Re: [boost] Boost.Variant binary visitation with modification

From: james.jones_at_[hidden]
Date: 2006-09-06 19:10:18

From: Yuval Ronen <ronen_yuval_at_[hidden]>
>
> Also notice the third operator(): if you wanted the opposite - convert 
> the int to float and store the plus result in the variant, thus changing 
> the type of the variant's content, then I don't know how to do it. I 
> mean, you could make the visitor derived from static_visitor<float>, 
> make the operators() return the result, and store it in the variant 
> later, but that would also make int+int result a float, which might be 
> not good for you...

I had the same problem with a visitor that I wanted to mutate the variant type. The solution I came up with was to wrap the variant in a class, then define my various methods in the class and forward them to the variant, then assign the result back to the class, like so:

typedef [...whatever...] private_variant_type;

class variant_type
{
private:
        private_variant_type _value;

public:
        variant_type 
        operator+= (const variant_type& lhs, const variant_type& rhs)
        {
                _value = boost::apply_visitor(addition_visitor(), lhs, rhs);
                return *this;
        }
};

The addition_visitor class returns private_variant_type.

I think this is a little clunky, but it DOES work. It would be great if there were a way to change the type of a variant from within a visitor!

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