The MRuby embedding API isn't a whole lot like Lua's. Lua is a fantastic experience to embed. You might have to futz with the registry to store Lua objects in a C struct, but the abstraction allows you to almost never have to actually worry about the VM internals or the GC directly. Mruby is a lot more like MRI Ruby's API. Raw objects are exposed to you, you have to turn off strict aliasing because inheritance is implemented by the old "common meta struct as first member" idiom, you have to manually trigger VM collecting in long-running C code ( https://github.com/mruby/mruby/blob/master/doc/guides/gc-are... ), getting args in a C function involves a variadic scanf-style function. The most striking difference is documentation. The documentation of the mruby C API is actually "read the headers". There are many seemingly redundant functions that look like they do the same thing, completely without explanatory comments, or minimal inscrutable comments:
/* mrb_gc_protect() leaves the object in the arena */
MRB_API void mrb_gc_protect(mrb_state *mrb, mrb_value obj);
/* mrb_gc_register() keeps the object from GC. */
MRB_API void mrb_gc_register(mrb_state *mrb, mrb_value obj);
/* mrb_gc_unregister() removes the object from GC root. */
MRB_API void mrb_gc_unregister(mrb_state *mrb, mrb_value obj);
I'd rather work with Ruby as a language than Lua, but I'd much rather work with Lua than Mruby for the documentation and API alone. If mruby had anything close to the Lua reference documentation, I'd be all over it. As-is, embedding mruby kind of sucks, which is a real shame.
> you have to turn off strict aliasing because inheritance is implemented by the old "common meta struct as first member" idiom
You shouldn't have to turn off strict aliasing for that, struct pointers are allowed to alias pointers to their first member. Unless I'm missing some awkward compatibility rule that mruby breaks.
You do need to turn off strict aliasing for that, because mruby uses `struct RObject { RB_OBJECT_HEADER; ... }` and `struct RHash { MRB_OBJECT_HEADER; ... }` (where MRB_OBJECT_HEADER begins with `RClass `). You can alias objects of both types as an `RClass `, but you can't alias them as one-another, converting RHash to RObject. According to my reading of the strict aliasing rules[0], the aliasing would be legal if one of the types literally contained the other, or if they were being accessed through a union. The "compatible types"[0] section requires the types to be exactly the same in layout, not just starting as the same. It's not safe to cast incompatible structs to one another just because they have the same initial members, unless you are accessing them through a union (C11 6.5.2.3p6). Optimization can cause UB when working with mruby if strict aliasing is enabled.
Oof, I get it now. I was expecting RHash to start with an actual RObject, not just the same members. I too read the "compatible first member" rule as not applying here, so I agree it breaks strict aliasing.
