Question

Here’s what I have done:
I’ve got a simple class:

class Person{
  public:
    Person();
}

And in my main:

int main() {
  Person myPer = NULL;
}

This is impossible since C++ does not allow that, however:

int main() {
  Person* perPtr = NULL;
  Person myPer = *perPtr; // corrected, it was &perPtr(typo error) before answers
}

This compiles fine and as I see I did able to have a NULL object. So isn’t it violating the rule that only pointers can be null in C++? Or is there such a rule in C++?
2nd one is after I wrote this code, I added a if statement checking whether myPer is NULL or not but that gave me error. So does it show that C++ does not really like the NULL object idea no matter what you do to make objects NULL…

Answer 1

Objects cannot be null, only pointers can. Your code is incorrect and does not compile, since its trying to initialize a Person from a pointer to a pointer to Person. If you were to change your code to

Person* perPtr = NULL;
Person myPer = *perPtr;

then it would be trying to initialize a Person out of a dereferenced null pointer to a Person, which is undefined behavior (and most likely a crash).

If you need to use the idioms where an object could be in a NULL state, you could use Boost.Optional:

boost::optional< Person > myPer = boost::none;
if( myPer )
{
    myPer->do_something();
}

It’s a generalization of what is usually done with pointers, except it does not use dynamic allocation.