Rust downcasting

downcast.rs

use std::any::Any;

fn main() {
    // Create boxes containing different types, boxed as Any
    let integer_box: Box<dyn Any> = Box::new(42);
    let string_box: Box<dyn Any> = Box::new(String::from("Hello"));
    let float_box: Box<dyn Any> = Box::new(3.14f64);

    // Demonstrate successful downcasting
    println!("Downcasting examples:");
    
    // Method 1: Using is() and downcast_ref()
    if integer_box.is::<i32>() {
        if let Some(value) = integer_box.downcast_ref::<i32>() {
            println!("Integer (ref): {}", value);
        }
    }

    // Method 2: Using downcast() which returns Result
    match string_box.downcast::<String>() {
        Ok(string_value) => println!("String (owned): {}", string_value),
        Err(box_any) => println!("Downcast failed: {:?}", box_any.type_id()),
    }

    // Demonstrate failed downcast
    println!("\nFailed downcast example:");
    match float_box.downcast::<i32>() {
        Ok(value) => println!("This won't print: {}", value),
        Err(_) => println!("Failed to downcast f64 to i32 (as expected)"),
    }

    // Example with a function that returns Box<dyn Any>
    let mystery_box = create_mystery_value(true);
    let mystery_box2 = create_mystery_value(false);

    println!("\nDowncasting mystery values:");
    match mystery_box.downcast::<i32>() {
        Ok(value) => println!("Mystery value was an integer: {}", value),
        Err(b) => println!("Mystery value was not an integer: {:?}", b.type_id()),
    }

    match mystery_box2.downcast::<String>() {
        Ok(value) => println!("Mystery value was a string: {}", value),
        Err(b) => println!("Mystery value was not a string: {:?}", b.type_id()),
    }
}

fn create_mystery_value(want_integer: bool) -> Box<dyn Any> {
    if want_integer {
        Box::new(42)
    } else {
        Box::new(String::from("Mystery solved!"))
    }
}