Access violation exception when calling a method

Question

I've got a strange problem here. Assume that I have a class with some virtual methods. Under a certain circumstances an instance of this class should call one of those methods. Most of the time no problems occur on that stage, but sometimes it turns out that virtual method cannot be called, because the pointer to that method is NULL (as shown in VS), so memory access violation exception occurs. How could that happen?

Application is pretty large and complicated, so I don't really know what low-level steps lead to this situation. Posting raw code wouldn't be useful.

UPD: Ok, I see that my presentation of the problem is rather indefinite, so schematically code looks like

void MyClass::FirstMethod() const { /* Do stuff */ }
void MyClass::SecondMethod() const
{
    // This is where exception occurs, 
    // description of this method during runtime in VS looks like 0x000000
    FirstMethod(); 
}

No constructors or destructors involved.

Answer 1

Heap corruption is a likely candidate. The v-table pointer in the object is vulnerable, it is usually the first field in the object. A buffer overflow for some kind of other object that happens to be adjacent to the object will wipe the v-table pointer. The call to a virtual method, often much later, will blow.

Another classic case is having a bad "this" pointer, usually NULL or a low value. That happens when the object reference on which you call the method is bad. The method will run as usual but blow up as soon as it tries to access a class member. Again, heap corruption or using a pointer that was deleted will cause this. Good luck debugging this; it is never easy.