Have a template function register(T)() instantiated and executed at startup for all sub classes of a specific base class?

[user1234]

On Sunday, 21 September 2025 at 08:52:48 UTC, TwoOfCups wrote:
> [...]
> 
> Is there any way to resolve this flaw? Is there some other 
> route I could take that doesn't require a mixin?
>
> I know I can just insert a mixin and do it that way but I want 
> things to look as clean as possible. Even that tiny amount of 
> boiler plate bothers me a lot and this is a foundational system 
> so that boiler plate will be multiplied hundreds of times I am 
> expecting. Probably in the end I will just have to get over my 
> aesthetic hang ups but I figured a forum post was worth a shot.

You can use the CRTP pattern [0] here:

```d
class Registerable(Derived, Base) : Base {

     pragma(crt_constructor)
     extern(C)
     static void reg(){
         writeln("Registering type ", Derived.stringof);
     }
}

class Base : Registerable!(Base, Object) {  }

class Derived1 : Registerable!(Derived1, Base) { }

class Derived2 : Registerable!(Derived2, Derived1) { }
```

The trick is that a a static ctor is instantiated for each 
instance. Note the example is a slight adaptation from link [1].

Otherwise solutions based on static intropsection will all 
involve mixins. Using mixins at the module level can be 
reasonable tho (vs using them in every new derived classes). For 
example:

```d
pragma(crt_constructor)
extern(C) void reg(){
     foreach (m; __traits(allMembers, mixin(__MODULE__))){
         static if (is(mixin(m)))
             static if (is(mixin(m) : Base))
                 writeln("Registering type ", m.stringof);
     }
}

class Base {}
class Sub0 : Base {}
class Sub1 : Sub0 {}
```

and then people just have mixin `reg()`, that your library 
provides as a `q{}` string, let's say.

Anyway as I think you'll like more the CRTP solution I wont 
develop more.

[0]: en.wikipedia.org/wiki/Curiously_recurring_template_pattern
[1]: 
https://forum.dlang.org/post/mailman.7481.1610735482.31109.digitalmars-d@puremagic.com