Where do "pure virtual function call" crashes come from?

Question

I sometimes notice programs that crash on my computer with the error: "pure virtual function call".

How do these programs even compile when an object cannot be created of an abstract class?

Answer 1

I'd guess there is a vtbl created for the abstract class for some internal reason (it might be needed for some sort of run time type info) and something goes wrong and a real object gets it. It's a bug. That alone should say that something that can't happen is.

Pure speculation

edit: looks like I'm wrong in the case in question. OTOH IIRC some languages do allow vtbl calls out of the constructor destructor.

Answer 2

They can result if you try to make a virtual function call from a constructor or destructor. Since you can't make a virtual function call from a constructor or destructor (the derived class object hasn't been constructed or has already been destroyed), it calls the base class version, which in the case of a pure virtual function, doesn't exist.

class Base
{
public:
    Base() { doIt(); }  // DON'T DO THIS
    virtual void doIt() = 0;
};

class Derived : public Base
{
    void doIt() {}
};

int main(void)
{
    Derived d;  // This will cause "pure virtual function call" error
}

Answer 3

Here is a great explanation of what can cause this:

http://www.artima.com/cppsource/pure_virtual.html

Answer 4

Usually when you call a virtual function through a dangling pointer--most likely the instance has already been destroyed.

There can be more "creative" reasons, too: maybe you've managed to slice off the part of your object where the virtual function was implemented. But usually it's just that the instance has already been destroyed.

Answer 5

Here is a sneaky way for it to happen. I had this essentially happen to me today.

class A
{
  A *pThis;
  public:
  A()
   : pThis(this)
  {
  }

  void callFoo()
  {
    pThis->foo(); // call through the pThis ptr which was initialized in the constructor
  }

  virtual void foo() = 0;
};

class B : public A
{
public:
  virtual void foo()
  {
  }
};

B b();
b.callFoo();

Answer 6

As well as the standard case of calling a virtual function from the constructor or destructor of an object with pure virtual functions you can also get a pure virtual function call (on MSVC at least) if you call a virtual function after the object has been destroyed. Obviously this is a pretty bad thing to try and do but if you're working with abstract classes as interfaces and you mess up then it's something that you might see. It's possibly more likely if you're using referenced counted interfaces and you have a ref count bug or if you have an object use/object destruction race condition in a multi-threaded program... The thing about these kinds of purecall is that it's often less easy to fathom out what's going on as a check for the 'usual suspects' of virtual calls in ctor and dtor will come up clean.

To help with debugging these kinds of problems you can, in various versions of MSVC, replace the runtime library's purecall handler. You do this by providing your own function with this signature:

int __cdecl _purecall(void)

and linking it before you link the runtime library. This gives YOU control of what happens when a purecall is detected. Once you have control you can do something more useful than the standard handler. I have a handler that can provide a stack trace of where the purecall happened; see here: http://www.lenholgate.com/archives/000623.html for more details.

(Note you can also call _set_purecall_handler() to install your handler in some versions of MSVC).