The move constructor doesn't allocate - it takes ownership of the GMP variable, and sets the variable in the moved-from object to a null state. The *destructor* then has an added check to ensure it doesn't try and clear null GMP objects: that's basically the only change. IMO the cost of the extra if statement in the destructor is worth it - and should be trivial compared to calling the external library routine to clear the GMP variable.
Well, what happens then if you do something to a moved-from object other than destructing it?
number a=2; number b=move(a); a=2; // or a=b; or a=b+1; or ...
It blows up your computer ;-) IMO It's a constraint (and the intent of move semantics), that moved-from objects can only be destroyed: if you require that the moved-from object remain usable, then move-assign become identical to a regular assign, and move semantics are basically useless. Not just for this lib, but for the std lib as well. John.