array of functions/delegates

[MoonlightSentinel]

On Tuesday, 24 December 2019 at 07:37:02 UTC, Rumbu wrote:
> I am trying to create an array of functions inside a struct.
>
> struct S {
>   void f1() {}
>   void f2() {}
>
>   alias Func = void function();
>
>  immutable Func[2] = [&f1, &f2]
>
> }
>
> What I got: Error: non-constant expression '&f1'
>
> Tried also with delegates (since I am in a struct context but I 
> got: no `this` to create delegate `f1`.
>
> So, is there any way to have an array of functions without 
> adding them at runtime?

You can set funcs within the constructor but not as a default 
initializer because they have to be compile time constants 
(https://dlang.org/spec/struct.html#default_struct_init)

struct S
{
     void f1() {}
     void f2() {}

     alias Func = void delegate();

     immutable Func[2] funcs;

     this(bool)
     {
         funcs = [&f1, &f2];
     }
}

&f1 creates a delegate which contain a function pointer and the 
context pointer (struct, closure, ...). In this example the 
latter contains a pointer to a concrete instance of S which is a 
usually a runtime value. 
(https://dlang.org/spec/type.html#delegates)