C++: object deleted (created with new) but member function still working (?)

Question

Just for curiosity and experimenting I wrote following code and now am trying to understand whats happening after delete... why is the cat object still meowing ??

the compiler version I use:

g++ (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609

and compile the code:

g++ cat.cpp -pedantic -Wall -o cat

With other compilers may crash when calling meou() after delete.

I would like to know

• why is not crashing
• which precautions should I take

the code:

#include <iostream>
using namespace std;


class Cat
{
    public:
        Cat()  { cout << "Cat construct" << endl; }
        ~Cat() { cout << "Cat destruct" << endl; }
        void meow(); 
};

void Cat::meow(void)
{
    cout << "meow..." << endl;
}    

int main()
{
    Cat * pCat = new Cat;    
    pCat->meow();    
    cout << "pCat = " << pCat << endl;    
    delete pCat;    
    pCat = NULL;    
    cout << "pCat = " << pCat << endl;    
    pCat->meow();    
    cout << "why still meowing?!" << endl;    
    return 0;
}

the output:

Cat construct
meow...
pCat = 0x2147030
Cat destruct
pCat = 0
meow...
why still meowing?!

Answer 1

why is not crashing

Because dereferencing nullptr or accessing a deleted object is undefined behaviour. C++ doesn't have required crashes, but crashes can be the result of undefined behaviour.

which precautions should I take

That's a rather broad topic. The most important thing in C++ is not to use dynamic allocation if you don't need to. Write:

Cat cat;
cat.meow();

If you cannot do that, use std::unique_ptr :

auto cat_ptr = std::make_unique<Cat>();
cat_ptr->meow();

If you need a collection, don't use new[] . Use std::vector :

std::vector<Cat> cats;
std::vector<std::unique_ptr<Cat>> cat_ptrs;