Re: sizeof an object that have same base class as member objects.

Note: This problem is related to gcc but after some back and forth in
group gnu.gcc.help it seems to have morph into more of a c++
specificiation question, hence the transplanting to this group.

The original post at gnu.gcc.help can be found at this link
http://groups.google.com/group/gnu.gcc.help/browse_thread/thread/ece55cbbd9c36270/2148a6c1ac6119e1?lnk=st&q=#2148a6c1ac6119e1

Here's the question:

class base {
public:
base(){};
~base(){};

};

class data : public base {
public:
data(){};
~data(){};
private:
int member;

}__attribute__((__packed__));

class group : public base {
public:
group(){};
~group(){};
private:
data d1;
data d2;
data d3;

} __attribute__((__packed__));

int main(int argc, char **argv) {
std::cout << "base = " << sizeof(base) << std::endl;
std::cout << "data = " << sizeof(data) << std::endl;
std::cout << "group = " << sizeof(group) << std::endl;
return (0);

}

The output of the program is:
base = 1
data = 4
group = 13

The result of sizeof(group) is puzzling as it should be 12 if EBO
(empty base optimization) worked for both class data and group.
Apparently EBO kicked in for _ONLY_ one of them. If EBO didn't work at
all, sizeof(group) should be 16.

Removing the extension of class base from either class group or data
will cause sizeof(group) to return 12. It seems that gcc is unable to
fully apply EBO when a class and its member inherits the same empty
base class.

The same code had been tested on microsoft msvc compiler and realview
arm compiler, both correctly optimizes the code and give the correct
value as 12.

Is this a known bug with gcc 3.4.5? (Note: I'm using MinGW)

I dug
through the bugbase but couldn't come up with anything. Maybe EBO
isn't the problem at all.