Written by Sangwan Kwon
on October 22, 2020

The serde with no_std

This post is about sample code about serde_json_core. Previous post about serde is here.

Serde with no_std

In order to use Serde in a no_std context default-features needs to be disabled. And derive feature needs to be added for Serialize, Deserialize trait.

# Cargo.toml
[dependencies]
serde = { version = "1.0", default-features = false, features = ["derive"] }

Serde_json no_std

Use serde-json-core instead of serde-json in a no_std context.

# Cargo.toml
[dependencies]
serde = { version = "1.0", default-features = false, features = ["derive"] }
serde-json-core = "0.1.0"
heapless = "0.5.5"

use heapless::consts::*;
use serde::{Serialize, Deserialize};
use serde_json_core;

#[derive(Serialize, Deserialize, Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let point = Point { x: 1, y: 2 };

    // Convert the Point to a JSON string.
    // U30 is the size of String (It should be specified for heapless String
    let serialized = serde_json_core::to_string::<U30, Point>(&point).unwrap();
    // Prints serialized = {"x":1,"y":2}
    println!("serialized = {}", serialized);

    // Another way to write above code
    let serialized: heapless::String<U30> = serde_json_core::to_string(&point).unwrap();
    println!("serialized = {}", serialized);

    // Convert the JSON string back to a Point.
    let deserialized: Point = serde_json_core::from_str(&serialized).unwrap();

    // Prints deserialized = Point { x: 1, y: 2 }
    println!("deserialized = {:?}", deserialized);
}

string type

Rust-core and serde-json-core do not support std::string::String type.

For string, we should use core::str like below.

let hello: &str = "Hello, world!";
let serialized: heapless::String<U30> = serde_json_core::to_string(&hello).unwrap();
let deserialized: &str = serde_json_core::from_str(&serialized).unwrap();
println!("{}", deserialized);

Check supported types.

Summary

It is reasonable to specify the size of the string in order not to use the rust heap allocator. But it makes the API inconvenience.

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