[sparkie]

The feature those languages are missing in order to do actors correctly, is enforced isolation of state. While you can emulate actors and message passing in nearly any language, you need this guarantee so that any actor can be executed anywhere at any time, without worrying that it is going to cause a race condition or hit a mutex somewhere. The big sin in Java and C++ which makes them quite unsuitable is "static"

It might be possible to achieve that isolation if you stick to strict coding practices, but the point where it's going to fail is when you import another library, written by someone else.

Because C++, Java and many other languages (including F#, Scala and others which have message passing libraries) do not have the enforced isolation of state, nor the ability to add notation to indicate that some piece of code is free of side effects (including all of it's dependencies), you're always going to have the risk of breakage when importing a library. The only way you can be sure that a library is actor-safe is to read it's source code - and if the source code is not available, then you're out of luck.

Java and C++ could have the potential to do actors correctly by adding the notation for side-effect-free code, using custom annotations/attributes on all functions which are actor-safe, and using some compiler extension or static analysis tools to prove correctness. I'm not aware of anything that does this for the mentioned languages though.

On the other hand, if you implement message passing in any purely functional programming language, your actors are automatically safe for free.