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Visitor

Visitor in Rust

Visitor is a behavioral design pattern that allows adding new behaviors to existing class hierarchy without altering any existing code.

Read why Visitors can’t be simply replaced with method overloading in our article Visitor and Double Dispatch.

Learn more about Visitor

Deserialization

A real-world example of the Visitor pattern is serde serialization framework and its deserialization model (see Serde data model).

Visitor should be implemented for a deserializable type.
Visitor is passed to a Deserializer (an “Element” in terms of the Visitor Pattern), which accepts and drives the Visitor in order to construct a desired type.

Let’s reproduce this deserializing model in our example.

visitor.rs

use crate::{TwoValuesArray, TwoValuesStruct};

/// Visitor can visit one type, do conversions, and output another type.
///
/// It's not like all visitors must return a new type, it's just an example
/// that demonstrates the technique.
pub trait Visitor {
    type Value;

    /// Visits a vector of integers and outputs a desired type.
    fn visit_vec(&self, v: Vec<i32>) -> Self::Value;
}

/// Visitor implementation for a struct of two values.
impl Visitor for TwoValuesStruct {
    type Value = TwoValuesStruct;

    fn visit_vec(&self, v: Vec<i32>) -> Self::Value {
        TwoValuesStruct { a: v[0], b: v[1] }
    }
}

/// Visitor implementation for a struct of values array.
impl Visitor for TwoValuesArray {
    type Value = TwoValuesArray;

    fn visit_vec(&self, v: Vec<i32>) -> Self::Value {
        let mut ab = [0i32; 2];

        ab[0] = v[0];
        ab[1] = v[1];

        TwoValuesArray { ab }
    }
}

main.rs

#![allow(unused)]

mod visitor;

use visitor::Visitor;

/// A struct of two integer values.
///
/// It's going to be an output of `Visitor` trait which is defined for the type
/// in `visitor.rs`.
#[derive(Default, Debug)]
pub struct TwoValuesStruct {
    a: i32,
    b: i32,
}

/// A struct of values array.
///
/// It's going to be an output of `Visitor` trait which is defined for the type
/// in `visitor.rs`.
#[derive(Default, Debug)]
pub struct TwoValuesArray {
    ab: [i32; 2],
}

/// `Deserializer` trait defines methods that can parse either a string or
/// a vector, it accepts a visitor which knows how to construct a new object
/// of a desired type (in our case, `TwoValuesArray` and `TwoValuesStruct`).
trait Deserializer<V: Visitor> {
    fn create(visitor: V) -> Self;
    fn parse_str(&self, input: &str) -> Result<V::Value, &'static str> {
        Err("parse_str is unimplemented")
    }
    fn parse_vec(&self, input: Vec<i32>) -> Result<V::Value, &'static str> {
        Err("parse_vec is unimplemented")
    }
}

struct StringDeserializer<V: Visitor> {
    visitor: V,
}

impl<V: Visitor> Deserializer<V> for StringDeserializer<V> {
    fn create(visitor: V) -> Self {
        Self { visitor }
    }

    fn parse_str(&self, input: &str) -> Result<V::Value, &'static str> {
        // In this case, in order to apply a visitor, a deserializer should do
        // some preparation. The visitor does its stuff, but it doesn't do everything.
        let input_vec = input
            .split_ascii_whitespace()
            .map(|x| x.parse().unwrap())
            .collect();

        Ok(self.visitor.visit_vec(input_vec))
    }
}

struct VecDeserializer<V: Visitor> {
    visitor: V,
}

impl<V: Visitor> Deserializer<V> for VecDeserializer<V> {
    fn create(visitor: V) -> Self {
        Self { visitor }
    }

    fn parse_vec(&self, input: Vec<i32>) -> Result<V::Value, &'static str> {
        Ok(self.visitor.visit_vec(input))
    }
}

fn main() {
    let deserializer = StringDeserializer::create(TwoValuesStruct::default());
    let result = deserializer.parse_str("123 456");
    println!("{:?}", result);

    let deserializer = VecDeserializer::create(TwoValuesStruct::default());
    let result = deserializer.parse_vec(vec![123, 456]);
    println!("{:?}", result);

    let deserializer = VecDeserializer::create(TwoValuesArray::default());
    let result = deserializer.parse_vec(vec![123, 456]);
    println!("{:?}", result);

    println!(
        "Error: {}",
        deserializer.parse_str("123 456").err().unwrap()
    )
}

Output

Ok(TwoValuesStruct { a: 123, b: 456 })
Ok(TwoValuesStruct { a: 123, b: 456 })
Ok(TwoValuesArray { ab: [123, 456] })
Error: parse_str unimplemented

Visitor in Other Languages