Is there any way to return a reference to a variable created in a function?

Question

I want to write a program that will write a file in 2 steps. It is likely that the file may not exist before the program is run. The filename is fixed.

The problem i

Answer 1

This is an elaboration on snnsnn's answer, which briefly explained the problem without being too specific.

Rust doesn't allow return a reference to a variable created in a function. Is there a workaround? Yes, simply put that variable in a Box then return it. Example:

fn run() -> Box {
    let x: u32 = 42;
    return Box::new(x);
} 

fn main() {
    println!("{}", run());
}

code in rust playground

As a rule of thumb, to avoid similar problems in Rust, return an owned object (Box, Vec, String, ...) instead of reference to a variable:

• Box instead of &T
• Vec instead of &[T]
• String instead of &str

For other types, refer to The Periodic Table of Rust Types to figure out which owned object to use.

Of course, in this example you can simply return the value (T instead of &T or Box)

fn run() -> u32 {
    let x: u32 = 42;
    return x;
}