Re: [boost] [variant] late "review"

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2006-11-06 07:31:01

David Abrahams wrote:
> I'm just looking through the variant library docs in detail and had
> the following commments:

Ah, if only boost::variant was actively maintained...

I had several issues raised in 
http://lists.boost.org/Archives/boost/2006/06/105801.php and some time 
later I encountered another issue - I think the default constructor for 
variant is a bad idea. The thought came to me after I made a mistake of 
writing

typedef variant< optional<X>, optional<Y> > SomeType;

when I really meant was

typedef optional< variant<X, Y> > SomeType;

later in my code, thinking I had the right typedef, I constructed an 
instance of SomeType using its default constructor, assuming I had an 
empty optional<variant> in my hands, while I actually had a variant with 
an empty optional<X>. Unfortunately, it compiled, and that mistake was 
only found in run-time, with the help of the QA department.

If variant didn't have a default constructor, my mistake wouldn't have 
compiled. IMO, this is also true from a more theoretical point of view - 
there is no reason to favor one of the variant types (the first one) 
over the others. Those types are basically equivalent and symmetric. If 
the user wishes to add a default constructor himself, he can derive from 
the variant, and add such a constructor, favoring the type of his 
choice. Or maybe the best solution would be to add a template parameter 
to boost::variant to indicate which of the types to use for the default 
constructor, with an option (the default one) to not supply a default 
constructor at all.

Of course I'm just saying all this here for Google's sake. Nothing real 
would probably be done. Sad...

Yuval