17. The dual operator[] idiom

• It is common to define two operator[ ]s

• Allows use on left-hand side, and on const objects:

  
  class MyDoubleArray
  {
      double& operator[](unsigned int i);
      double operator[](unsigned int i) const;
      //...
  };
  
  void copy( MyDoubleArray& dest, MyDoubleArray const& src )
  {
      for ( size_t idx = 0; idx < src.size(); ++idx ) {
          // Use non-const op[] on left, const op[] on right:
          dest[ idx ] = src[ idx ];
      }
  }