pure virtual method called错误-zieckey-ChinaUnix博客

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 
#include 
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(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.

3 comments

In my case I have a base class that provides a thread for asynchronous processing. This is started in the base class constructor.

A derived class defines a thread function as well as pure virtual functions. The thread function can sometimes start calling the pure virtual functions before the later constructors are called in the ‘main’ thread.

My fix is inelegant - put a delay into the thread startup code.
There must be a better way?