Rust’s Ownership concept becomes easier to understand once we look at how Rust stores data.
In Rust, values with a fixed size known at compile time can usually be stored directly on the stack. We use Heap when the amount of data may change while the program is running in source
Let’s look at an example:
let age = 25;
let name = String::from("Alice");Rust knows the size of an integer such as i32. That size won’t change depending on the number stored inside it, hence this will be stored in a Stack data structure
But for the String, its different. It may contain five characters, fifty characters or several megabytes of text. We don’t know the exact size of this at compile time, hence it’ll use heap for data storage.
The String value keeps track of three pieces of information:
- A pointer to the string data
- The current length
- The capacity of the allocation
The metadata has a known size, so it can stay on the stack but the actual string bytes are stored on the heap
Transclude of rust-stack-heap
For this String “Alice”, the owner is name. Once this goes out of scope, Rust can release the String “Alice” allocation from the heap