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< h2 > < a class = "header" href = "#references-and-the-dot-operator" id = "references-and-the-dot-operator" > References and the dot operator< / a > < / h2 >
< p > We learned that when you have a reference, you need to use < code > *< / code > to get to the value. A reference is a different type, so this won't work:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let my_number = 9;
let reference = & my_number;
println!(" {}" , my_number == reference); // ⚠️
}
< / code > < / pre > < / pre >
< p > The compiler prints:< / p >
< pre > < code class = "language-text" > error[E0277]: can't compare `{integer}` with `& {integer}`
--> src\main.rs:5:30
|
5 | println!(" {}" , my_number == reference);
| ^^ no implementation for `{integer} == & {integer}`
< / code > < / pre >
< p > So we change line 5 to < code > println!(" {}" , my_number == *reference);< / code > and now it prints < code > true< / code > because it's now < code > i32< / code > == < code > i32< / code > , not < code > i32< / code > == < code > & i32< / code > . This is called dereferencing.< / p >
< p > But when you use a method, Rust will dereference for you. The < code > .< / code > in a method is called the dot operator, and it does dereferencing for free.< / p >
< p > First, let's make a struct with one < code > u8< / code > field. Then we will make a reference to it and try to compare. It will not work:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > struct Item {
number: u8,
}
fn main() {
let item = Item {
number: 8,
};
let reference_number = & item.number; // reference number type is & u8
println!(" {}" , reference_number == 8); // ⚠️ & u8 and u8 cannot be compared
}
< / code > < / pre > < / pre >
< p > To make it work, we need to dereference: < code > println!(" {}" , *reference_number == 8);< / code > .< / p >
< p > But with the dot operator, we don't need < code > *< / code > . For example:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > struct Item {
number: u8,
}
fn main() {
let item = Item {
number: 8,
};
let reference_item = & item;
println!(" {}" , reference_item.number == 8); // we don't need to write *reference_item.number
}
< / code > < / pre > < / pre >
< p > Now let's create a method for < code > Item< / code > that compares < code > number< / code > to another number. We don't need to use < code > *< / code > anywhere:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > struct Item {
number: u8,
}
impl Item {
fn compare_number(& self, other_number: u8) { // takes a reference to self
println!(" Are {} and {} equal? {}" , self.number, other_number, self.number == other_number);
// We don't need to write *self.number
}
}
fn main() {
let item = Item {
number: 8,
};
let reference_item = & item; // This is type & Item
let reference_item_two = & reference_item; // This is type & & Item
item.compare_number(8); // the method works
reference_item.compare_number(8); // it works here too
reference_item_two.compare_number(8); // and here
}
< / code > < / pre > < / pre >
< p > So just remember: when you use the < code > .< / code > operator, you don't need to worry about < code > *< / code > .< / p >
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