactly: we didn't want to clone the whole String. But a clone of an < code > Rc< / code > just clones the pointer - it's basically free. It's like putting a name sticker on a box of books to show that two people own it, instead of making a whole new box.< / p >
< p > You can clone an < code > Rc< / code > called < code > item< / code > with < code > item.clone()< / code > or with < code > Rc::clone(& item)< / code > . So calgary.city_history has 2 owners. We can check the number of owners with < code > Rc::strong_count(& item)< / code > . Also let's add a new owner. Now our code looks like this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::rc::Rc;
#[derive(Debug)]
struct City {
name: String,
population: u32,
city_history: Rc< String> , // String inside an Rc
}
#[derive(Debug)]
struct CityData {
names: Vec< String> ,
histories: Vec< Rc< String> > , // A Vec of Strings inside Rcs
}
fn main() {
let calgary = City {
name: " Calgary" .to_string(),
population: 1_200_000,
// Pretend that this string is very very long
city_history: Rc::new(" Calgary began as a fort called Fort Calgary that..." .to_string()), // Rc::new() to make the Rc
};
let canada_cities = CityData {
names: vec![calgary.name],
histories: vec![calgary.city_history.clone()], // .clone() to increase the count
};
println!(" Calgary's history is: {}" , calgary.city_history);
println!(" {}" , Rc::strong_count(& calgary.city_history));
let new_owner = calgary.city_history.clone();
}
< / code > < / pre > < / pre >
< p > This prints < code > 2< / code > . And < code > new_owner< / code > is now an < code > Rc< String> < / code > . Now if we use < code > println!(" {}" , Rc::strong_count(& calgary.city_history));< / code > , we get < code > 3< / code > .< / p >
< p > So if there are strong pointers, are there weak pointers? Yes, there are. Weak pointers are useful because if two < code > Rc< / code > s point at each other, they can't die. This is called a " reference cycle" . If item 1 has an Rc to item 2, and item 2 has an Rc to item 1, they can't get to 0. In this case you want to use weak references. Then < code > Rc< / code > will count the references, but if it only has weak references then it can die. You use < code > Rc::downgrade(& item)< / code > instead of < code > Rc::clone(& item)< / code > to make weak references. Also, you use < code > Rc::weak_count(& item)< / code > to see the weak count.< / p >
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