c++编写程序【问题描述】声明-个基类BaseClass,从它派生出类DerivedClass,BaseClass有成员2数fnl()、fn2(),DerivedClass也有成员函数fnl()、fn2(),在主函数中声明向DerivedClass的对象，分别用DerivedClass的对象以及BaseClass和DerivedClass的指针来调用fnl()、fn2(),观察运行结果。 【输入形式】无输入。 【输出形式】分别输出调用指定函数之后的指定信息。 【样例输入】无输入 【样例输出】 the fn1 founction of the DerivedClass the fn2 founction of the DerivedClass the fn1 founction of the baseclass the fn2 founction of the baseclass the fn1 founction of the DerivedClass the fn2 founction of the DerivedClass 【程序说明】各个函数的实现中，只需要有相应的函数调用输出信息就可以了。 主函数中函数调用的依次为：DerivedClass对象调用函数fnl()、fn2()；BaseClass类型指针指向DerivedClass对象调用函数fnl()、fn2()；DerivedClass类型指针指向DerivedClass对象调用fnl()、fn2()；

以下是C++代码实现：

#include <iostream>
using namespace std;

class BaseClass {
public:
    void fn1() {
        cout << "the fn1 founction of the baseclass" << endl;
    }
    void fn2() {
        cout << "the fn2 founction of the baseclass" << endl;
    }
};

class DerivedClass : public BaseClass {
public:
    void fn1() {
        cout << "the fn1 founction of the DerivedClass" << endl;
    }
    void fn2() {
        cout << "the fn2 founction of the DerivedClass" << endl;
    }
};

int main() {
    DerivedClass d;
    d.fn1();
    d.fn2();

    BaseClass* pb = &d;
    pb->fn1();
    pb->fn2();

    DerivedClass* pd = &d;
    pd->fn1();
    pd->fn2();

    return 0;
}

输出结果为：

the fn1 founction of the DerivedClass
the fn2 founction of the DerivedClass
the fn1 founction of the baseclass
the fn2 founction of the baseclass
the fn1 founction of the DerivedClass
the fn2 founction of the DerivedClass

可以看出，当直接调用DerivedClass对象的成员函数时，会优先调用DerivedClass中的同名函数；当使用基类的指针指向DerivedClass对象，并调用基类中的同名函数时，会调用基类中的函数；当使用DerivedClass类型的指针指向DerivedClass对象时，会优先调用DerivedClass中的同名函数。