Re: Capturing by reference with "visit" from std.variant

[Radu via Digitalmars-d-learn]

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