c++ - gcc list node swap implementation overcomplicated? -


i'm using sgi stl (gcc) reference custom library, , while digging std::list::swap() came across following implementation,

note: method not handle adjacent nodes properly.

// namespace std { // namespace __detail { void _list_node_base::swap(_list_node_base& __x, _list_node_base& __y) throw() {   if ( __x._m_next != &__x )     {       if ( __y._m_next != &__y )         {           // both __x , __y not empty.           std::swap(__x._m_next,__y._m_next);           std::swap(__x._m_prev,__y._m_prev);           __x._m_next->_m_prev = __x._m_prev->_m_next = &__x;           __y._m_next->_m_prev = __y._m_prev->_m_next = &__y;         }       else         {           // __x not empty, __y empty.           __y._m_next = __x._m_next;           __y._m_prev = __x._m_prev;           __y._m_next->_m_prev = __y._m_prev->_m_next = &__y;           __x._m_next = __x._m_prev = &__x;         }     }   else if ( __y._m_next != &__y )     {       // __x empty, __y not empty.       __x._m_next = __y._m_next;       __x._m_prev = __y._m_prev;       __x._m_next->_m_prev = __x._m_prev->_m_next = &__x;       __y._m_next = __y._m_prev = &__y;     } }

this looks me if simplified to,

void _list_node_base::swap(_list_node_base& __x, _list_node_base& __y) throw() {   _list_node_base* __xnext = __x._m_next;   _list_node_base* __xprev = __x._m_prev;   _list_node_base* __ynext = __y._m_next;   _list_node_base* __yprev = __y._m_prev;    __xnext->_m_prev = __xprev->_m_next = &__y;   __ynext->_m_prev = __yprev->_m_next = &__x;   std::swap(__x._m_next,__y._m_next);   std::swap(__x._m_prev,__y._m_prev); }

i've tested cases (empty/empty, empty/not-empty, etc.), including __x , __y referencing same node, , seems work, however, trust in sgi codebase making me doubt myself.

so question is: correct? , if there benefit using longer version?

thank you.

self-assignment checks rage. they're known pessimizations , bug-hiding now. may want find more modern source of inspiration.


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