Accessing class members on a NULL pointer

Question

I was experimenting with C++ and found the below code as very strange.

class Foo{
public:
    virtual void say_virtual_hi(){
        std::cout << \"Vi         


        

Answer 1

It is undefined behaviour. But most of compilers made instructions which will handle this situation correctly if you don't accessing to member variables and virtual table.

let see disassembly in visual studio for understand what happens

   Foo* foo = 0;
004114BE  mov         dword ptr [foo],0 
    foo->say_hi(); // works well
004114C5  mov         ecx,dword ptr [foo] 
004114C8  call        Foo::say_hi (411091h) 
    foo->say_virtual_hi(); // will crash the app
004114CD  mov         eax,dword ptr [foo] 
004114D0  mov         edx,dword ptr [eax] 
004114D2  mov         esi,esp 
004114D4  mov         ecx,dword ptr [foo] 
004114D7  mov         eax,dword ptr [edx] 
004114D9  call        eax  

as you can see Foo:say_hi called as usual function but with this in ecx register. For simplify you can assume that this passed as implicit parameter which we never use in your example.
But in second case we calculating adress of function due virtual table - due foo addres and gets core.