[boost] [variant] passing visitors to apply_visitor()

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2005-06-23 09:16:22

Hi.

When I want to apply a visitor to a variant, I currently have 2 choices:

    boost::apply_visitor( my_visitor(), my_variant );

and

    my_visitor mv;
    my_variant.apply_visitor( mv );

A tempting 3rd choice of

    my_variant.apply_visitor( my_visitor() );

doesn't work because the variant::apply_visitor member function accept a
const visitor ref, which means it can't be a temporary object.

Moreover, the first option (boost::apply_visitor freestanding function) also
has a problem - it only accepts non-const variants.

I'm not really sure, but I guess that all these limitation are originated
from the desire to pass the visitor by ref, and it's not always possible to
provide both const-ref and non-const-ref overloads. My question is: why by
reference and not by value? I see visitors as a kind of function objects,
and I think that's how it was meant to be seen. Function objects are usually
passed by value, so why should visitors be any different? I couldn't find
anything relevant to this in the docs. Passing by value will allow the much
more appealing syntax I presented as the 3rd option, and it will remove the
limitation from the freestanding function.

Makes sense or complete nonsense?

Thanks,
Yuval