Class Base

base_code/pure_virtual.cpp:4–14

using namespace std;

class Base
{
protected:
    int x;
public:
    virtual void fun() = 0; // 纯虚函数
    explicit Base(int i) { x = i; }
    virtual ~Base() {
        cout << "Destructor: Base." << endl;
    }
};

class Derived: public Base
{