Formatted read consumes input

[Steven Schveighoffer]

On Thu, 23 Aug 2012 07:33:13 -0400, monarch_dodra <monarchdodra at gmail.com>  
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?
>
> 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);
> }
> ----
> main.d(9): Error: template std.format.formattedRead does not match any  
> function template declaration
> C:\D\dmd.2.060\dmd2\windows\bin\..\..\src\phobos\std\format.d(526):  
> Error: template std.format.formattedRead(R,Char,S...) cannot deduce  
> template function from argument types !()(string,string,int*)
> ----
>
> 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);
>
> Is there a particular reason for this pass by ref? It is inconsistent  
> with the rest of phobos, or even C's scanf?
>
> Is this a file-able bug_report/enhancement_request?

I believe it behaves as designed, but could be designed in such a way that  
does not need ref input range.  In fact, I think actually R needing to be  
ref is a bad thing.  Consider that if D didn't consider string literals to  
be lvalues (an invalid assumption IMO), then passing a string literal as  
the input would not work!

The only issue is, what if you *do* want ref behavior for strings?  You  
would need to wrap the string into a ref'd range.  That is not a good  
proposition.  Unfortunately, the way IFTI works, there isn't an  
opportunity to affect the parameter type IFTI decides to use.

I think a reasonable enhancement would be to add a formattedReadNoref (or  
better named alternative) that does not take a ref argument.

-Steve