"Cannot convert parameter" using boost::variant iterator

Answer 1

I suspect you are missing cases on your do_compare static_visitor. Remeber, the variants might have anything, so you need all possible combinations, like comparing a FooList::const_iterator to a FooMap::const_iterator. It's complaining because the compiler is trying to find some match for that case, and can't convert a FooMap::const_iterator to a FooList::const_iterator.

Hammering it out:

struct do_compare : boost::static_visitor<bool>
{
    bool operator() (
        const FooMap::const_iterator & a,
        const FooMap::const_iterator & b) const
    { return a != b; }

    bool operator() (
        const FooList::const_iterator & a,
        const FooList::const_iterator & b) const
    { return a != b; }

    bool operator() (
        const FooMap::const_iterator & a,
        const FooList::const_iterator & b) const
    { return false; }

    bool operator() (
        const FooList::const_iterator & a,
        const FooMap::const_iterator & b) const
    { return false; }
};

Here's a version with templates:

template <typename A, typename B>
bool operator() (
    const A & a,
    const B & b) const
{ return false; }

template <typename A>
bool operator() (
    const A & a,
    const A & b) const
{ return a != b; }

It's compiling on comeau, but I'm not 100% it will work, so some testing is required. Other than cleaner, more versatile code, it shouldn't have any effect, as long as it works.