Question

According to C++ Standard, it’s perfectly acceptable to do this:

class P
{
    void Method() {}
};

...

P* p = NULL;
p->Method();

However, a slight change to this:

class P
{
    virtual void Method() {}
};

...

P* p = NULL;
p->Method();

produces an access violation when compiled with Visual Studio 2005.

As far as I understand, this is caused by some quirk in Microsoft’s compiler implementation and not by my sheer incompetence for a change, so the questions are:

1) Does this behavior persist in more recent versions of VS?

2) Are there any, I don’t know, compiler settings that prevent this access violation?

Answer 1

According to C++ Standard, it’s perfectly acceptable to do this

No it is not!
Dereferencing a NULL pointer is Undefined Behavior as per the C++ Standard.^[#1]

However, If you do not access any members inside a non virtual member function it will most likely work on every implementation because for a non virtual member function the this only needs to be derefernced for accessing members of this since there are no members being accessed inside the function hence the result.
However, just because the observable behavior is okay does not mean the program is well-formed. correct.
It still is ill-formed.
It is an invalid program nevertheless.

The second version crashes because while accessing a virtual member function, the this pointer needs to be dereferenced just even for calling the appropriate member function even if there are no members accessed within that member function.

A good read:
What’s the difference between how virtual and non-virtual member functions are called?

---

^[#1]Reference:

C++03 Standard: §1.9/4

Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer). [Note: this International Standard imposes no requirements on the behavior of programs that contain undefined behavior. ]