My number 1 missing feature in Java is the ability to set object references to be 'copy on write'.

I'm doing numerical/scientific programming. Say I have an object which contains an array, and a 'get' function to return that array. Currently I have two choices: I can return a pointer to my object's array, or make a copy of the array and return that.

Returning a pointer is very fast, but now my class is at the mercy of callers which might write into my array. Returning a copy is safe, but so long as the callers behave themselves and don't try to write to it, is a waste of time and memory. If I could return a "copy-on-write-reference" to my array, I'd get the best of both worlds.

Any reference reached via a copy-on-write-reference would also need to be copy-on-write. If you make copy-on-write a qualifier on a variable, this could be all enforced by the compiler.

Are there any languages which do something like this?