jeff-mitchell.dev

Today I asked myself a random question about Rust programming.

Do I always have to use the println! macro to write output to stdout?

The answer is no. You can use the Write trait from the standard library and output quite neatly without using a macro. Macros are a time saving convenience, but increasingly I'd like to not use them so that I understand a little better what's going on underneath the hood.

It's actually pretty simple to output to stdout and can be done in a one-liner. Let's take a look.

// dependencies
use std::io::{Result, stdout, Write};

// main function
fn main() -> Result<()> {
  let msg = "Hello, World!";
  stdout().write_all(msg.as_bytes());

  Ok(())
}

That's pretty much it. What's going on here

• we bring some things into scope from the standard library, namely the Result<T, E> type, the stdout function and Write trait
• we bind a string slice with a message to a variable named msg
• we use the stdout() function to give us a handle to stdout which enables output to our console
• we call the write_all() method, giving it as a parameter our message, the message is converted to bytes, as required by write_all
• the main function returns a Result<T, E> type because the I/O operation could fail in any number of ways.

If we save this program as main.rs and then run it using the rust compiler, like this: rustc main.rs then the message, "Hello, World!" will be visible in the console.

Macros are convenient, but they obscure details that you might want to know. The println! macro does bring some overhead, and in trivial code like above it doesn't matter, but in a larger CLI program, writing to stdout manually might be something you need to do for performance reasons.

Exercises like this are a little silly, but you can learn a lot by asking yourself "what if" kinds of questions.

Thanks for reading! I hope this small little byte was useful to you. Yes, my humour is terrible...I'll show myself out.

References

The Rust Standard Library