Odd behavior without virtual inheritance

Question

I am working on a piece of code that presents a very odd behavior. I managed to replicate it in a simple hello world style program, following is the code:

#include <iostream>
using namespace std;

class Test
{
public:
    virtual ~Test() = default;

protected:
    virtual void SetUp() { }
};

class ICallbackReceiver
{
public:
    virtual ~ICallbackReceiver() = default;
    virtual void onReady() = 0;
};

// C-style callback
void readyReceiver(void* userdata)
{
    cout << "3) readyReceiver\n";
    static_cast<ICallbackReceiver*>(userdata)->onReady();
}

using callback_t = void(*)(void*);
callback_t myCallback;
void* myUserData;

void registerCallback(callback_t callback, void* userData)
{
    cout << "2) registerCallback\n";
    myCallback = callback;
    myUserData = userData;
}

class ConfigurableTest : public /*virtual*/ Test, public ICallbackReceiver
{
public:

    void SetUp() override
    {
        cout << "1) ConfigurableTest::SetUp\n";
        registerCallback(&readyReceiver, static_cast<void*>(this));
    }

    void onReady() override
    {
        cout << "4) ConfigurableTest::onReady\n";
    }
};

int main()
{
    ConfigurableTest test;
    test.SetUp();

    myCallback(myUserData);

    return 0;
}

Whenever myCallback is called something has to be tested. And this is the output that should be displayed:

1) ConfigurableTest::SetUp
2) registerCallback
3) readyReceiver
4) ConfigurableTest::onReady

But, unless I specify a virtual inheritance for the Test class, this is the output that I see:

1) ConfigurableTest::SetUp
2) registerCallback
3) readyReceiver
1) ConfigurableTest::SetUp
2) registerCallback

As you can see ConfigurableTest::onReady is never called, but ConfigurableTest::SetUp is actually called twice!

What is the origin of this behavior? How can I re-factor the code to reproduce the correct behavior without using virtual inheritance?

What compiler did you use? Anyway, replicated on `g++.exe (Rev1, Built by MSYS2 project) 7.2.0` — underscore_d, Dec 21 '17 at 15:45
I can also replicate it by not using `C++11` (with minor changes to the code). — Nick, Dec 21 '17 at 15:57
This works as expected if `ICallbackReceiver *` is put in place of `void *`. Maybe some type information is getting lost. — dbush, Dec 21 '17 at 16:05
@Nick I expected as much. Just trying to narrow down the problem. — dbush, Dec 21 '17 at 16:11

score 4 · Accepted Answer · answered Dec 21 '17 at 16:13

The problem is because you make conversions ConfigurableTest -> void * -> ICallbackReceiver, what is not permitted. You need to convert to the same type ConfigurableTest back from void * - no error in this case.

See also multiple inheritance: unexpected result after cast from void * to 2nd base class

Odd behavior without virtual inheritance

1 Answers1