The rule of thumb I've always heard for C++ is that anything non-primitive should be passed as a reference (or a pointer if you really have to) unless you have

The rule of thumb I've always heard for C++ is that anything non-primitive should be passed as a reference (or a pointer if you really have to) unless you have a good reason to pass by value (and normally you won't). Partly because of the ABI, partly to avoid copy or move constructors. It's a tedious low level detail, but that's what you have to pay attention to in C++ if you want the best performance. And to be clear, it is just a performance optimization - the struct passing code will work fine, just not as fast.