Table of contents📚
- What is a reference❔
- How to create a reference❔
- How to use a reference🤹
- Modifying a reference✏️
- Warning⚠️
References🔗
What is a reference❔
A reference is just another way to refer to a variable, it is similar to a pointer in other languages.
For exampe if your 🐕 Dog is called Domenic, and you call him Dom, you’re still referring to him. So Dom is a reference to Domenic.
Dom ----------> Domenic
🐕 Your dog
This is the same with variables.
How to create a reference❔
First, we need to create a variable.
let domenic = "Dog 🐶";
Then, we can create a reference to the variable by adding the & symbol before the variable name.
let mut dom = &domenic;
The & symbol means that we want to get a reference to the variable next to it.
So now, the variable Dom is a reference to Domenic.
How to use a reference🤹
You can use a reference to access the value of the variable.
For example, if you want to print the value of the variable dom, you can do it like this:
println!("The value of Dom is : {}", dom);
println!("The value of Domenic is : {}", domenic);
The output will be:
The value of Dom is : Dog 🐶
The value of Domenic is : Dog 🐶
ℹ️ We can have multiple references to the same variable.
Modifying a reference✏️
dom being a reference to Domenic, it looks like we can modify it’s value by doing
dom = "Doggo 🐶";
But this is not possible.
Firstly
Because by defaults, references are immutable, so we can’t modify the value of the variable.
To make a reference mutable, we need to add the mut keyword before the variable name.
let dom = &mut domenic;
Now, we can modify the value of the variable.
Secondly
When we do
dom = "Doggo 🐶";
We don’t modify the value of the variable Domenic, but we modify the value of the variable Dom that is a reference to Domenic.
Before
Dom Domenic
--------------> "Dog 🐶"
After
Dom Domenic
"Doggo 🐶" "Dog 🐶"
To modify the value of the variable Domenic, we need to use the * symbol before the variable name.
*dom = "Doggo 🐶";
Because *dom means that we are referring to the value of domenic not the value of dom.
Warning⚠️
Firstly
We can’t have more than one mutable reference to the same variable.
Secondly
You can only use one mutable reference at the same time depending on the rustc version.
In the following example :
let mut username = "Super cool username 💪";
let username_ref = &mut username;
println!("The username is : {}", username);
We will encounter an error because the println! macro borrows the variable username and we already have a mutable reference to it.