When using opDispatch()

• how can I tell the compiler that I do not want to handle some calls? Some code is testing for range methods (empty, front, popFront) and I don't know where and which side effects it causes.

• how can I dismiss calls from __traits(compiles)?

On Friday, 14 May 2021 at 22:39:29 UTC, frame wrote:

Put a template constraint on it. void opDispatch(string s)() if(s == "whatever") or minimally like if(s != "popFront")

This kind of thing is why I always put opDispatch on a helper thing now though, and still do the != "popFront" just to stop the ducktypers.

On Friday, 14 May 2021 at 22:39:29 UTC, frame wrote:

Use a template constraint. For example:

auto opDispatch(string method, Args...)(Args args)
    if (shouldHandle!method)
{
    // ...
}

...where shouldHandle checks the method name and returns true if you want to handle it, or false if you don't.

You can't.

On Friday, 14 May 2021 at 22:39:29 UTC, frame wrote:

Thanks! I stumbled around with static asserts and mixins... totally forgot about the constraints but they will to the trick.

On Friday, 14 May 2021 at 23:02:22 UTC, frame wrote:

Now I run into the error

"class Foo member s is not accessible"
"template instance Base.opDispatch!("s", int, Foo) error instantiating"

and I have no clue why.

I swear the original code is just simple as that and satisfyDispatch works fine (there is no problem if I change s to a simple field).

abstract class Base {
  void opDispatch(string property, S, this T)(auto ref S v) // if (satisfyDispatch!(T, property))
  {
     __traits(getMember, cast(T) this, property) = v; // error reported here
  }

  auto opDispatch(string property, this T)() // if (satisfyDispatch!(T, property))
  {
    return __traits(getMember, cast(T) this, property);
  }
}

// other module
class Foo : Base {
  protected:
    int _s;

    // int s; // would work

    void s(int v) {
        _s = v;
    }

    int s() {
        return _s;
    }
}

// called from a method like this:
void updateValue(T, V)(V value, Object object) {
   (cast(T) object).opDispatch!(property)(value);
}

I first thought the compiler does a recursive call on __traits(getMember) but it has no problems with this simple code above.

On Saturday, 15 May 2021 at 23:41:19 UTC, frame wrote:

Does it work if you remove protected from the derived class?

On Saturday, 15 May 2021 at 23:59:49 UTC, Paul Backus wrote:

Yes, but why should the derived class not have access to it?

On Sunday, 16 May 2021 at 00:04:39 UTC, frame wrote:

Possibly because you're accessing it from an opDispatch method defined in the base class. I'm not 100% sure how protected works in that case, but just from reading the spec it seems like it could be an issue.

On Sunday, 16 May 2021 at 00:04:39 UTC, frame wrote:

I don't think that's your problem. From the template docs:

Your opDispatch templates are instantiated in the scope of Base, from which _s is not accessible when it's protected.

On Sunday, 16 May 2021 at 03:19:04 UTC, Mike Parker wrote:

Indeed, the error goes away if each derived class has its own implementation.

But the same with fields work? They are also protected.