C++ why you need to have a good copy constructor

shallow copy vs deep copy

Do you see any problem?

If you just compile and run, you will not notice a big problem here.

However, if you compile with address sanitizer with an option of -fsanitize=address then you will see a big problem of double free !!!

This because basically, a default constructor will just copy the value of a member

// something similar to this
Foo(const Foo& other) : p(other.p) {}

So, addresses of foo.p and bar.p are same and deleting same address twice will end up with a double free.

To avoid this, you need to create a good copy constructor like

Foo(const Foo &other) { 
    std::cout << "copy constructor\n";
    if (other.p != nullptr)
        p = new int(*other.p); 
}

This will allocate a separate memory location for p and performs a deep copy.