"Anonyomous" Objects

From:
none <none@none.none>
Newsgroups:
comp.lang.c++
Date:
Tue, 10 Nov 2009 00:40:18 GMT
Message-ID:
<Xns9CBEB3B6E727Bnonenonenone@69.16.186.8>
I am having great troubles with "anonymous" objects.
That might not be the correct term, but I just mean an
object created on the stack but never given a name.
These objects apparently do not follow the rules of
virtual inheritance.

The code below outputs the following:

B::func()
B::func()
A::func()
A::func()

The behavior is the same on both g++ and in MS Visual
Studio 2005.

The most obvious workaround would be to avoid using
anonymous objects. Unfortunately, I really NEED to use
them. I can explain the details of why I need to do
this, if it matters.

I've spent a day experimenting and I've found that the
behavior is very fickle. For example, removing the
constructor and destructor from class A changes the
output to:

B::func()
B::func()
B::func()
B::func()

Can anyone explain what's going on? Thanks in advance.

----- begin code: -----

#include "stdio.h"

class A
{
public:
    A() {}
    virtual ~A() {}
    virtual void func() { printf("A::func()\n"); }
};

class B: public A
{
public:
    void func() { printf("B::func()\n"); }
};

class C: public B
{
};

C x()
{
    C new_C;
    return new_C;
}

int main(int argc, char **argv)
{
    // Call a method in an anonyomus object.
    // This works.
    x().func();

    // Call a method in an anonymous object through a
pointer.
    // This looks a little strange, but works.
    (&(x()))->func();

    // Should be exactly the same as above, but split
into two lines.
    // This calls A::func() even though func() is
virtual!
    A *A_ptr = &(x());
    A_ptr->func();

    // This also calls A::func(), even though the
pointer
    // is of type "pointer-to-C"!!
    C *C_ptr = &(x());
    C_ptr->func();

    return 0;
}

----- end code -----

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