get weakptr point from this

get weakptr point from this

和enable_shared_from_this刚好相反， http://www.cppblog.com/pizzx/archive/2014/07/02/207477.html
内部持有shared_ptr作为成员变量，生命周期和类相同，通过weak_ptr可以判断当前对象生命周期是否已经结束（可以在其他线程判断，线程安全）。

#include  < iostream >
#include  < boost / weak_ptr.hpp >
#include  < boost / shared_ptr.hpp >
#include  < vector >

using   namespace  std;
using   namespace  boost;

class  null_deleter
{
public:
void operator()(void *)
{}

} ;


class  X
{
private:

    shared_ptr<X> this_;
    int i_;

public:

    explicit X(int i): this_(this, null_deleter()), i_(i)
    {
    }

    // repeat in all constructors (including the copy constructor!)

    X(X const & rhs): this_(this, null_deleter()), i_(rhs.i_)
    {
    }

    // do not forget to not assign this_ in the copy assignment

    X & operator=(X const & rhs)
    {
            i_ = rhs.i_;
    }

    weak_ptr<X> get_weak_ptr() const { return this_; }
} ;

int  main()
{
   vector<weak_ptr<X> >  m_vcwptr;

    {
     X x(1);
    }

    {
        shared_ptr<X>  x(new X(1));
        m_vcwptr.push_back(x->get_weak_ptr());
        std::cout<<"x use count: "<<x.use_count()<<"\n";

        weak_ptr<X>  xinscope = m_vcwptr[0];

        if(!xinscope.lock())
           std::cout<<"xy is already expired"<<"\n";
        else
           std::cout<<"xy is exist "<<"\n";
    }

    weak_ptr<X>  xy = m_vcwptr[0];

    if(!xy.lock())
       std::cout<<"xy is already expired"<<"\n";
}