Why not "if (px != rpx) " ?

From: Louis Lavery (Louis_at_[hidden])
Date: 2001-12-14 05:30:43

Hello,

The following is use by the boost shared_ptr assignment operators
(see boost\smart_ptr.hpp)...

   void share(T* rpx, long* rpn) {
      if (pn != rpn) { // Q: why not px != rpx? A: fails when both == 0
         ++*rpn; // done before dispose() in case rpn transitively
                 // dependent on *this (bug reported by Ken Johnson)
         dispose();
         px = rpx;
         pn = rpn;
      }
   } // share

...I curious to know what it means by "fails when both == 0".

I mean, I can't see as the sky would fall in if the test was
"if (px != rpx)". Am I missing something?

Thanks,

Louis.