Declaring a bufferedReader, what is the different between these?

[scarlet drift]

Yesterday I spent a long time trying to declare a bufferedReader. I eventually got it but I'd like to understand what is going on.

First question:

This works:

    var stdin_buf_ = std.io.bufferedReader(std.io.getStdIn().reader());
    var stdin_buf = stdin_buf_.reader();

This doesn't work:

var stdin_buf = std.io.bufferedReader(std.io.getStdIn().reader()).reader();

The error I get is basically: expected type "*io.buffered_reader...", found "*const io.buffered_reader...".

I'm guessing what's going on is basically that a pointer is being created in bufferedReader and since we don't explicitly store it in a variable it is const by default?

Is there some other notation I could use or is my first code the way to do it?

Second Question

What is much more confusing is why the following doesn't work:

    var stdin = std.io.getStdIn().reader();
    var stdin_buf = std.io.bufferedReader(&stdin).reader();

I get the error:

/usr/lib/zig/std/io/buffered_reader.zig:14:37: error: type '*io.GenericReader(fs.File,error{InputOutput,SystemResources,IsDir,OperationAborted,BrokenPipe,ConnectionResetByPeer,ConnectionTimedOut,NotOpenForReading,SocketNotConnected,WouldBlock,AccessDenied,Unexpected},(function 'read'))' has no members
        pub const Error = ReaderType.Error;

The BufferedReader "template" defines its "Error" field to be the "Error" field of the the type of the reader you passed into it
But I don't get how just putting the stdin outside of the function removes "Error" from it. How are the two declarations any different? Is there some hidden cast going at some point?

[sacred ridge]

• yes, temporaries are const
• in your particular example, you couldn't even use stdin_buf correctly since you couldn't call flush as you'd need to call that on the temporary that you're throwing away
• for the second question, you shouldn't be passing a pointer to stdin; std.io.bufferedReader(stdin) would work (the return of reader() is a struct that contains a pointer, you pass that struct around by value)

https://github.com/ziglang/zig/blob/bbd13ab961ab49e01d5699eec195929cae7bf25a/lib/std/io/buffered_reader.zig#L43-L45

[scarlet drift]

Thanks a lot for the help!

• Makes sense
• I don't think BufferedReader has flush... Only BufferedWriter
• Here is the code passing it by value:

    const stdin = std.io.getStdIn().reader();
    var stdin_buf = std.io.bufferedReader(stdin).reader();

I had tried it like this before I got an error complaining about some pointer being const and assumed it was stdin's fault
Now I understand it that it's complaining that about BufferedReader being const (since it's a temporary)

So changing to this works:

    const stdin = std.io.getStdIn().reader();
    var stdin_buf_ = std.io.bufferedReader(stdin);
    var stdin_buf = stdin_buf_.reader();

I was confused about the fact that std.io.getStdIn().reader(); works since it looks like there's a temporary there too, but I'm seeing that getStdIn() is a File that gets passed by value to reader, so I guess it's OK