Data and derive macro

Data segregation

Rust doesn't have inheritance like OOP languages. If you want to "inherit" some fields, you can use structural composition. If you want to "inherit" some implementation, you can use traits. Traits can have a default implementation or a generic implementation. The traits in Rust can't contain fields, it is pure interfaces.

Based on that information we propose you the following concept of smart contract development:

Storage trait

Extract the logic of data storing into a separate trait to have the ability to define the default implementation without knowing what contract will inherit that. You can use that separate trait as a bound in your generic implementation(below we will describe).

You can define your own storage trait like:

pub trait PointStorage {
    fn get(&self) -> & PointData;
    fn get_mut(&mut self) -> &mut PointData;
}

Or you can use openbrush::traits::Storage trait from OpenBrush.

Storage is a generic trait, so you can use it to work with different storage. For example, if in your default implementation you need to have psp22::extensions::metadata::Data and psp22::Data, you can add bounds T: Storage<metadata::Data> + Storage<psp22::Data>. It allows you to work with two independent storage.

Data of the trait

That trait returns some data with fields that can be used in the implementation. The data is a simple struct with fields. Later that struct can be embedded into the contract struct.

pub struct PointData {
    pub x: u32,
    pub y: u32,
}

Also, you can use the openbrush::storage_item macro that implements that trait by default, and also prepare the storage to be upgradeable.

#[openbrush::storage_item]
pub struct PointData {
    pub x: u32,
    pub y: u32,
}

Upgradeable struct

For struct to be upgradeable, every field that is not Mapping or other lazy loaded type, it should be marked with #[lazy] attribute.

#[openbrush::storage_item]
pub struct PointData {
    #[lazy]
    pub x: u32,
    #[lazy]
    pub y: u32,
}

Default implementation

Define the default or generic implementation for your main trait with the restriction that Self should also implement storage trait.

A default implementation with impl trait:

pub trait PointImpl: PointStorage {
    fn x(&self) -> u32 {
        PointStorage::get(self).x
    }

    fn y(&self) -> u32 {
        PointStorage::get(self).y
    }

    fn name(&self) -> String {
        "AlphaPoint".to_string()
    }
}

or a generic implementation:

pub trait Point: PointImpl {
    fn x(&self) -> u32;

    fn y(&self) -> u32;

    fn name(&self) -> String;
}

A default implementation with openbrush::traits::Storage:

pub trait Point: openbrush::traits::Storage<PointData> {
    fn x(&self) -> u32 {
        self.data().x
    }

    fn y(&self) -> u32 {
        self.data().y
    }

    fn name(&self) -> String {
        "AlphaPoint".to_string()
    }
}

or a generic implementation with openbrush::traits::Storage:

pub trait Point {
    fn x(&self) -> u32;

    fn y(&self) -> u32;

    fn name(&self) -> String;
}

"Inheritance" of the implementation

When someone wants to "inherit" implementation and fields, he can embed the data structure, implement the storage trait, and define an impl section of the main trait:

struct PointContract {
    point: PointData,
}

impl PointStorage for PointContract {
    fn get(&self) -> & PointData {
        &self.point
    }
    fn get_mut(&mut self) -> &mut PointData {
        &mut self.point
    }
}

impl PointImpl for PointContract {}

impl Point for PointContract {
    #[ink(message)]
    fn x(&self) -> u32 {
        PointImpl::x(self)
    }
    
    #[ink(message)]
    fn y(&self) -> u32 {
        PointImpl::y(self)
    }
    
    #[ink(message)]
    fn name(&self) -> String {
        PointImpl::name(self)
    }
}

If you are using openbrush::traits::Storage trait, then you can use derive macro to automate the implementation of the trait. Each field for which you want to implement the Storage trait should be marked with #[storage_field].

use openbrush::traits::Storage;

#[derive(Storage)]
struct PointContract {
    #[storage_field]
    point: PointData,
}