Member method of NULL pointer

[namcs]

Hi all,
Recently my friend found out that the following code wouldn't crash:

struct aStruct
{
  print()
  {
    printf("hahaha\n")
  }
};
int main()
{
   aStruct* p= 0;
   p->print();
}

I guess it is because p has 2 memory parts, 1 is for member functions, the other is for data. The assignment (p=0) only affects the part reserved for data. So by right it still works normally if we access the other part (p->print()).

Am I right or is it the compiler is just so smart that it altered the object?

[hamsterman]

A struct does not point to it's functions. That code works the same way as

struct aStruct{};
void print( aStruct* ptr ){
    printf("hahaha\n")
}
int main()
{
   aStruct* p= 0;
   print( p );
}

It would crash, however, if print tried to access members of ptr (or this, in your case).

[namcs]

Hi hamsterman,
Thanks. The program will not crash even if we use class instead of struct.. Does your explaination still apply?

[namcs]

And may I know I can learn about these tips?

[hamsterman]

In C++ structs and classes are identical, except for default access modifier.
What tips are you talking about?

[bluecoder]

Hi hamsterman,
I have tried both the was , what mancs is descussing about and what you are descussing about . But both seems working with out the crash .
I am not able to get that .. can any one of you explain that ?

[hamsterman]

They are supposed to work without a crash.

If you want something to crash, try this:

struct aStruct {
  int x;
  void print() {
    x = 5;
    printf("hahaha\n")
  }
};

int main() {
   aStruct* p= 0;
   p->print();
}

[namcs]

Thanks hamsterman, I meant where I can learn about things like the compiler would refactor struct method into a separate function.

[hamsterman]

I wouldn't call it refactoring. There is just one way to represent a function (sort of. you may want to google "calling conventions"). I only meant that the compiled code is (nearly) the same. The compiler does not transform methods to global functions.

As for learning, I can't say. Just ask here when you see something you don't know. Eventually some things will become apparent. Learning assembly might help some.

[namcs]

Great, thanks hamsterman.

[1yR4jE8b]

Member functions always implicity pass the calling object as the this pointer. The program doesn't crash because you aren't using the this pointer. That's it.

[namcs]

Thanks darkesfriengt.

[Disch]

Note that this might crash or do weird things (if, for example, the function you are trying to call is virtual), so I don't recommend you ever think you can do it. Technically the language prohibits it.