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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 = "cow" > < a class = "header" href = "#cow" > Cow< / a > < / h2 >
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< p > Cow is a very convenient enum. It means " clone on write" and lets you return a < code > & str< / code > if you don't need a < code > String< / code > , and a < code > String< / code > if you need it. (It can also do the same with arrays vs. Vecs, etc.)< / p >
< p > To understand it, let's look at the signature. It says:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > pub enum Cow< 'a, B>
where
B: 'a + ToOwned + ?Sized,
{
Borrowed(& 'a B),
Owned(< B as ToOwned> ::Owned),
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}
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fn main() {}
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< / code > < / pre > < / pre >
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< p > You know right away that < code > 'a< / code > means it works with references. The < code > ToOwned< / code > trait means that it is a type that can be turned into an owned type. For example, < code > str< / code > is usually a reference (< code > & str< / code > ) and you can turn it into an owned < code > String< / code > .< / p >
< p > Next is < code > ?Sized< / code > . This means " maybe Sized, but maybe not" . Almost every type in Rust is Sized, but types like < code > str< / code > are not. That is why we need a < code > & < / code > for a < code > str< / code > , because the compiler doesn't know the size. So if you want a trait that can use something like a < code > str< / code > , you add < code > ?Sized.< / code > < / p >
< p > Next are the < code > enum< / code > variants. They are < code > Borrowed< / code > and < code > Owned< / code > .< / p >
< p > Imagine that you have a function that returns < code > Cow< 'static, str> < / code > . If you tell the function to return < code > " My message" .into()< / code > , it will look at the type: " My message" is a < code > str< / code > . This is a < code > Borrowed< / code > type, so it chooses < code > Borrowed(& 'a B)< / code > . So it becomes < code > Cow::Borrowed(& 'static str)< / code > .< / p >
< p > And if you give it a < code > format!(" {}" , " My message" ).into()< / code > then it will look at the type. This time it is a < code > String< / code > , because < code > format!< / code > makes a < code > String< / code > . So this time it will select " Owned" .< / p >
< p > Here is an example to test < code > Cow< / code > . We will put a number into a function that returns a < code > Cow< 'static, str> < / code > . Depending on the number, it will create a < code > & str< / code > or a < code > String< / code > . Then it uses < code > .into()< / code > to turn it into a < code > Cow< / code > . When you do that, it will choose either < code > Cow::Borrowed< / code > or < code > Cow::Owned< / code > . Then we will match to see which one it chose.< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::borrow::Cow;
fn modulo_3(input: u8) -> Cow< 'static, str> {
match input % 3 {
0 => " Remainder is 0" .into(),
1 => " Remainder is 1" .into(),
remainder => format!(" Remainder is {}" , remainder).into(),
}
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}
fn main() {
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for number in 1..=6 {
match modulo_3(number) {
Cow::Borrowed(message) => println!(" {} went in. The Cow is borrowed with this message: {}" , number, message),
Cow::Owned(message) => println!(" {} went in. The Cow is owned with this message: {}" , number, message),
}
}
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}
< / code > < / pre > < / pre >
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< p > This prints:< / p >
< pre > < code class = "language-text" > 1 went in. The Cow is borrowed with this message: Remainder is 1
2 went in. The Cow is owned with this message: Remainder is 2
3 went in. The Cow is borrowed with this message: Remainder is 0
4 went in. The Cow is borrowed with this message: Remainder is 1
5 went in. The Cow is owned with this message: Remainder is 2
6 went in. The Cow is borrowed with this message: Remainder is 0
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< / code > < / pre >
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< p > < code > Cow< / code > has some other methods like < code > into_owned< / code > or < code > into_borrowed< / code > so you can change it if you need to.< / p >
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