c++ - On the implementation of atomic_exchange_strong_explicit for shared_ptr -

please see this link:

template<typename _tp>   bool   atomic_compare_exchange_strong_explicit(shared_ptr<_tp>* __p,                                           shared_ptr<_tp>* __v,                                           shared_ptr<_tp> __w,                                           memory_order,                                           memory_order)   {     shared_ptr<_tp> __x; // goes out of scope after __lock     _sp_locker __lock{__p, __v};     owner_less<shared_ptr<_tp>> __less;     if (*__p == *__v && !__less(*__p, *__v) && !__less(*__v, *__p))       {         __x = std::move(*__p);         *__p = std::move(__w);         return true;       }     __x = std::move(*__v);     *__v = *__p;     return false;   } 

to me looks *__p == *__v , !__less(*__p, *__v) && !__less(*__v, *__p) both state fact pointers *__p , *__v equal. why both used there?

thanks.

as touched on by great answer need both sure both shared_ptrs share same object , share same ownership of object ( both shared pointers use same ref counter ? ).

*__p == *__v checks first part comparing *__p.get() == *__v.get() , !__less(*__p, *__v) && !__less(*__v, *__p) makes sure both shared pointers share internal ref counter, satisfying both parts.

*__p == *__v checks equality of owned objects, not shared state of internal ref counter need here, why need construct.