Can I serialize a subset of fields and use a function to deserialize the other fields without manually implementing Serialize or Deserialize?

Question

I have the following struct

struct S { value: f32, square: f32, }

I would like to derive square from value in the new(value):

impl S { fn new(value: f32) -> Self { Self { value, square: value.powi(2) } } }

Can I use value field only to serialize the struct and use new(value) to deserialize the whole thing without manually implementing Serialize/Deserialize traits?

what is the point of that? if you do that then deserialization wouldn't be safe — Daniel A. White
– Daniel A. White, Commented Jul 23, 2021 at 14:54
It looks like your question might be answered by the answers of How to transform fields during deserialization using Serde?. If not, please edit your question to explain the differences. Otherwise, we can mark this question as already answered. — Shepmaster
– Shepmaster, Commented Jul 23, 2021 at 14:56
@hombit yes, and the fully working example I provided shows that exactly. — Shepmaster
– Shepmaster, Commented Jul 23, 2021 at 15:03
Does this answer your question? How to transform fields during deserialization using Serde? — Stargateur
– Stargateur, Commented Jul 23, 2021 at 15:14

jonasbb · Accepted Answer · 2021-07-23 16:04:26Z

You can implement deserialization with the #[serde(from = "FromType")] attribute. If the transformation can fail, there is also the try_from attribute.

Deserialize this type by deserializing into FromType, then converting. This type must implement From<FromType>, and FromType must implement Deserialize.

Serializing a subset of fields can be achieved by applying the #[serde(skip)] annotation on the fields you want to omit.

Applied to your use case a solution looks like this:

#[derive(Debug, Serialize, Deserialize)] #[serde(from = "DeserS")] struct S { value: f32, #[serde(skip)] square: f32, } #[derive(Deserialize)] struct DeserS { value: f32, } impl From<DeserS> for S { fn from(tmp: DeserS) -> Self { Self { value: tmp.value, square: tmp.value.powi(2), } } }

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