when is _declspec( novtable ) unsafe?

Question

Under what circumstances will _declspec( novtable ) cause an access violation?

For example, this code doesn't:

class __declspec(novtable) Base
{
public:
    virtual ~Base() { };
    virtual int Foo() const = 0;
    virtual int Bar() const { return 2; };
};

class A : public Base
{
public:
    int Foo() const { return 1; };
};

int main(int argc, char* argv[])
{
    A a;
    volatile int a1 = a.Foo();
    volatile int a2 = a.Bar();

    Base* c = new A();
    volatile int c1 = c->Foo();
    volatile int c2 = c->Bar();
    delete c;

    return 0;
}

This code also does not:

class __declspec(novtable) Base
{
public:
    virtual ~Base() { };
};

int main(int argc, char* argv[])
{ 
    Base a;
}

But, this code will:

int main(int argc, char* argv[])
{ 
    Base* a = new Base();
    delete a; // access violation
}

Why does the code in the first two example not throw on the destructors?

Answer 1

http://msdn.microsoft.com/en-us/library/k13k85ky%28v=vs.71%29.aspx

If you attempt to instantiate a class marked with novtable and then access a class member, you will receive an access violation (AV).

The code where you get an access violation is the code where you're explicitly calling "delete" on the novtable class.