I spent about two hours today trying to debug a race condition in a multi-threaded C++ app today… definitely not a fun thing to do. The worst part? The runtime diagnostics weren’t giving me anything useful to work with! Sometimes things just worked, sometimes I got segmentation faults inside old, well-tested parts of the application. At one point, I saw this error pop up:

pure virtual method called
terminate called without an active exception
Aborted

What? I know I can’t instantiate a class that has any pure-virtual methods, so how did this error show up? To debug it, I decided to replace all of the potentially-erroneous pure virtuals with stub functions that printed warnings to stderr. Lo and behold, I confirmed that polymorphism wasn’t working in my application. I had a bunch of Deriveds sitting in memory, and yet, the Base methods were being called.

Why was this happening? Because I was deleting objects while they were still in use. I don’t know if this is GCC-specific or not, but something very curious happens inside of destructors. Because the object hierarchy’s destructors get called from most-derived to least-derived, the object’s vtable switches up through parent classes. As a result, at some point in time (nondeterministic from a separate thread), my Derived objects were all really Bases. Calling a virtual member function on them in this mid-destruction state is what caused this situation.

Here’s about the simplest example I can think of that reproduces this situation:


#include <pthread.h>
#include <unistd.h>
struct base
{
    virtual ~base() { sleep(1); }
    virtual void func() = 0;
};
struct derived : public base
{
    virtual ~derived() { }
    virtual void func() { return; }
};
static void *thread_func(void* v)
{
    base *b = reinterpret_cast<base*>(v);
    while (true) b->func();
    return 0;
}
int main()
{
    pthread_t t;
    base *b = new derived();
    pthread_create(&t, 0, thread_func, b);
    delete b;
    return 0;
}

So what’s the moral of the story? If you ever see the error message pure virtual method called / terminate called without an active exception, check your object lifetimes! You may be trying to call members on a destructing (and thus incomplete) object. Don’t waste as much time as I did.

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