How can I test stdin and stdout? - testing

Question

I'd like to write a prompt function that sends a passed-in string to stdout and then returns the string that it reads from stdin. How could I test it?
Here is an example of the function:
fn prompt(question: String) -> String {
let mut stdin = BufferedReader::new(stdin());
print!("{}", question);
match stdin.read_line() {
Ok(line) => line,
Err(e) => panic!(e),
}
}
And here is my testing attempt
#[test]
fn try_to_test_stdout() {
let writer: Vec<u8> = vec![];
set_stdout(Box::new(writer));
print!("testing");
// `writer` is now gone, can't check to see if "testing" was sent
}

Answer 1

Use dependency injection. Coupling it with generics and monomorphism, you don't lose any performance:
use std::io::{self, BufRead, Write};
fn prompt<R, W>(mut reader: R, mut writer: W, question: &str) -> String
where
R: BufRead,
W: Write,
{
write!(&mut writer, "{}", question).expect("Unable to write");
let mut s = String::new();
reader.read_line(&mut s).expect("Unable to read");
s
}
#[test]
fn test_with_in_memory() {
let input = b"I'm George";
let mut output = Vec::new();
let answer = prompt(&input[..], &mut output, "Who goes there?");
let output = String::from_utf8(output).expect("Not UTF-8");
assert_eq!("Who goes there?", output);
assert_eq!("I'm George", answer);
}
fn main() {
let stdio = io::stdin();
let input = stdio.lock();
let output = io::stdout();
let answer = prompt(input, output, "Who goes there?");
println!("was: {}", answer);
}
In many cases, you'd want to actually propagate the error back up to the caller instead of using expect, as IO is a very common place for failures to occur.
This can be extended beyond functions into methods:
use std::io::{self, BufRead, Write};
struct Quizzer<R, W> {
reader: R,
writer: W,
}
impl<R, W> Quizzer<R, W>
where
R: BufRead,
W: Write,
{
fn prompt(&mut self, question: &str) -> String {
write!(&mut self.writer, "{}", question).expect("Unable to write");
let mut s = String::new();
self.reader.read_line(&mut s).expect("Unable to read");
s
}
}
#[test]
fn test_with_in_memory() {
let input = b"I'm George";
let mut output = Vec::new();
let answer = {
let mut quizzer = Quizzer {
reader: &input[..],
writer: &mut output,
};
quizzer.prompt("Who goes there?")
};
let output = String::from_utf8(output).expect("Not UTF-8");
assert_eq!("Who goes there?", output);
assert_eq!("I'm George", answer);
}
fn main() {
let stdio = io::stdin();
let input = stdio.lock();
let output = io::stdout();
let mut quizzer = Quizzer {
reader: input,
writer: output,
};
let answer = quizzer.prompt("Who goes there?");
println!("was: {}", answer);
}