Re: [boost] [variant] Basic rvalue and C++11 features support

Subject: Re: [boost] [variant] Basic rvalue and C++11 features support
From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2013-01-14 12:11:27

On Mon, Jan 14, 2013 at 4:37 PM, Larry Evans <cppljevans_at_[hidden]>wrote:

> [...]

Thus, there is no indication in the variant that anything has gone
> wrong other than the fact that the variant's value has not changed
> from it's state before the assignment.  This test for the exact same
> value seems to be an "additional complexity-of-use" which, according
> to:
>
>
>
> http://www.boost.org/doc/libs/1_52_0/doc/html/variant/design.html#variant.design.never-empty.guarantee
>
> the never-empty guarantee was supposed to insulate the user from.
>
> Wouldn't the "implicit" boost::blank default construction be a "less
> complexity-of-use" alternative?  IOW, when an exception occurs during
> an assignment of two distinct types, the variant's type is changed to
> boost::blank even when that is not among the explicit bounded types.
> This would be a flag to the user that something unusual had happened
> and would easily be tested for with the which() function which would
> return some value (e.g. -2 or -999 or whatever) indicating that the
> value is "undefined".
>

An exception being thrown (well, propagated) is a fairly strong indication
that something unusual has happened. Variant does not swallow the exception
that caused the rollback.

--
gpd