使用boost :: weak_ptr时编译错误

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;
}

大家好，当我编译上面的代码以熟悉boost :: weak_ptr时，我遇到了以下编译错误：variable ‘boost::weak_ptr<int> q’ has initializer but incomplete type。任何人都可以帮我弄清楚哪里出错了？提前致谢！