c++-多态和vptr指针

多态的原理

#define _crt_secure_no_warnings
#include <iostream>


using namespace std;

class parent
{
public:
    parent(int a) {
        this->a = a;
    }

    virtual void func(int a)
    {
        cout << "parent::func(int)..." << endl;
    }

    virtual void func(int a, int b, int c)
    {
        cout << "parent::func(int ,int ,int )...." << endl;
    }
private:
    int a;
};

class child :public parent
{
public:
    child(int a, int b) :parent(a)
    {
        this->b = b;
    }
    virtual void func(int a) 
    {
        cout << "child: func(int)..." << endl;
    }

    void func(int a, int b) {
        cout << "child :func(int ,int )..." << endl;
    }

    virtual void func(int a, int b, int c)
    {
        cout << "child ::func(int ,int ,int )..." << endl;
    }
private:
    int b;
};

void myfunc(parent *pp)
{
    pp->func(10);
}

int main(void)
{
    //parent *pp = new parent(10);
    //parent *cp = new child(100, 200);


    parent *pp = new child(100, 200);

    pp->func(10);//parent ？ child
                    //如果调用一个普通函数，编译器根本就不会查找虚函数表。
                    //只有你调用的函数，是虚函数的时候，才会去查找虚函数表
    // myfunc(pp);
    pp->func(10, 20, 30);

    return 0;
}

验证vptr指针的存在

#define _crt_secure_no_warnings
#include <iostream>


using namespace std;

class parent
{
public:
    virtual void func()
    {
        cout << "parent::func().." << endl;
    }
    virtual void func(int a)
    {
        cout << "parent::func().." << endl;
    }
private:
    int a;
};


class parent2
{
public:
    void func()
    {
        cout << "parent2::func().." << endl;
    }
private:
    int a;
};
int main(void)
{
    parent p1;
    parent2 p2;


    cout << "sizeof(p1) " << sizeof(p1) << endl;//多出来的4个字节就是vptr指针所占用的空间。
    cout << "sizeof(p2) " << sizeof(p2) << endl;

    
    return 0;
}

vptr指针分布初始化

#define _crt_secure_no_warnings
#include <iostream>


using namespace std;

class parent
{
public:
    parent(int a)
    {
        cout << "parent(int ..)" << endl;
        this->a = a;

        //print();//是调用父类的print（） 还是 子类的print（）？
                //通过结果 此时调用的父类的print 并不是子类的print
    }

    virtual void print()
    {
        cout << "parent::print()...a = "<<a << endl;
    }
private:
    int a;
};

class child :public parent
{
public:
    child(int a, int b) :parent(a) //在调用父类的构造器的时候，会将vptr指针当做父类来处理。
                                    //此时会临时指向父类的虚函数表


    {
        //将子类对象的空间有编程子类对象处理，vptr指针就从指向父类的表 变成 指向子类的表

        cout << "child (int ,int )" << endl;
        this->b = b;
        print();//此时vptr指针已经回到了 子类的表， 调用的是子类的print函数。

    }

    virtual void print() {
        cout << "child ::print()..b = " << b << endl;
    }
private:
    int b;
};

int main(void)
{
    parent *pp = new child(10, 20);
    // pp->print();//发生多态

    delete pp;
    
    return 0;
}

父类指针和子类指针的步长

#define _crt_secure_no_warnings
#include <iostream>

using namespace std;


class parent
{
public:
    parent(int a)
    {
        this->a = a;
    }
    virtual void print()
    {
        cout << "parent::print a=  " << a << endl;
    }

    int a;
};

class child :public parent
{
public:
    child(int a) :parent(a)
    {

    }

    virtual void print()
    {
        cout << "child::print a=  " << a << endl;
    }

    int b;
};


int main(void)
{

    child array[] = { child(0), child(1), child(2) };
    
    parent *pp = &array[0];
    child *cp = &array[0];

    pp++;

    pp->print();
    cp->print();

#if 0
    pp++;//pp +sizeof(parent)
    cp++;//cp +sizeof(child)

    pp->print();
    cp->print();
#endif
    cout << "-----" << endl;

    int i = 0;
    for (cp= &array[0], i = 0; i < 3; i++, cp++) {
        cp->print();
    }

    return 0;
}