如何将迭代器写入File或Stdout（实现Write的任何方式）？

Question

I have an iterator full of u8 's that I want to write to a file or stdout. Calling io::stdout().write_all(foo) on my iterator gives me an expected type &[u8] , and that I have an iterator type instead. I understand why this doesn't work.

What I don't understand is how to change it so that it does work. At first I tried adding a .collect() to the end of my iterator, but then it says the trait bound &[u8]: std::iter::FromIterator<u8> is not satisfied and a collection of type &[u8] cannot be built from an iterator over elements of type u8 .

It seems odd to me that Write doesn't provide a way of writing with an iterator, when Read provides a bytes() function that returns an iterator. What is the idiomatic way to do this?

Here is my main function's contents:

io::stdout().write_all(
    io::stdin().bytes().map(
        |x| match x {
            Ok(b) => b,
            _ => panic!("There was an error reading from stdin"),
        }
    ).repeat(3).collect()
);

Answer 1

The problem is that you want to build a Vec<u8> , but are attempting to create a Vec<&u8> . You can do the following:

fn main() {
    let array = [1u8, 2, 3];
    let vector: Vec<u8> = array.iter().map(|i| *i).collect();
}

Note the .map(|i| *i) part that allows to go from &u8 references to u8 values.

Answer 2

It seems odd to me that Write doesn't provide a way of writing with an iterator, when Read provides a bytes() function that returns an iterator. What is the idiomatic way to do this?

It does feel inconsistent, indeed, but you can certainly write the equivalent yourself.

Something like:

fn write_all<W: Write, I: Iterator<Item=u8>>(writer: &mut W, iter: I) {
    const SIZE: usize = 1024;

    let mut buffer = [0u8; SIZE];
    let mut index = 0;

    for i in iter {
        buffer[index] = i;

        index += 1;
        if index == SIZE {
            writer.write_all(&buffer);
        }
    }

    writer.write_all(&buffer[..index]);
}

There are probably ways to make it more idiomatic, and I haven't tested the boundary conditions, but hopefully it should get you going.