Convert fastnum decimal number to usize

Question

Given a fastnum decimal number, how to convert it to a primitive integer like usize? The fastnum source file to_primitive.rs contains a mention of:

    to_uint_impl!(to_usize, usize);

But when I try to use it as in the following:

use fastnum::{dec256};

fn main() {
    let a = dec256!(10);
    let b = a.to_usize();
    println!("{}", b);
}

It gives this error message:

error[E0599]: no method named `to_usize` found for struct `Decimal` in the current scope
 --> src\main.rs:5:15
  |
5 |     let b = a.to_usize();
  |               ^^^^^^^^ method not found in `Decimal<4>`

What am I missing?

Answer 1

There is a TryFrom<Decimal<N>> for usize implementation available:

use fastnum::dec256;

fn main() {
    let a = dec256!(10);
    let b = usize::try_from(a);
    println!("{:?}", b);

    let a = dec256!(-10);
    let b = usize::try_from(a);
    println!("{:?}", b);
}

Ok(10)
Err(PosOverflow)

---

The .to_usize() method is provided via the ToPrimitive trait implementation which is from the num-traits crate. To use it, you would need to enable the "numtraits" feature and import the trait:

[dependencies]
fastnum = { version = "0.2.2", features = ["numtraits"] }
num-traits = "0.2.19"

use fastnum::dec256;
use num_traits::ToPrimitive;

fn main() {
    let a = dec256!(10);
    let b = a.to_usize();
    println!("{:?}", b);
}

Some(10)