Self

self is used to represent the specific instance of a Contract. It is used to access variables that are owned by that specific instance. This works the same way for Fe contracts as for, e.g. self in the context of classes in Python, or this in Javascript. self gives access to constants from the contract code and state variables from contract storage.

Note: Here we focus on functions defined inside a contract, giving access to contract storage; however, self can also be used to read and write struct fields where functions are defined inside structs.

If a function takes self as a parameter, the function must be called via self. For example:

#![allow(unused)]
fn main() {
let ok: bool = self.transfer(from, to, value)
}

Mutability

self is immutable and can be used for read-only operations on the contract storage (or struct fields). In order to write to the contract storage, you must use mut self. This makes the contract instance mutable and allows the contract storage to be updated.

Examples

Reading contract storage

contract example {

    value: u256;

    pub fn check_value(self) -> u256 {
        return self.value;
    }
}

Writing contract storage

contract example {

    value: u256;

    pub fn update_value(mut self) {
        self.value += 1;
    }
}