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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. That trait can have a simple naming like NAME_OF_ORIGINAL_TRAIT + Storage suffix.

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

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,}

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:

pub trait Point: 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 {    fn x(&self) -> u32;
    fn y(&self) -> u32;
    fn name(&self) -> String;}
impl<T: PointStorage> Point for T {    default fn x(&self) -> u32 {        PointStorage::get(self).x    }
    default fn y(&self) -> u32 {        PointStorage::get(self).y    }
    default fn name(&self) -> String {        "AlphaPoint".to_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 Point for PointContract {}

Macros from OpenBrush#

Some macros from OpenBrush allows to remove boilerplate code and simplify the development:

use brush::declare_storage_trait;declare_storage_trait!(PointStorage, PointData);
  • impl_storage_trait! macro implements the storage trait for the contract and return the field from that contract of the data type
use brush::impl_storage_trait;impl_storage_trait!(PointStorage, PointContract, point, PointData);
  • Also, if you are familiar with derive macro:

You can create a derive macro for your storage trait by yourself with declare_derive_storage_trait! from OpenBrush. To define a derive macro you need a separate directory(let's call it derive). This directory contains the standard stuff of a Cargo folder - .gitignore, Cargo.toml, and the file, inside of which we will define a derive. So in the end, our file will look like this:

#![cfg_attr(not(feature = "std"), no_std)]
extern crate proc_macro;
use brush_derive::declare_derive_storage_trait;
declare_derive_storage_trait!(derive_point_storage, PointStorage, PointStorageField);

In the Cargo.toml of the derive folder you need to import brush_derive dependencies:

[dependencies]syn = { version = "1.0" }quote = "1.0"proc-macro2 = "1"brush_derive = { tag = "v1.2.0", git = ""}
[lib]name = "point_derive"path = ""proc-macro = true
[features]default = ["std"]std = []

After importing that derive crate into your main contract, you can use derive(PointStorage) instead of impl_storage_trait!.

use point_derive::PointStorage;
#[derive(PointStorage)]struct PointContract {    #[PointStorageField]    point: PointData,}
impl Point for PointContract {}