[gpderetta]

Sure, if calling any external function or writing though any pointer would force the callee to copy the object then yes you can have only the callee do the copy, but then it seems that this optimization would apply only to a very small subset of functions.

[mort96]

Right. That was my understanding, but I now see that there are more ways to understand it. I don't know which is correct, so I wrote a response to moonchild's comment here: https://news.ycombinator.com/item?id=27091726