Dereferncing NULL pointer to member function of class

Question

I am just curious that how is this program able to print the statement of func() of class.

#include <iostream>
using namespace std;
class foo
{
    public:
        void func ()
        {
            cout << "In func" << endl;
        }
    private:
        int a;
};

int main () {
    foo *f1 = NULL;
    f1->func();
}

How is that I am able to get the print of method func()?

You need to either create an instance of Foo, or make the function static. e.g. http://cpp.sh/4dyt — Eamonn McEvoy, Oct 13 '15 at 09:44

score 0 · Answer 1 · answered Oct 13 '15 at 09:37

0

Dereferencing a null pointer yields Undefined Behaviour.

I can guess why this still works for you (but again it's UB!), but if you try to access a member variable in func() I'm pretty sure it will stop working.

answered Oct 13 '15 at 09:37

David Haim

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skyking · Answer 2 · 2015-10-13T09:40:21.290

Dereferencing a NULL pointer means that you have undefined behavior.

However in this case on most implementations you will end up in correct function with this being NULL, the normal implementation of calling a non-virtual method is to set the hidden parameter this as being the pointer to the object (in this case NULL) and then just call the function. If you don't access member variables or call virtual methods you should be fine in most implementations.

Since you can't access members or call virtual functions or in any other way do anything useful with this pointer you're pretty close to the scenario of a static method and I'll suggest one use that instead if the this pointer is not used.

For the corner case where you want to be able to do nothing else with the this pointer than check for NULLness you could still use a static method and explicitely pass the pointer to the object:

  static void func(foo* diz) {
       cout << "In func" << endl;

       if( diz != NULL ) {
           diz->actual_work_on_diz();
       }
  }

local variables probably should be "member variables" – David Haim Oct 13 '15 at 09:38 — David Haim, Oct 13 '15 at 09:38
@DavidHaim Yes of course, I'll change that. – skyking Oct 13 '15 at 09:38 — skyking, Oct 13 '15 at 09:38

Dereferncing NULL pointer to member function of class

2 Answers2