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—or shouldn't—be 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);
}