C++ 实现不能被继承的类

#include <iostream>

using namespace std;

class A
{
public:
    static A* getInstance();
    static void deleteInstance(A* pA);

private:
    A() { cout << "construct A\n";}
    ~A() { cout << "destruct A\n"; }
};

A* A::getInstance()
{
    return new A;
}

void A::deleteInstance(A* pA)
{
    delete pA;
    pA = nullptr;
}

int  main()
{
    A* pA = A::getInstance();
    A::deleteInstance(pA);
    cin.get();
}

#include <iostream>

using namespace std;

template <typename T>
class A
{
    friend T;
private:
    A(int data) : m_data(data) { cout << "construct A. data: " << m_data << endl; }
    ~A() {}

    int m_data;
};

class B : virtual public A<B>
{
public:
    B(int data) : A(data) { cout << "construct B\n"; }
    ~B() {}
};


int main(void)
{
    B b(4);
    cin.get();
}