WHY does an uninitialised pointer work in C++ programs?

Question

Guys plz check the code below:

class Human
{ 
public:
    void chat(Human h)
    {
        cout << "human";
    }

    void chat(ComputerScientist c)
    {
        cout << "computer";
    }
};

class ComputerScientist : public Human
{
};

//Main function below
int main()
{
    Human* p, p1;
    //Uninitialized pointer above;

    p->chat(p1); //It shows perfectly the result without ANY error!
}

However, things go tricky if I make a function in derived class ComputerScientist which overrides the Human's one.

class Human
{
public:
    virtual void chat(Human* h)
    {
        cout << "about the weather";
    }

    virtual void chat(ComputerScientist* c)
    {
        cout << "about their own computer illiteracy";
    }
};

class ComputerScientist : public Human
{
public:
    virtual void chat(Human* h)
    {
        cout << " about computer games";
    }

    virtual void chat(ComputerScientist* c)
    {
        cout << " about others’ computer     illiteracy";
    }
};

And I use the same main function, it appears to be a segmentation fault in the null pointer line. But why?

Things have make change in two places in the second example:

1. I used the overriding function by making it virtual.
2. The function is taking a pointer as argument.

Answer 1

The code you've written exhibits undefined behavior. Code that is experiencing undefined behavior may:

• segmentation fault
• output weird characters to the screen.
• vary over different optimization levels and compilers
• work exactly as you wanted it to.

Answer 2

In first case, your class is a POD (plain old data) and your chat function does not dereference/access any member variable- thus it appears to work okay (this is a bad practice though- due to undefined behavior). In case of virtual functions each object has vtable, which needs a valid pointer to work- thus leads to fault.

Here is how you can make your first case fault as well:

class Human 
{ 
    private:
        int n;
    public:

    void chat(Human h) 
    {
        cout << "human #" << n << endl;
    }

    void chat(ComputerScientist c) 
    {
        cout << "computer";
    }
};

class ComputerScientist:public Human{};

int main()
{
    Human* p,p1;
    //Uninitialized pointer above;
    p->chat(p1);//It shows perfectly the result without ANY error!

    return 0;
}