Re: [boost] several messages

Subject: Re: [boost] several messages
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-08-05 08:05:09

>> 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.