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.