C++总结6——继承与多态的笔试题

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#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;    }    virtual ~Base()      {        cout<<"~Base()"<<endl;    }    virtual void Show(int i= 10)//虚函数    {        cout<<"Base::Show(), i="<<i<<endl;    }private:    int _ma;};class Derive : public Base{public:    Derive(int data1, int data2):Base(data1),_mb(data2)      {        cout<<"Derive()"<<endl;    }    ~Derive()      {        cout<<"~Derive()"<<endl;    };    virtual void Show(int i= 20)//虚函数    {        cout<<"Derive::Show(), i="<<i<<endl;    }private:    int _mb;};int main(){    Base *p = new Derive(10,10);    p->Show();    delete p;    return 0;}

p->Show();函数有默认的参数，在调用过程中没有传递参数。编译期进行形参压栈，已经将基类的Show()函数的10压进栈了，调用派生类中的Show时，不再对其进行初始化。若要对虚函数中的值进行赋值，不要在基类虚函数的参数列表中赋默认值。

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#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;    }    virtual ~Base()    {        cout<<"~Base()"<<endl;    }    virtual void Show()    {        cout<<"Base::Show()"<<endl;    }    void Show(int i)    {        cout<<"Base:Show(int)"<<endl;    }private:    int _ma;};class Derive : public Base{public:    Derive(int data1, int data2):Base(data1),_mb(data2)      {        cout<<"Derive()"<<endl;    }    ~Derive()      {        cout<<"~Derive()"<<endl;    }private:    virtual void Show()    {        cout<<"Derive::Show()"<<endl;    }private:    int _mb;};int main(){    Base *p = new Derive(10,20);    p->Show();    delete p;    return 0;}

在编译时，发现是基类的指针指向派生类的对象，并且该基类中有虚函数，故采用动态绑定。在运行时，Derive调用自己的私有Show()函数。

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#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;        Clear();    }    void Clear()    {        memset(this, 0 ,sizeof(*this));    }    virtual ~Base()    {        cout<<"~Base()"<<endl;    }    virtual void Show()    {        cout<<"Base::Show()"<<endl;    }pr`vate:    int _ma;};int main(){    Base b(10);    Base *p = &b;    p->Show();    delete p;    return 0;}

程序崩溃！
在Base类的构造函数中调用clear()函数，将刚构造的对象b清空了。p调用show()，非法访问内存，程序崩溃！

看下面一段代码段，结果什么呢？

#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;        Clear();    }    void Clear()    {        memset(this, 0 ,sizeof(*this));    }    virtual ~Base()    {        cout<<"~Base()"<<endl;    }    virtual void Show()    {        cout<<"Base::Show()"<<endl;    }private:    int _ma;};class Derive : public Base{public:    Derive(int data1, int data2):Base(data1),_mb(data2)      {        cout<<"Derive()"<<endl;    }    ~Derive()      {        cout<<"~Derive()"<<endl;    }    virtual void Show()    {        cout<<"Derive::Show()"<<endl;    }private:    int _mb;};int main(){    Base *p = new Derive(10,10);    p->Show();    delete p;    return 0;}

对象d中，基类Base的内存都被清空了，但是在Derive的构造函数中，又将虚表的地址写在虚表指针的内存里，故可以访问Show()函数。

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#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;        Show();    }    virtual ~Base()    {        cout<<"~Base()"<<endl;    }    virtual void Show()    {        cout<<"Base::Show()"<<endl;    }private:    int _ma;};class Derive : public Base{public:    Derive(int data1, int data2):Base(data1),_mb(data2)      {        cout<<"Derive()"<<endl;        Show();    }    ~Derive()      {        cout<<"~Derive()"<<endl;    }    virtual void Show()    {        cout<<"Derive::Show()"<<endl;    }private:    int _mb;};int main(){    Derive d(10,10);    return 0;}

在构造函数里调用虚函数，都是静态绑定。Base类构造函数调用Base::Show()；Derive类构造函数调用Derive::Show()

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#include <iostream>using namespace std;class Base{public:    Base(int data):_ma(data)      {        cout<<"Base()"<<endl;    }    virtual ~Base()    {        cout<<"~Base()"<<endl;    }    void Show()    {        cout<<"Base::Show()"<<endl;    }private:    int _ma;};class Derive : public Base{public:    Derive(int data1, int data2):Base(data1),_mb(data2)      {        cout<<"Derive()"<<endl;    }    ~Derive()      {        cout<<"~Derive()"<<endl;    }    virtual void Show()    {        cout<<"Derive::Show()"<<endl;    }private:    int _mb;};int main(){    Base *p = new Derive(10,10);    p->Show();    delete p;    return 0;}

Base类的Show函数不是虚函数，Derive类的Show函数是虚函数。p->Show();采用静态绑定，在编译时已经确定调用的是Base类的Show函数。