Why does a destructor in base class need to be declared virtual?

1. 父类的destructor是非虚函数

#include <iostream>
using namespace std;

class A
{
public:
~A()
{
cout << "I am in A" << endl;
}
};

class B:public A
{
public:
~B()
{
cout << "I am in B" << endl;
}
};

int main()
{
A* a = new B();
delete a;
}

运行结果，

>> ./a.out
I am in A

2. 父类的destructor是虚函数

>> cat dest.C
#include <iostream>
using namespace std;

class A
{
public:
virtual ~A()
{
cout << "I am in A" << endl;
}
};

class B:public A
{
public:
~B()
{
cout << "I am in B" << endl;
}
};

int main()
{
A* a = new B();
delete a;
}

运行结果：

>> ./a.out
I am in B
I am in A

we should use virtual destructors if we call delete on a base class pointer which points to a derived class.