Re: [boost] Subobjects placement.

From: Jason Kankiewicz (jkankiewicz_at_[hidden])
Date: 2005-09-17 11:51:35

A struct with no methods should, I believe, have a straightforward 
memory mapping in order to be backwards-compatible with C.

In order to provide a struct-like interface to a class without losing 
the benefits of data hiding by declaring its members as public, I like 
to use a nested struct, like so:

class point2d
{
public:
   struct data
   {
     double x,
            y;
   };

   point2d( data const& Data ):
     Data_(Data)
   {;}

   operator data( void ) const
   {
     return Data_;
   }

   point2d( double X, double Y ):
   {
     Data_.x= X;
     Data_.y= Y;
   }

   double
   X( void ) const
   {
     return Data_.x;
   }

   double
   setX( double Value )
   {
     Data_.x= Value;
   }

   double
   Y( void ) const
   {
     return Data_.y;
   }

   void
   setY( double Value )
   {
     Data_.y= Value;
   }

private:
   data Data_;

};

The non-explicit constructor taking an instance of the nested struct as 
argument and a conversion method to the nested struct type provides 
seamless interoperability between instances of the class and its nested 
struct like so:

point2d::data Data;
Data.x= 6.0;
Data.y= 8.0;
point2d Point2D= Data;

Point2d.setX(Point2d.X() * Point2d.Y());
Point2d.setY(0.0)
point2d::data Data= Point2D;

When you want a full-blown object interface to the data, use the class, 
when you want a struct-like interface to the data, use the nested 
struct instead.

Adam Badura wrote:
>     At first english is not my primary language and all my books on C++ are
> in polish so I have to translate. I'm afraid I'm not using correct words or
> terms but I will try to be as descripting as possible to be understood.
> 
>     I have a question on placement of subobjects in a class (or struct -
> what I know of standard this will be the same). In S. Lippman's "Inside the
> C++ Object Model" I found that standard only guaranties that subobjects are
> placed in specific sequence (not to get inside details) but there is no
> guarante that compiler doesn't place some additional objects (for his
> internal use) at the begining of object in between subobjects or at the end.
> (I looked also in Stroustrup's "The C++ Programming Language" and "The
> Design and Evolution of C++" but have found nothing - but I wasn't looking
> that hard).
> 
>     So, if I have a class:
> 
> class point2d {
> public:
>  double x;
>  double y;
> };
> 
> and make an object of it
> 
> point2d pp = new point2d;
> 
> than ther is no guaranty that
> 
> reinterpret_cast<double*>(pp) will give me addres of point2d::x.
> 
>     Normally it no difference but if I want to write a class that for
> example warps comunication with some hardware than I have to be SURE how
> data would be allocated in memory becouse it must be correct acording to
> this hardware specification (if hardware keeps in memory a buffer with a
> char [for command] and int [for argument]) than I my class members must be
> so placed in this mamory).
>     Another example is color. If I want to write a class that warps color
> managemant I want it to be more useable I have to be sure that class
> 
> class color_argb {
> public:
>  unsigned char a;
>  unsigned char r;
>  unsigned char g;
>  unsigned char b;
> };
> 
> will use ONLY 4 chars (bytes) and ther will not be any other data placed in
> memory. If one would be added (for example virtual table pointer) than
> color_argb could not be used to warp data recevied from and given to many
> libraries and hardware.
> 
>     I suppose that if I don't use any polimorphism (as virtual functions),
> inherit only in single line and without virtual base classes and don't use
> any runtime support that compiler doesn't have any reason to place
> additional data (but still can use more than 4 bytes to align data for
> example). But what I know it still MAY do this and relaying on that that it
> does not have to is dangarous.
>     So how is it?
> 
>     Thanks in advance.
> 
>     Adam Badura
> 
> 
> 
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