Re: [boost] Re: class properties: request forpromotionand/orfastreview.

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-05-24 06:34:44

Peter Dimov wrote:
>It doesn't matter. Properties represent degrees of freedom. They don't
>describe dependent data well.
>
>As a relatively simple example, consider a pair of integers (x, y) that has
>an invariant of x*x + y*y = 25, and try to move from (3, 4) to (4, 3) by
>using the X and Y properties.

Since the invariant is on both variables, neither can be set independant of 
each other, since you'd assume that changing one would invalidate the 
invariant. Therefore, the properties must be *readonly* to prevent 
modification. You'd have a function to set both values at the same time 
(e.g. moveto).

class coord
{
   private:
      int x, y;
   public:
      inline int get_X(){ return( x ); }
      inline int get_Y(){ return( y ); }
      inline void moveto( int x_, int y_ )
      {
         assery(( x_ * x_ ) + ( y_ * y_ ) == 25 ); // invariant
         x = x_;
         y = y_;
      }
   public: // data access
      property_readonly< int, &get_X > X;
      property_readonly< int, &get_Y > Y;
   public:
      // ...
};

Regards,
Reece

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