create a closure in C++

main.cpp

struct db_sum: public std::unary_function<boost::any, void> {
private:
    double& sum_;

public:
    explicit db_sum(double& sum)
        : sum_(sum)
    {}

    void operator()(const cell_t& value) {
        const std::type_info& ti = value.type();

        if (ti == typeid(int)) {
            sum_ += boost::any_cast<int>(value);
        } else if (ti == typeid(float)) {
            sum_ += boost::any_cast<float>(value);
        }
    }
};