How do I tell Serde to deserialize an object in the order of a struct definition?

Question

I'm trying to store the value of a field in a global context, but this doesn't work when the JSON object is in the wrong order.

How can I make it so the output is always, a then b?

Playground

use serde::{Deserialize, Deserializer};
use std::error::Error;

fn a_deserialize<'de, D>(deserializer: D) -> Result<u32, D::Error>
where
    D: Deserializer<'de>,
{
    let num = Deserialize::deserialize(deserializer)?;

    println!("a");

    Ok(num)
}

fn b_deserialize<'de, D>(deserializer: D) -> Result<u32, D::Error>
where
    D: Deserializer<'de>,
{
    let num = Deserialize::deserialize(deserializer)?;

    println!("b");

    Ok(num)
}

#[derive(Deserialize)]
struct Test {
    #[serde(deserialize_with = "a_deserialize")]
    a: u32,
    #[serde(deserialize_with = "b_deserialize")]
    b: u32,
}

fn main() -> Result<(), Box<dyn Error>> {
    // notice the order is b then a
    let str = "{ \"b\": 1, \"a\": 2 }";

    let test: Test = serde_json::from_str(&str)?;
    // it prints out b then a, but I want a then b -- the same as in the struct definition

    Ok(())
}

Answer 1

Serde was designed to incorporate a stateful deserializer which processes the various items in sequence, as the source data is consumed. As such, it is unrealistic to tell this deserializer to deserialize content in a different order. Given the JSON object

{"b": 1, "a": 2}

b appears first, so that will be processed first. Anything else would have required the deserializer to keep track of potentially all data read in the past, keeping it "on hold" to visit it again at a later stage, making it more inefficient.

It is more realistic and feasible to adjust your program accordingly: visit the fields of a struct after it has been fully deserialized.

#[derive(Deserialize)]
struct Test {
    a: u32,
    b: u32,
}

fn main() -> Result<(), Box<dyn Error>> {
    let str = "{ \"b\": 1, \"a\": 2 }";

    let test: Test = serde_json::from_str(&str)?;
    
    println!("a: {}", test.a);
    println!("b: {}", test.b);

    Ok(())
}

See also:

• Access struct field by variable
• How do I create mutable iterator over struct fields