Formatted read consumes input

[Dmitry Olshansky]

On Thursday, 23 August 2012 at 11:33:19 UTC, monarch_dodra wrote:
> As title implies:
>
> ----
> import std.stdio;
> import std.format;
>
> void main()
> {
>   string s = "42";
>   int v;
>   formattedRead(s, "%d", &v);
>   writefln("[%s] [%s]", s, v);
> }
> ----
> [] [42]
> ----
>
> Is this the "expected" behavior?

Yes, both parse family and formattedRead are operating on ref 
argument. That means they modify in place. Also ponder the 
thought that 2 consecutive reads should obviously read first and 
2nd value in the string not the same one.


> Furthermore, it is not possible to try to "save" s:
> ----
> import std.stdio;
> import std.format;
> import std.range;
>
> void main()
> {
>   string s = "42";
>   int v;
>   formattedRead(s.save, "%d", &v);
>   writefln("[%s] [%s]", s, v);
> }
> ----

Yes, because ref doesn't bind r-value.

> The workaround is to have a named backup:
>   auto ss = s.save;
>   formattedRead(ss, "%d", &v);
>
>
> I've traced the root issue to formattedRead's signature, which 
> is:
> uint formattedRead(R, Char, S...)(ref R r, const(Char)[] fmt, S 
> args);
>

As I explained above the reason is because the only sane logic of 
multiple reads is to consume input and to do so it needs ref.

> Is there a particular reason for this pass by ref? It is 
> inconsistent with the rest of phobos, or even C's scanf?

C's scanf is a poor argument as it uses pointers instead of ref 
(and it can't do ref as there is no ref in C :) ). Yet it doesn't 
allow to read things in a couple of calls AFAIK. In C scanf 
returns number of arguments successfully read not bytes so there 
is no way to continue from where it stopped.

BTW it's not documented what formattedRead returns ... just ouch.