Crate erased_serde

Expand description

This crate provides type-erased versions of Serde’s Serialize, Serializer and Deserializer traits that can be used as trait objects.

The usual Serde Serialize, Serializer and Deserializer traits cannot be used as trait objects like &dyn Serialize or boxed trait objects like Box<dyn Serialize> because of Rust’s “object safety” rules. In particular, all three traits contain generic methods which cannot be made into a trait object.

This library should be considered a low-level building block for interacting with Serde APIs in an object-safe way. Most use cases will require higher level functionality such as provided by typetag which uses this crate internally.

The traits in this crate work seamlessly with any existing Serde Serialize and Deserialize type and any existing Serde Serializer and Deserializer format.

Serialization

use erased_serde::{Serialize, Serializer};
use std::collections::BTreeMap as Map;
use std::io;

fn main() {
    // Construct some serializers.
    let json = &mut serde_json::Serializer::new(io::stdout());
    let cbor = &mut serde_cbor::Serializer::new(serde_cbor::ser::IoWrite::new(io::stdout()));

    // The values in this map are boxed trait objects. Ordinarily this would not
    // be possible with serde::Serializer because of object safety, but type
    // erasure makes it possible with erased_serde::Serializer.
    let mut formats: Map<&str, Box<dyn Serializer>> = Map::new();
    formats.insert("json", Box::new(<dyn Serializer>::erase(json)));
    formats.insert("cbor", Box::new(<dyn Serializer>::erase(cbor)));

    // These are boxed trait objects as well. Same thing here - type erasure
    // makes this possible.
    let mut values: Map<&str, Box<dyn Serialize>> = Map::new();
    values.insert("vec", Box::new(vec!["a", "b"]));
    values.insert("int", Box::new(65536));

    // Pick a Serializer out of the formats map.
    let format = formats.get_mut("json").unwrap();

    // Pick a Serialize out of the values map.
    let value = values.get("vec").unwrap();

    // This line prints `["a","b"]` to stdout.
    value.erased_serialize(format).unwrap();
}

Deserialization

use erased_serde::Deserializer;
use std::collections::BTreeMap as Map;

fn main() {
    static JSON: &'static [u8] = br#"{"A": 65, "B": 66}"#;
    static CBOR: &'static [u8] = &[162, 97, 65, 24, 65, 97, 66, 24, 66];

    // Construct some deserializers.
    let json = &mut serde_json::Deserializer::from_slice(JSON);
    let cbor = &mut serde_cbor::Deserializer::from_slice(CBOR);

    // The values in this map are boxed trait objects, which is not possible
    // with the normal serde::Deserializer because of object safety.
    let mut formats: Map<&str, Box<dyn Deserializer>> = Map::new();
    formats.insert("json", Box::new(<dyn Deserializer>::erase(json)));
    formats.insert("cbor", Box::new(<dyn Deserializer>::erase(cbor)));

    // Pick a Deserializer out of the formats map.
    let format = formats.get_mut("json").unwrap();

    let data: Map<String, usize> = erased_serde::deserialize(format).unwrap();

    println!("{}", data["A"] + data["B"]);
}