Why doesn't the program crash when I call a member function through a null pointer in C++?

Question

#include "iostream"
using namespace std;
class A
{
public:
    void mprint()
    {
        cout<<"\n TESTING NULL POINTER";
    }
};

int main()
{
    A *a = NULL;
    a->mprint();
    return 0;
}

I am getting output as "TESTING NULL POINTER". Can anyone please explain why this program is printing the output instead of crashing. I checked it on Dev C++ and aCC compiler both gave same result.

Answer 1

You're not using any member variables of A - the function is completely independent of the A instance, and therefore the generated code happens to not contain anything that dereferences 0. This is still undefined behavior - it just may happen to work on some compilers. Undefined behavior means "anything can happen" - including that the program happens to work as the programmer expected.

If you e.g. make mprint virtual you may get a crash - or you may not get one if the compiler sees that it doesn't really need a vtable.

If you add a member variable to A and print this, you will get a crash.

Answer 2

According to the C++ spec, This program has undefined behavior because you're invoking a member function on a null receiver.

The reason that this works, though, is that non virtual member functions are typically implemented as regular functions that take the "this" pointer as an implicit first argument. Consequently, if you call a member function on a null pointer, as long as you don't use the this pointer, your program will not crash. Of course, you cannot rely n this; a valid C++ compiler could cause this to crash.

However, virtual functions are a different story because the function that actually gets called needs to be resolved at runtime. This usually involves introspecting on the receiver's virtual function table. Thus if you try calling a virtual member function on a null pointer, even if te function doesn't access this, it will still cause a crash. Try this out if you're curious!