Question

Based on C++ Primer 4th edition (i.e. pp 105),

bitset<32> bitvec;

bitvec[0].flip(); // reverses the first bit.

My question is why the second line works? Based on

http://www.cplusplus.com/reference/stl/bitset/operator[]/

     bool operator[] ( size_t pos ) const;
reference operator[] ( size_t pos );

How can bitset::flit can be used on bool or reference?

Thank you

Answer 1

“reference” returned by operator[] is not a C++ reference, it is a special proxy class that has flip() method.

Pseudo code:

class bitset
{
   class reference
   {
   public:
      reference& flip()
      {
         bitset_.flip(pos_);
         return *this;
      }

      reference(bitset& obj)
      : bitset_(obj), pos_(pos)
      {
      }

   bitset& bitset_;
   size_t pos_;
   };

   reference operator[](size_t pos)
   {
      return reference(*this, pos);
   }
}

See source code of your STL implementation for details.