Does serde_json handle f64 in a special way?

help
1

Hi, experts.

serde_json parses the 1.23456e30 as f64 which is different from let x = 1.23456e30.
Why?

Here are my depedencies:

[dependencies]
serde = { version = "1.0.219", features = ["derive"] }
serde_json = "1.0.140"

Here is the code:

use serde::Deserialize;

#[derive(Deserialize, Debug)]
struct f64s {
    values: Vec<f64>
}

    fn test_serde_json_f64() {
        let bytes = 1.23456e30_f64.to_le_bytes();
        println!("expected:   {:02x}{:02x}{:02x}{:02x}{:02x}{:02x}{:02x}{:02x}", 
            bytes[0], bytes[1], bytes[2], bytes[3], bytes[4],
            bytes[5], bytes[6], bytes[7]);
    
         let data = r#"
        {
            "values": [1.23456e30]
        }"#;
        let v: f64s = serde_json::from_str(data).unwrap();
        let bytes = v.values[0].to_le_bytes();
        println!("serde_json: {:02x}{:02x}{:02x}{:02x}{:02x}{:02x}{:02x}{:02x}", 
            bytes[0], bytes[1], bytes[2], bytes[3],
            bytes[4], bytes[5], bytes[6], bytes[7]);
    }

Here is the output:

expected:   d55eae31282a2f46
serde_json: d65eae31282a2f46

Many thanks in advance.

2

I can't reproduce this on the playground, which gives me

expected:   d55eae31282a2f46
serde_json: d55eae31282a2f46

as output. What platform are you running locally?

3

Thank you for your kind reply. It is Windows 10 x64 with msvc. vs2022 installed.

4

This might be the same problem you are having:

maybe try enabling the float_roundtrip feature on serde_json and see if it deserialises correctly?

Edit: this would explain why I can't reproduce it on the playground, where the float_roundtrip feature is enabled.

3 Likes
5

Thank you! That does fix the issue.

serde_json = { version = "1.0.140", features = ["float_roundtrip"] }
2 Likes