Capturing by reference with "visit" from std.variant

[Radu]

On Tuesday, 20 February 2018 at 16:01:11 UTC, Smaehtin wrote:
> I'm trying to understand why the following doesn't work:
>
> import std.stdio;
> import std.variant;
>
> void main()
> {
>     Algebraic!(string, int) test = "Test";
>
>     test.tryVisit!(
>         (ref string s) { s = "Why does this not work?"; }
>     );
>
>     writeln(test);
> }
>
> But this works fine:
> *test.peek!string = "Works fine";
>
> As far as I can tell, the "visit" template expands to something 
> that ends up calling my handler like this:
> if (auto ptr = variant.peek!T)
> {
>     handler(*ptr);
> }
>
> But seeing as the handler in my case takes a reference, 
> shouldn't that work just fine? What am I missing?

Your lambda is called, but you can't change the variant value 
trough that reference.

Test with:

import std.stdio;
import std.variant;

void main()
{
     Algebraic!(string, int) test = "Test";

     test.tryVisit!(
         (ref string s)
         {
             writeln("Why does this work? ", s);
         }
     );

     writeln(test);
}


You should get:

Why does this work? Test
Test