虚函数的意外输出

Question

有人可以解释为什么以下代码的输出是B:2？当没有调用x

时，变量num如何获得A::print(int)的值

#include <iostream>

class A
{
  public:
    void virtual print(int num = 2) {
         std::cout << "A:" << num << std::endl;
    }
};

class B : public A
{
  public:
    void virtual print(int x = 22) {
        std::cout << "B:" << x << std::endl;
    }
};

int main()
{
    A *a = new B;
    a->print();
    delete a;
    return 1;
}