The problem is illustrated via something like
a = move(a) + b;
Of course, the above is better written as "a += b", but it's exemplary of more elaborate assignments, e.g., "a = c * move(a) + b". Indeed, an even simpler example is just "a = move(a)", given John's implementation of operator+ above and ignoring the arithmetic (which is not relevant). And the problem with "a = move(a)" in a generic context is that T::operator=(T&& x) may freely assume that this != &x [1], mostly for efficiency purposes (Dave, correct me if I'm wrong here).
So, in other words, don't return an rvalue reference from a function which is a reference to one of its arguments unless this is explicitly intended and documented (e.g., move and forward).
Note that changing the return type from Number&& to Number cancels the
allocation gain when using a type like GMP that doesn't have an empty state.
Huh, really? That's no good.
Don't panic it's OK ;-) The current sandbox code, does have these rvalue ref operator overloads, does return by value for safety, and still manages to avoid the extra allocations - so for example in my horner test case, evaluating: Real result = (((((a[6] * x + a[5]) * x + a[4]) * x + a[3]) * x + a[2]) * x + a[1]) * x + a[0]; Reuslts in just one allocation even when expression templates are turned off - the first operator overload called generates a temporary, which then gets moved and reused, eventually ending up in the result. Of course rvalue refs can't help in simple cases such as: a = b * c; For that you still need expression templates if you want to avoid temporaries. I've posted these results before, but here are the current Bessel function evaluation tests using mpf_t: With rvalue refs: Testing Bessel Functions at 50 digits..... Time for mpf_float_50 = 5.00486 seconds Total allocations for mpf_float_50 = 2592797 Time for mpf_float_50 (no expression templates = 5.30183 seconds Total allocations for mpf_float_50 (no expression templates = 4174980 And again with BOOST_NO_RVALUE_REFERENCES defined Time for mpf_float_50 = 4.93078 seconds Total allocations for mpf_float_50 = 2594701 Time for mpf_float_50 (no expression templates = 5.68103 seconds Total allocations for mpf_float_50 (no expression templates = 6498294 As expected the biggest hit is in the no-ET code, where the number of allocations rises dramatically. Cheers, John.