In gtkD one can use a lambda directly:

    X.addOnButtonPress((GdkEventButton* e, Widget w) ...

but if I try move the lambda in to a variable so I can use it with multiple handlers, I get an error:

    // x is a function and does not work
    auto x = (GdkEventButton* e, Widget w) ...
    X.addOnButtonPress(x);

etc...

It's because the second case treats x as a function while the first it is a delegate and addOnButtonPress requires a delegate...

Why is x not a delegate?

Now, if I reference a variable outside the scope of the lambda it turns magically in to a delegate and works!

But that is an ugly hack!

// now x is a delegate and works
auto x = (GdkEventButton* e, Widget w) { auto q = X; ...


Now sure I can hack x and use some template to turn it in to a fake delegate but then that is dangerous.

I suppose one could have a template that only converts it if it is a function and then that will deal with both cases and might work...

But why?
Is there any way to force it to not reduce the delegate to a function which is obviously an optimization when nothing accesses the outside context.






			

On Wed, May 22, 2019 at 10:33:52PM +0000, Alex via Digitalmars-d-learn wrote:
> In gtkD one can use a lambda directly:
> 
>     X.addOnButtonPress((GdkEventButton* e, Widget w) ...
> 
> but if I try move the lambda in to a variable so I can use it with multiple handlers, I get an error:
> 
>     // x is a function and does not work
>     auto x = (GdkEventButton* e, Widget w) ...
>     X.addOnButtonPress(x);
> 
> etc...
> 
> It's because the second case treats x as a function while the first it is a delegate and addOnButtonPress requires a delegate...
[...]

You could try explicitly declaring it as a delegate:

	void delegate(GdkEventButton*, Widget) x = (e, w) { ... };
	X.addOnButtonPress(x);


T

-- 
I am Ohm of Borg. Resistance is voltage over current.

On Wednesday, 22 May 2019 at 22:33:52 UTC, Alex wrote:
>     auto x = (GdkEventButton* e, Widget w) ...
>     X.addOnButtonPress(x);
>
> Why is x not a delegate?

Because you didn't ask for one and it didn't have to be.

Just add the delegate keyword to ask for one:

auto x = delegate(GdkEventButton* e, Widget w) {};
         ^^^^^^^^

On Wednesday, 22 May 2019 at 23:54:47 UTC, Adam D. Ruppe wrote:
> On Wednesday, 22 May 2019 at 22:33:52 UTC, Alex wrote:
>>     auto x = (GdkEventButton* e, Widget w) ...
>>     X.addOnButtonPress(x);
>>
>> Why is x not a delegate?
>
> Because you didn't ask for one and it didn't have to be.
>
> Just add the delegate keyword to ask for one:
>
> auto x = delegate(GdkEventButton* e, Widget w) {};
>          ^^^^^^^^

That is what I was looking for.

Thanks.