Confused with delete keyword operation in C++

Question

I would like to know how delete works? In main function I have deleted the cfact object. But still the cfact->Hello() works instead of throwing an error. While debugging I found while delete happens, cfact releases the memory. as soon as factory* c2fact = newfun.Newfun("c2_fact"); line executes cfact gets some memory location.

class factory{

public:
    virtual void Hello() = 0;
};
class c_fact: public factory
{
public:
    void Hello(){
    cout << "class c_fact: public factory"<<endl;
    }
};
class c2_fact: public factory
{
public:
    void Hello(){
    cout << "class c2_fact: public factory"<<endl;
    }
};

class callFun{
public:
    virtual factory* Newfun(string data)
    {
        if(data == "c_fact")
            {return new c_fact;}
        else
            {return new c2_fact;}
    }
};
class newFun:public callFun{
public:
    factory* Newfun(string data)
    {
        if(data == "c_fact")
            {return new c_fact;}
        else if (data == "c2_fact")
            {return new c2_fact;}
    }
};
int main()
{
    newFun newfun;
    factory* cfact = newfun.Newfun("c_fact");
    delete cfact;                              //Deleted the instance
    factory* c2fact = newfun.Newfun("c2_fact");
    cfact->Hello();//Still it prints the output
    c2fact->Hello();
    system("pause");
    return 0;
}

Answer 1

delete doesn't actually invalidate what it points to. It just tells the OS that the memory can be used for something else and that the program doesn't need it anymore.

If it not overwritten by other data your data will still be in memory and will still be accessible. This is a cause of many bugs that go undetected during development phase and later show up.

The fact that is is working now doesn't mean it will always work. For example if you move the code to another machine or if you restart your computer the code might segfault.

It is always a good practice to set pointers to NULL after delete. Or even better use smart pointers.