How to recover the concrete type from Any

Question

Code:

use std::fmt::Debug;
use std::any::Any;

fn any_to_u16(value: &dyn Any)
{
    let v = value as u16;
}

fn main() 
{
    let x = true;
    any_to_u16(&x);
}

Erorr :

error[E0606]: casting `&(dyn std::any::Any + 'static)` as `u16` is invalid
 --> src/lib.rs:6:13
  |
6 |     let v = value as u16;
  |             ^^^^^^^^^^^^
  |
  = help: cast through a raw pointer first

Playground How to fix?

What are you trying to do? The code does not match the title, and what if `value` is a vector of strings, for example? — Boiethios, Feb 21 '20 at 16:01
`Any` can be anything: you do understand that you cannot convert anything to an `u16` or a `bool`, right? The conversion must actually make sense — Boiethios, Feb 21 '20 at 16:07
It sounds like you want a predicate function instead, which is the most reasonable way to map a value to either `true` or `false`. — E_net4, Feb 21 '20 at 16:11
I want to convert bool-> u16. For example: If the value var_bool == true then var_u16 == 0x01. — Michał Hanusek, Feb 21 '20 at 16:13

score 1 · Answer 1 · answered Feb 21 '20 at 16:25

1

You must use Any::downcast_ref:

use std::any::Any;

fn any_to_u16(value: &dyn Any)
{
    if let Some(value) = value.downcast_ref::<bool>().map(|b| *b as u16) {
        // value is a `bool`
        assert_eq!(value, 1);
    }
}

fn main() 
{
    let x = true;
    any_to_u16(&x);
}

answered Feb 21 '20 at 16:25

Boiethios

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Given the comments I think the OP actually wants to use ˋTryInto`. In any case, this function in no way works for any type. – mcarton Feb 21 '20 at 18:36

How to recover the concrete type from Any

1 Answers1