如何把this指针转换成boost的shared_ptr

在使用shared_ptr的时候碰到了这个问题，直接上代码示例，编译： g++ shared_ptr.cpp -I ~/usr_spider/include/ -L ~/usr_spider/lib64  

#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <iostream>

using namespace std;

class test: public boost::enable_shared_from_this<test> {
    public:
        int num;

        boost::shared_ptr<test> get_sharedPtr_from_this(){
             return shared_from_this();
        }   
};

typedef boost::shared_ptr<test> testPtr;

main(){
    testPtr a(new test);
    a->num = 33; 
    testPtr b = a->get_sharedPtr_from_this();
    cout<<b->num<<endl;
}

参考：http://www.boost.org/doc/libs/1_47_0/libs/smart_ptr/enable_shared_from_this.html