Visitor pattern for boost::any in C++11

any_visitor.cpp

#include <boost/any.hpp>
#include <unordered_map>
#include <functional>
#include <iostream>
#include <vector>

struct type_info_hash {
    std::size_t operator()(std::type_info const & t) const {
        return t.hash_code();
    }
};

struct equal_ref {
    template <typename T>
    bool operator()(std::reference_wrapper<T> a,
                    std::reference_wrapper<T> b) const {
        return a.get() == b.get();
    }
};

struct any_visitor {
    using type_info_ref = std::reference_wrapper<std::type_info const>;
    using function = std::function<void(boost::any&)>;
    std::unordered_map<type_info_ref, function, type_info_hash, equal_ref> fs;

    template <typename T>
    void insert_visitor(std::function<void(T&)> f) {
        fs.insert(std::make_pair(
            std::ref(typeid(T)),
            function([&f](boost::any & x) { f(boost::any_cast<T&>(x)); })));
    }

    bool operator()(boost::any & x) {
        auto it = fs.find(x.type());
        if (it != fs.end()) {
            it->second(x);
            return true;
        } else {
            return false;
        }
    }
};

struct abc {};

void fa(int i) { std::cout << "fa(" << i << ")" << std::endl; }
void fb(abc) { std::cout << "fb(abc())" << std::endl; }

int main() {
    any_visitor f;
    f.insert_visitor<int>(fa);
    f.insert_visitor<abc>(fb);
    
    std::vector<boost::any> xs;
    xs.push_back(1);
    xs.push_back(abc());
    xs.push_back(1.5);
    
    for (auto & x : xs) {
        if (!f(x)) std::cout << "no visitor registered" << std::endl;
    }
}

output.txt

fa(1)
fb(abc())
no visitor registered