Method weak_ptr

system/lib/libcxx/include/__memory/shared_ptr.h:1283–1286

template <class _Tp>
inline weak_ptr<_Tp>::weak_ptr(weak_ptr const& __r) _NOEXCEPT : __ptr_(__r.__ptr_), __cntrl_(__r.__cntrl_) {
  if (__cntrl_)
    __cntrl_->__add_weak();
}

template <class _Tp>
template <class _Yp, __enable_if_t<__compatible_with<_Yp, _Tp>::value, int> >