Question

I want to make following code snippet to work

typedef boost::function<result_type ()> functor_type;
typedef boost::variant<int, functor_type> result_type;

two types depend on each other, how to break the circular dependencies?

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The Motivation

Basicly, I want to implement tail call in c++, like this

template<typename ResultT>
struct TailCall
{
    typedef ResultT result_type;
    typedef boost::function<ret_type ()> continuation_type;
    typedef boost::variant<result_type, continuation_type> ret_type;

    TailCall(const continuation_type& init) : next(init) {}

    result_type operator()()
    {
         while (true) {
            ret_type r = next();
            result_type *result = boost::get<result_type>(&r);
            if (result)
                return *result;
            else
                next = boost::get<continuation_type>(r);
         }
    }

private:
    continuation_type next;
};

TailCall<int>::ret_type fibonacci_impl(int term, int val, int prev)
{
    if (term == 0) return prev;
    if (term == 1) return val;
    return boost::bind(fibonacci_impl, term-1, val+prev, val);
}

int fibonacci(int index)
{
    TailCall<int> fib(boost::bind(fibonacci_impl, index, 1, 0));
    return fib();
}

Answer 1

Boost variant has two specific utilities (recursive_variant and recursive_wrapper) to do that.

Here an example using recursive_wrapper

struct functor_type;

typedef boost::variant<int,
            boost::recursive_wrapper<functor_type> > result_type;

struct functor_type : public boost::function<result_type()> {
      using boost::function<result_type()>::boost::function<result_type()>;
};