How can I use the format! macro in a no_std environment?

Question

How could I implement the following example without using std?

let text = format!("example {:.1} test {:x} words {}", num1, num2, num3);

text has type &str and num1, num2 and num3 have any numeric type.

I've tried using numtoa and itoa/dtoa for displaying numbers but numtoa does not support floats and itoa does not support no_std. I feel like displaying a number in a string is fairly common and that I'm probably missing something obvious.

Answer 1

In general, you don't. format! allocates a String, and a no_std environment doesn't have an allocator.

If you do have an allocator, you can use the alloc crate. This crate contains the format! macro.

#![crate_type = "dylib"]
#![no_std]

#[macro_use]
extern crate alloc;

fn thing() {
    let text = format!("example {:.1} test {:x} words {}", 1, 2, 3);
}

See also:

• How to format output to a byte array with no_std and no allocator?