Runnable implementation using packaged_task in c++11 - multithreading

Question

I am trying to create a Runnable interface in c++11 using packaged_task, with child class overriding run() function. I don't know why this code is not compiling. Its giving error related to type argument.
/usr/include/c++/4.8.1/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of()>’
typedef typename result_of<_Callable(_Args...)>::type result_type;
Below is my code snippet. Could someone plz give me some information on this error and whether implementing Runnable this way is a right way to proceed ?
class Runnable {
public:
explicit Runnable() {
task_ = std::packaged_task<int()>(&Runnable::run);
result_ = task_.get_future();
std::cout << "starting task" << std::endl;
}
virtual int run() = 0;
int getResult() {
task_();
return result_.get();
}
virtual ~Runnable() {
std::cout << "~Runnable()" << std::endl;
}
private:
std::future<int> result_;
std::packaged_task<int()> task_;
};
class foo : public Runnable {
int fib(int n) {
if (n < 3) return 1;
else return fib(n-1) + fib(n-2);
}
public:
explicit foo(int n) : n_(n) {}
int run() {
cout << "in foo run() " << endl;
int res = fib(n_);
cout << "done foo run(), res = " << res << endl;
return res;
}
~foo() {}
private:
int n_;
};
int main(int argc, char*argv[]) {
stringstream oss;
oss << argv[1];
int n;
oss >> n;
shared_ptr<foo> obj(new foo(n));
obj->run();
cout << "done main" << endl;
return 0;
}