[boost] Subobjects placement.

From: Adam Badura (abadura_at_[hidden])
Date: 2005-09-17 10:55:44

    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