Converting C-Like Enums to String values in Rust

Note: The contents of this post are based on an API that is now deprecated. For latest guidance over Rust usage of enums from a C/C++ perspective, I recommend reading fasterthanlime’s Peeking inside a rust enum guide.

Suppose you have a C-like enumerator in Rust:

enum Foo {
    SomeValue,
    OtherValue
}

Note: C-Like enumerations in Rust have no parameters (i.e. SomeValue(~str)), and can have their discriminator values explicitly set to a constant value (i.e. SomeValue = 0). See the Rust book section 4.13 Enums for details.

If you want to obtain the string value (i.e. “SomeValue”) given an unsigned integer, the trick is to first cast it as a Foo value, then use format to obtain its string representation. For the former step, first we need to make Foo implement the CLike trait:

extern mod extra;

use std::cast;
use extra::enum_set::CLike;

#[repr(uint)]
enum Foo {
    SomeValue,
    OtherValue
}

impl CLike for Foo {
    fn to_uint(&self) -> uint {
        *self as uint
    }
    fn from_uint(v: uint) -> Foo {
        unsafe { cast::transmute(v) }
    }
}

The latter step is easy, we just need to format the output as string. The simplest way is to make Foo derive from ToStr:

extern mod extra;

use std::cast;
use extra::enum_set::CLike;
use std::to_str::ToStr;

#[repr(uint)]
#[deriving(ToStr)]
enum Foo {
    SomeValue,
    OtherValue
}

impl CLike for Foo {
    fn to_uint(&self) -> uint {
        *self as uint
    }
    fn from_uint(v: uint) -> Foo {
        unsafe { cast::transmute(v) }
    }
}

fn main() {
    let i : Foo = CLike::from_uint(1);
    println!("{}", i.to_str());
}

The output for this code is SomeValue.