deserialize_null_and_missing_value.rs

// serde = "1.0"
// serde_derive = "1.0"
// serde_json = "1.0"
use serde::{Deserialize, Deserializer};
use serde_derive::Deserialize;

#[derive(Debug, Deserialize)]
struct Hoge {
    #[serde(deserialize_with="deserialize_option")]
    val: Option<u64>,
}

// deny if entry is not exist
// (null is acceptable)
fn deserialize_option<'de, D, T>(deserializer: D) -> Result<Option<T>, D::Error>
    where D: Deserializer<'de>,
          T: Deserialize<'de>
{
    Deserialize::deserialize(deserializer)
}

fn main() {
    let json_1 = r#"{ "val": 334 }"#;
    let ans_1: Hoge = serde_json::from_str(json_1).unwrap();
    assert_eq!(ans_1.val, Some(334));

    let json_2 = r#"{ "val": null }"#;
    let ans_2: Hoge = serde_json::from_str(json_2).unwrap();
    assert_eq!(ans_2.val, None);

    let json_3 = r#"{}"#;
    let ans_3: Result<Hoge, _> = serde_json::from_str(json_3);
    assert!(ans_3.is_err());
}