C++单冒号表示继承以及虚函数virtual

#include <iostream>
using namespace std;

class Base
{
public:
    virtual void fun(int x = 0)
    {
        cout << "Base::fun(), x = " << x << endl;
    }
};

class Derived : public Base
{
public:
    // 这里的virtual关键字可以省略，因为只要基类里面被声明为虚函数，那么在子类中默认都是虚的
    // 或者定义为 virtual void fun ( int x = 10)
    virtual void fun(int x)
    {
        cout << "Derived::fun(), x = " << x << endl;
    }
};


int main()
{
    Derived d1;
    Base *bp = &d1;
    bp->fun();
    return 0;
}