Make a safe Reader/Writer vector

Inspired by this code, I am trying to implement a Reader/Writer vector that can safely call push_back() concurrently by threads.

Once this class is in place, I might then create method erase() by calling std::swap(), which swaps the target item and the last item and then erase the last item in the collection. In this way, I assume that the performance should be fair because deleting an item in the middle of collection does not invoke moving all items following the target item in the collection.

Unfortunately, the following code:

#include <vector>
#include <boost/thread/shared_mutex.hpp> //shared_mutex
#include <memory> //shared_ptr
#include <utility> //swap()

template <class T>
class readers_writer_vector
{
    std::shared_ptr<boost::shared_mutex> pm;
    std::vector<T> data;
public:
    readers_writer_vector() :
        pm(new std::shared_ptr<boost::shared_mutex>){}
    void push_back(const T& item){
        boost::unique_lock<boost::shared_mutex> lock(*pm); //wrong design
        data.push_back(item);
    }
};

int main()
{
    readers_writer_vector<int> db;
    db.push_back(1);
    return 0;
}

yields the following compliation errors:

/usr/include/c++/4.9/bits/shared_ptr_base.h:871:39: error: cannot convert ‘std::shared_ptr<boost::shared_mutex>*’ to ‘boost::shared_mutex*’ in initialization
         : _M_ptr(__p), _M_refcount(__p)

//g++ -std=c++11 -Iboost -lboost t.cpp

How do I fix it? Please!