What is the effect of calling a virtual method by a base class pointer bound to a derived object that has been deleted

Question

The fllowing questions are:

1. p->test() should not work after b is destroyed. However, the code is running without any issue, the dynamic binding still works;
2. when the destructor of A is defined, the dynamic binding doesnot work anymore. What is the logic behind it?

#include <iostream>
using namespace std;
struct A {
    //~A() {} 
    virtual void test() {        cout << 0 << endl;    }
};

class B :public A {
    void test() {        cout << 1 << endl;    }
};

int main() {
    A* p;
    {
        B b;
        p = &b;
    }
    p->test(); // the method called will be different if the destructor of A is removed
}

Answer 1

1. p->test() should not work after b is destroyed. However, the code is running without any issue, the dynamic binding still works;

It does not "work". p->test() invokes undefined behavior.

2. when the destructor of A is defined, the dynamic binding doesnot work anymore. What is the logic behind it?

There is no logic behind it, other than implementation details of the compiler you are using, because the C++ standard does not mandate what a compiler should do with code that has undefined behavior.

For more details on undefined behavior I refer you to https://en.cppreference.com/w/cpp/language/ub

Compilers cannot detect all undefined behavior, but some of it. With gcc you can try -fsanitize=address to see the issue more clearly: https://godbolt.org/z/qxTs4sxcW.