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

Possibly you're calling the function (directly or indirectly) from a constructor of a base class which itself doesn't have that function.

Possibly there's a broken cast somewhere (such as a reinterpret_cast of a pointer when there's multiple inheritance involved) and you're looking at the vtable for the wrong class.

Possibly (but unlikely) you have somehow trashed the vtable.

Is the pointer to the function null just for this object, or for all other objects of the same type? If the former, then the vtable pointer is broken, and you're looking in the wrong place. If the latter, then the vtable itself is broken.