Understanding basic Ownership and Borrowing in Rust
In this lesson we'll briefly discuss the concept of Ownership and Borrowing in Rust, and how expressions can be passed by value and by reference.
Pascal Precht: [00:00] Let's have a look of this function say_first_name(). say_first_name() takes a first_name of type string and then outputs the name using println! And the placeholder syntax. We then call this function in our main function by passing it a variable first_name, which has the value "Pascal." If I run this function, we'll see that it will properly output my name.
[00:26] However, if we now go ahead and call this function again, we'll see that the compiler complains. In fact, it tells us that when we try to call say_first_name the second time, the value passed to that function has been used after a move. It also tells us that the move has happened the first time we called say_first_name.
[00:55] What does that mean? Rust comes with a feature called ownership, which aims to prevent us from writing memory unsafe code. What this means is that every variable in Rust owns its value. This variable first_name owns the value string "Pascal."
[01:15] However, if we pass the value to a function like we do here in say_first_name, we move the value and therefore the ownership to the function first_name(). This means that after this function has been called with this value, we can no longer use this variable.
[01:37] The reason for that is after this function has been called the first time, Rust will drop the value. What this means is we no longer have access to first_name after line four when the program is executed.
[01:51] If we do need access to first_name after this function has been called the first time, we will have to pass it by reference instead of by value. This is done by using the & symbol and by updating the function's signature the same way.
[02:06] Using the &, we're basically saying that this is a string reference and not a string value. Instead of passing ownership from this variable to this function, we're saying, "Here's a reference to this value." This is also called borrowing because we don't pass ownership to the function, but we borrow the value.
[02:28] We update the function here as well, save the file, run the program and we'll see everything runs as expected.