例题一

#include <iostream>
using namespace std;class Animal
{
public:Animal(string name) :name_(name) {}virtual void bark() = 0;
protected:string getName() { return name_; }
private:string name_;
};class Cat : public Animal {
public:Cat(string name) : Animal(name){}void bark() {cout << "Cat " << getName() << "miao miao miao" << endl;}
};
class Dog : public Animal {
public:Dog(string name) : Animal(name) {}void bark() {cout << "Dog " << getName() << "wang wang wang" << endl;}
};
int main()
{Animal* cat = new Cat("罗小黑");Animal* dog = new Dog("布鲁托");int* p = (int*)cat;int* q = (int*)dog;int tmp = p[0];p[0] = q[0];q[0] = tmp;cat->bark();dog->bark();
}

运行结果: Dog 罗小黑wang wang wang
Cat 布鲁托miao miao miao

原因:将Cat与Dog的虚表指针vfptr交换了，导致调用的函数互换了。

例题二

class Base {
public:virtual void show(int i = 10){cout << "Base::show() " << i << endl;}
};class Drived : public Base
{
public:virtual void show(int i = 20){cout << "Drived::show() " << i << endl;}
};
int main()
{Base* b = new Drived;b->show();
}

运行结果 Drived::show() 10

原因：编译阶段，会将基类的show()函数，如果该函数有默认形参那么将其形参默认值入栈。但是运行时期，是调用的虚函数表的函数地址。

具体汇编代码如下，

b->show();
00007FF66DED2692  mov         rax,qword ptr [b]  
00007FF66DED2696  mov         rax,qword ptr [rax]  
00007FF66DED2699  mov         edx,0Ah   // 基类的show()默认形参10
00007FF66DED269E  mov         rcx,qword ptr [b]  
00007FF66DED26A2  call        qword ptr [rax]

例题三

class Base {
public:virtual void show(){cout << "Base::show() " << endl;}
};class Drived : public Base
{
private:virtual void show(){cout << "Drived::show() " << endl;}
};
int main()
{Base* b = new Drived;b->show();delete b;
}

运行结果：Drived::show()

原因：编译阶段，编译器首先是否拥有函数访问权限。代码中，使用的是基类指针，所以编译器检查的是基类中show()的权限。而具体调用的show()是在运行阶段，因为基类使用虚函数，所以调用show()的时候动态绑定。尽管派生类的show()函数是私有的，但是其地址是在虚函数表中，因此从虚表中调用派生类的show()函数。

例题四

class Base {
public:Base(){cout << "Base Construct" << endl;clear();}void clear(){memset(this, 0, sizeof(*this));}virtual void show(){cout << "Base::show() " << endl;}
};class Drived : public Base
{public:Drived() {cout << "Drived Construct" << endl;}virtual void show(){cout << "Drived::show() " << endl;}
};
int main()
{/*Base* b1 = new Base(); // 运行错误b1->show();*/Base* b2 = new Drived;   // 运行失败b2->show();
}