GMP and C++11 move constructors

[Marco Bodrato]

>From gmp-discuss:

Il Lun, 21 Maggio 2018 12:15 pm, Marc Glisse ha scritto:
> If m is a
> moved-from object, it is fine to assign a new value to it with m =
> whatever. That's even what std::swap does. It should also be fine to
> read from it, although you can't rely on any specific value so that's
> not very useful.

Should the non specific value be somehow valid?

With lazy allocation, we should define NaN as an admissible value for mpq,
and use it whenever we init a variable that will immediately be
overwritten.

I did this, internally in mini-mpq:
https://gmplib.org/repo/gmp/rev/8e0a2e94ae5a

The idea might be useful also for some mpq constructor from double, from
string... if not for move.
Wile we are looking into this, is there a motivation why we have the
assignment "operator=(mpz_class &&z)" for mpq_class, but not a constructor
from mpz_class&& ?

Ĝis,
m

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