Item 43. Const-Correctness

Difficulty: 6

const is a powerful tool for writing safer code. Use const as much as possible, but no more. Here are some obvious and not-so-obvious places where const should梠r shouldn't梑e used.

Don't comment on or change the structure of this program; it's contrived and condensed for illustration only. Just add or remove const (including minor variants and related keywords) wherever appropriate.

Bonus question: In what places are the program's results undefined/uncompilable due to const errors?

class Polygon 


{


public:


  Polygon() : area_(-1) {}


  void  AddPoint( const Point pt ) { InvalidateArea();


                                     points_.push_back(pt); }


  Point GetPoint( const int i )    { return points_[i]; }


  int   GetNumPoints()             { return points_.size(); }


  double GetArea()


  {


    if( area_ < 0 ) // if not yet calculated and cached


    {


      CalcArea();     // calculate now


    }


    return area_;


  }


private:


  void InvalidateArea() { area_ = -1; }


  void CalcArea()


  {


    area_ = 0;


    vector<Point>::iterator i;


    for( i = points_.begin(); i != points_.end(); ++i )


      area_ += /* some work */;


  }


  vector<Point> points_;


  double        area_;


};


Polygon operator+( Polygon& lhs, Polygon& rhs )


{


  Polygon ret = lhs;


  int last = rhs.GetNumPoints();


  for( int i = 0; i < last; ++i ) // concatenate


  {


    ret.AddPoint( rhs.GetPoint(i) );


  }


  return ret;


}





void f( const Polygon& poly )


{


  const_cast<Polygon&>(poly).AddPoint( Point(0,0) );


}





void g( Polygon& const rPoly ) { rPoly.AddPoint( Point(1,1) ); }





void h( Polygon* const pPoly ) { pPoly->AddPoint( Point(2,2) ); }





int main()


{th


  Polygon poly;


  const Polygon cpoly;


  f(poly);


  f(cpoly);


  g(poly);


  h(&poly);


}