std.format.formattedRead docs example does not work with a string literal as input, why?

[H. S. Teoh via Digitalmars-d-learn]

On Thu, Mar 31, 2016 at 06:23:21PM +0000, ParticlePeter via Digitalmars-d-learn wrote:
> Example from docs:
> string s = "hello!124:34.5";
> string a;
> int b;
> double c;
> formattedRead(s, "%s!%s:%s", &a, &b, &c);
> assert(a == "hello" && b == 124 && c == 34.5);
> 
> now changing the first formattedRead argument to a string literal:
> formattedRead("hello!124:34.5", "%s!%s:%s", &a, &b, &c);
> 
> results in this compiler error:
> Error: template std.format.formattedRead cannot deduce function from
> argument types !()(string, string, string*, int*, double*), candidates are:
> ..\..\src\phobos\std\format.d(588,6):
> std.format.formattedRead(R, Char, S...)(ref R r, const(Char)[] fmt, S args)
> 
> I am not getting the point that in both cases the argument is a
> string, but in the first case it is interpreted as a Range, and in the
> second case not.
> Why?

Because in the second case the string is an rvalue, whereas in the first
case it gets stored in a variable first, so it's an lvalue.  The first
parameter of formattedRead is 'ref', meaning that it requires an lvalue.

(Arguably, it should be `auto ref` instead, then literals would work,
but that belongs in an enhancement request.)


T

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