Table of contents📚
Pass by reference🖇️
The problem❌
Let’s try to create a function that takes one argument.
struct Zoo {
lions: u8,
bears: u8,
elephants: u8,
penguins: u8,
seals: u8,
}
fn main() {
let my_zoo = Zoo {lions: 2, bears: 1, elephants: 3, penguins: 8, seals: 5};
print_zoo(my_zoo);
}
fn print_zoo(zoo: Zoo) {
println!("🐯 {}", zoo.lions);
println!("🐻 {}", zoo.bears);
println!("🐘 {}", zoo.elephants);
println!("🐧 {}", zoo.penguins);
println!("🦭 {}", zoo.seals);
}
the function print_zoo takes one argument of type Zoo (my_zoo).
The code works as expected.
Output:
🐯 2
🐻 1
🐘 3
🐧 8
🦭 5
But if we try to call the function again:
print_zoo(my_zoo);
print_zoo(my_zoo);
we get the following error:
Simply because when we call print_zoo, the variable my_zoo is transferred from the scope of the main function to the scope of the print_zoo function.
The solution💡
We can solve this problem by passing a reference to my_zoo instead of the variable itself when we call the function.
And we will have to make the function wait for the variable to be passed by reference.
struct Zoo {
...
}
fn main() {
let my_zoo = Zoo {lions: 2, bears: 1, elephants: 3, penguins: 8, seals: 5};
print_zoo(&my_zoo);
print_zoo(&my_zoo);
}
fn print_zoo(zoo: &Zoo) {
...
}
Output:
🐯 2
🐻 1
🐘 3
🐧 8
🦭 5
<------ Second call of the function
🐯 2
🐻 1
🐘 3
🐧 8
🦭 5