2.纯虚析构函数必须有定义

class Base
{
public:
    virtual void f() = 0;
    virtual ~Base() = 0;
};
class Derived: public Base
{
public:
    void f() {
        cout << "hello\n";
    }
    ~Derived() {};
};
int main() {
    Derived d; // 连接器报错，要对Base的析构函数有定义
    return 0;
}

要加上

Base::~Base() { };