• realhet (44/44) Aug 31 Hi,
• =?UTF-8?Q?Ali_=C3=87ehreli?= (9/15) Aug 31 Unless that's an optimization, that part would be the same: on the
• realhet (5/7) Aug 31 Very cool feature.

realhet <real_het hotmail.com> writes:

Hi,

I noticed that the local area of the myUi() function was 
allocated on the heap, not on the stack as it would be the usual 
case with functions.
Please explain me how the D compiler detects that. Does it knows 
that the myUi() function returns something, that contains a 
delegate which is referencing to a variable on the function's 
local area?
What is the trigger of this heap allocated function storage area?


```d
import std;

class Button
{
     string title;
     void delegate() onPress;

     this(string title, void delegate() onPress)
     {
         this.title = title, this.onPress = onPress;
     }

     void draw(){ writeln("Drawing ["~title~"]"); }
     void press(){ writeln("Pressing ["~title~"]"); onPress(); }
}

auto myUi()
{
     auto titles = ["Hello", "Hi", "Greetings"].cycle;
     Button btn;
     btn = new Button(titles.front, (){
         titles.popFront; btn.title = titles.front;
         writeln("Title changed");
     });
     return btn;
}

void main()
{
     auto ui = myUi;
     auto ui2 = myUi;

     foreach(i; 0..10){
         writeln("1---------");
         ui.draw;if(i%3) ui.press;
         writeln("2---------");
         ui2.draw;if(i%2) ui2.press;
     }
}
```

=?UTF-8?Q?Ali_=C3=87ehreli?= <acehreli yahoo.com> writes:

On 8/31/25 2:36 PM, realhet wrote:
 Hi,

 I noticed that the local area of the myUi() function was allocated on
 the heap, not on the stack as it would be the usual case with functions.

Unless that's an optimization, that part would be the same: on the 
stack. However, because the delegate necessitates long-term lifetime, 
its context has to be on GC memory.

So, either the compiler allocates on the heap and uses that for local 
stack frame, or allocates on the stack and then copies to heap. I think 
the former is not complicated. I am guessing...

 Please explain me how the D compiler detects that. Does it knows that
 the myUi() function returns something, that contains a delegate which is
 referencing to a variable on the function's local area?

Yes.

Ali

realhet <real_het hotmail.com> writes:

On Sunday, 31 August 2025 at 22:11:26 UTC, Ali Çehreli wrote:
 On 8/31/25 2:36 PM, realhet wrote:
 ...long-term lifetime...

Very cool feature.
Even a function body can be used to make a composition of 
multiple objects. It acts like a class constructor, but with 
cleaner, simpler syntax, and automatically deallocated by GC.