Compile error when using boost::weak_ptr

Question

#include <iostream>
#include <boost/shared_ptr.hpp>
using namespace std;
using boost::shared_ptr;
using boost::weak_ptr;

int main() {
  shared_ptr<int> p(new int(5));
  weak_ptr<int> q = p;
  if(shared_ptr<int> r = q.lock())
  {
    cout << r << endl;
  }
  return 1;
}

Hi all, when I compile above codes to be familiar with boost::weak_ptr, I encountered the following compiling error: variable ‘boost::weak_ptr<int> q’ has initializer but incomplete type. Could anyone help me figure out where is wrong? Thanks in advance!