On 12/24/19 8:52 AM, MoonlightSentinel wrote:
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)
Just FYI, doing this is very suspicious. Storing a delegate to a struct
means you have a context pointer to that struct. And structs can usually
move around freely. Meaning that f1 and f2 are still going to refer to
the original struct, even if it has been copied, and even if the struct
has been deallocated (i.e. it was on a stack frame that no longer exists).
-Steve
