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

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 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" wrote in message news:eoq76s$is8$1@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