is inheritence possible? #679

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1. khvzak commented on Feb 9, 2026

   

   khvzak

   on Feb 9, 2026

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   You can do something like:

   struct Instance(String);
   
   impl UserData for Instance {
       fn register(registry: &mut UserDataRegistry<Self>) {
           registry.add_field_method_get("Name", |_, this| Ok(this.0.clone()));
       }
   }
   
   struct WorldInstance {
       parent: AnyUserData,
       position: f32,
   }
   
   impl UserData for WorldInstance {
       fn register(registry: &mut UserDataRegistry<Self>) {
           registry.add_field_method_get("Position", |_, this| Ok(this.position));
   
           registry.add_meta_method("__index", |_, this, key: String| {
               use mlua::ObjectLike as _;
               this.parent.get::<Value>(key)
           });
       }
   }
   
   let instance = lua.create_userdata(Instance("example".to_string()))?;
   lua.globals().set("Instance", &instance)?;
   lua.globals().set(
       "WorldInstance",
       lua.create_userdata(WorldInstance {
           parent: instance,
           position: 1.23,
       })?,
   )?;
2. SDFTDusername commented on Feb 10, 2026

   

   SDFTDusername

   on Feb 10, 2026

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   that does work, though the only issue that i have is that if the key doesn't exist, it would return nil instead of giving me an error. this also makes it unknown if its a field with a value of nil, or if the field doesn't exist.
   since i can't find any method for checking if a key exists, is there another way make it return an error for when the parent field doesn't exist?
3. khvzak commented on Feb 10, 2026

   

   khvzak

   on Feb 10, 2026

   Last edited by khvzak

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   if the key doesn't exist, it would return nil instead of giving me an error

   you sure? Accessing WorldInstance.NonExistent raise an exception runtime error: attempt to get an unknown field 'NonExistent'.

   This can happen though for static fields. In Lua nil and non-existent is the same (for example you cannot have a table with a key set to nil).
4. mlua-rs

   locked and limited conversation to collaborators on Feb 10, 2026
5. khvzak

   converted this issue into a discussion #680 on Feb 10, 2026