[FIXED] What is Vec<_>?

Issue

I have seen Vec<_> a couple of times already. For example:

let a = "line1\r\nline2\nline3";
println!("{:?}", a.lines().collect::<Vec<_>>());

But what does that 'uncertain face' <_> mean?

I'm used to a typename in angle brackets, but what type can that be? The only meaning of underscore that I'm aware of is from Python as a name for an unused variable.

Solution

It means "Rust compiler, infer what type goes into the Vec". And it is indeed analogous to the unused variable in Python (and in Rust itself), in that it represents a placeholder for a type, like it can represent a placeholder for a variable name.

You can find an explanation in The Rust Programming Language chapter about iterator consumers:

Using a _ will let you provide a partial hint:

let one_to_one_hundred = (1..101).collect::<Vec<_>>(); This says "Collect into a Vec<T>, please, but infer what the T is for me." _ is sometimes called a "type placeholder" for this reason.

Answered By - Paolo Falabella

Answer Checked By - Marilyn (PHPFixing Volunteer)