[Finally going through my back-log of MP discussion emails...] On Sat, Jun 30, 2012 at 10:17 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
On Sat, 30 Jun 2012, John Maddock wrote:
I've started to work through some of the points that came up in the
multiprecision review, starting with further rvalue reference support.
The idea (due to Marc Glisse) was to add operator overloads that look something like:
Number&& operator+(Number&&a, const Number& b) { return static_cast<Number&&>(a += b); }
The safe solution is returning Number, not Number&&. If Dave reads this message, he can probably point us to previous discussions about this. Being able to return Number&& would be great...
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. - Jeff [1] http://cpp-next.com/archive/2009/09/your-next-assignment/