[gmpi] Re: C++ detail question

On Wed, Mar 02, 2005 at 04:00:15PM -0500, Paul Davis wrote:
> >> its guaranteed to be C compatible, which leads to the data elements
> >> coming first.
> >
> >OK, so second question.  If I cast a "struct Foo *" to a "class CFoo *",
> >what happens?  Can I use non-virtual methods on it?  Can I use virtual
> >methods on it?
> >
> >Obviously, I could not access any data members that are part of CFoo but
> >not Foo, right?
> 
> not sure if i understand the question completely. which one is
> declared in what language?

/* defined in C */
struct Foo
{
        int foo1;
        int foo2;
};

/* defined in C++ */
class CFoo : public Foo
{
public:
        int method1(void) { return 0; }
        virtual int method2(void) { return 0; }
        int cfoo1;
};

void func(struct Foo *foo)
{
        CFoo* cfoo = static_cast<CFoo*>(foo);

        cfoo->method1();        /* safe? */
        cfoo->method2();        /* not safe */
        cfoo1 = 12345;          /* not safe */
}


My assumption is that it is safe to call CFoo::method1 because the
compiler will do static binding.  Now, if method1 does something unsafe,
that's a different story.  Assuming method1 is safe internally, is this
safe to do?

It seems pretty clear that CFoo::method2 is illegal, as the CFoo object
does not have a valid vtable.  Accessing the cfoo1 member is likewise
bad because it doesn't exist.

The question is whether it is safe to access method1?

Specifically, there is a C struct that I want to wrap in C++, including
a couple of operator methods (needed to be put into a map).  We could
create a new CFoo and initialize it with the Foo, but it seems like
there will be a lot of construct/destruct cycles wasted there...



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