void DoSomething(const boost::function<bool ()>& condition, other stuff);

boost::function<bool (size_t)> ge = boost::bind(std::greater_equal<size_t>(),
                                                _1, threshold);
boost::function<size_t ()> size = boost::bind(&std::vector<std::string>::size,
                                              data);
DoSomething(boost::lambda::bind(ge, boost::lambda::bind(size)), other stuff);

DoSomething(delegate() { return data.size() >= threshold; }, other stuff);
DoSomething(() => (data.size() >= threshold), other stuff);

boost::function<size_t ()> size = boost::bind(&std::vector<std::string>::size,
                                              boost::ref(data));

boost::function<bool ()> cond =
    (boost::bind(&std::vector<std::string>::size, boost::ref(data))
        >= threshold);

DoSomething(cond, other stuff);