虚函数在构造函数中失效

class Base
{
public:
    Base() { sayHello(); }
    virtual void sayHello() { cout << "Base says hello\n"; }
};

class Derived : public Base
{
public:
    void sayHello() { cout << "Derived says hello\n"; }
};

int main(int argc, char *argv[])
{
    Derived d;
    return 0;
}

输出结果:
   Base says hello