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Subject: Re: [boost] several messages
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-08-09 11:22:13

>> > OK I'll have to rethink, and make sure everything other than
>> > arithmetic is supported for moved-from types.
>> Actually, in principle, a moved-from object should be a valid object,
>> which just has an unspecified value.
> IMHO any code that reads such unspecified values is buggy.


> I wonder if the compiler could produce warnings for reading an unspecified
> value. It seems like a bad idea 99.9% of the time. It'd be cool if
> Valgrind could be given a hint to mark such data as non-inited.
> If I had a movable class with a double I'd set it's value signaling NAN at
> least for a debug build.

Currently, any attempt to "use" a moved-from value results in an assertion,
the only things you can safely do are destruct, assign-to, swap, or copy
such values.

There is a slight inconsistency though - not all number type Backends
support (or need to support) move semantics. So if you switch from a fixed
precision integer which simply copies (no move), to an arbitrary precision
integer (moves) then you get a change in behavior. Of course any code that
relies on an object having a specific value after it's been moved was IMO
buggy to begin with, it's just that you might not see the asserts until you
test with a move-enabled type. This is unfortunate, but I really don't want
to hobble the fixed precision integers by giving them a special singular
state (more code more checking), when most of the negative review comments
were of the opinion that they're too heavyweight already.


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