Re: [Boost-users] boost::apply_visitor(const& visitor, ...) - why const?

From: Chris Russell (cdr_at_[hidden])
Date: 2007-01-19 11:04:55

Thanks Yuval - this allows me to do exactly what I want.

It would be useful to add this information to the high-level class overview 
http://boost.org/doc/html/variant/reference.html#header.boost.variant.hpp 
and to the Basic Usage tutorial section 
http://boost.org/doc/html/variant/tutorial.html#variant.tutorial.basic which 
doesn't mention this useful detail.

To your point about passing the visitor by value: I don't understand why 
this would be useful. One could for example simply call like:

typedef boost::variant<int,long,string> my_variant;
class my_visitor : public boost::static_visitor<>
{ //... };
my_variant v(5);
v.apply_visitor(my_visitor());

... which constructs the visitor as a temporary but doesn't incur the 
overhead of an extra copy constructor.

Anyway - thanks very much for pointing out that I can call apply_visitor 
directly on the variant. This saves me time and makes my code more readable 
than the previous suggestion of using boost::bind to adapt the unary visitor 
functors into binary functors that accept a non-const reference to the data 
I want to operate on in my visitor's operator().

- Regards

Chris

"Yuval Ronen" <ronen_yuval_at_[hidden]> wrote in message 
news:eoq76s$is8$1_at_sea.gmane.org...
> Accepting the visitor by non-const reference will make calling
> apply_visitor with a temporary visitor not compile, so there's a
> downside to that too. You can use the apply_visitor member function of
> variant:
>
> v1.apply_visitor(my_visitor);
> v2.apply_visitor(my_visitor);
>
> which accepts the visitor by non-const reference.
>
> Not that I'm saying that any if these solutions is best. IMO, a
> static_visitor is some kind of a functor, and as such should be accepted
> by value, making it possible to pass temporaries, and write code as you
> described. However, I couldn't convince about it...
>
> Yuval