• Alex (26/26) May 22 2019 In gtkD one can use a lambda directly:
• H. S. Teoh (8/23) May 22 2019 [...]
• Adam D. Ruppe (5/8) May 22 2019 Because you didn't ask for one and it didn't have to be.
• Alex (3/12) May 23 2019 That is what I was looking for.

Alex <AJ gmail.com> writes:

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.






			

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

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.

Adam D. Ruppe <destructionator gmail.com> writes:

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) {};
          ^^^^^^^^

Alex <AJ gmail.com> writes:

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.