Re: sequence of inheritance from virtual base class?
Am 16.05.13 14:55, schrieb Ralf Fassel:
* Richard Damon <Richard@Damon-Family.org>
| The order of inheritance should not matter for conforming code. A
| likely issue is code that assumes &Derived == &Base and does
| invalid casting on it.
That's indeed involved. I have two libraries, which need to pass
objects of class Base. For some reason I have to pass these via a
string representation. I do:
library 1:
Base *ptr = &Derived;
ptr->foo(); // ok
std::ostrstream ptrstr1 << (void*) ptr; // assume ptrstr1 is a
global variable
library 2:
std::istrstream ptrstr2(ptrstr1.str());
void *ptr;
ptrstr2 >> ptr;
Base *bptr = (Base*)ptr;
bptr->foo(); // crash if Base is not the first in inheritance list
of Derived
I have checked that the bptr in lib2 has the same value as ptr in
lib1. I thought that casting to/from void* using the same class in
both directions was ok, but maybe I'm missing something fundamental
here...
I ran the attached code, and it worked just fine. This indicates that
your marshalling code is probably not the source of the error, and
neither is the layout of your inheritance hierarchy.
The fact that you use operator& to take the address of Derived in your
code above leads me to believe that Derived may be a local variable
that may get destroyed after the function in library 1 has
finished. So it may be the case that library 2 receives a pointer that
is no longer valid. If you allocate Derived in the heap, you shouldn't
run into such problems.
Regards,
Stuart
#include <iostream>
#include <strstream>
class Base {
public:
Base() {}
virtual ~Base() {}
virtual void foo() = 0;
};
class Unrelated1 {};
class Unrelated2 {};
class Derived1 : public Base, public Unrelated1, public Unrelated2 {
public:
Derived1() {}
~Derived1() {}
void foo() {}
};
class Derived2 : public Unrelated1, public Unrelated2, public Base {
public:
Derived2() {}
~Derived2() {}
void foo() {}
};
int main() {
Derived2 d1;
Base *ptr = &d1;
ptr->foo(); // ok
std::ostrstream ptrstr1;
ptrstr1 << (void*) ptr; // assume ptrstr1 is a global variable
std::istrstream ptrstr2(ptrstr1.str());
void *ptr2;
ptrstr2 >> ptr2;
Base *bptr = (Base*)ptr2;
bptr->foo(); // crash if Base is not the first in inheritance list
// doesn't crash in this example.
}
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