C++多态本质 虚表

01 Dec 2021

#include<windows.h>
#include<stdio.h>


class Test
{
public:

    int a = 1;
    int b = 2;

    void func1()
    {
        return;
    }
    virtual void func2()
    {
        return;
    }
};

class Base
{
public:
    int a = 1;
    int b = 2;
    virtual void Func1()
    {
        puts("Func1");
    }
    virtual void Func2()
    {
        puts("Func2");
    }
    virtual void Func3()
    {
        puts("Func3");
    }

private:

};

class Sub:Base
{
public:
    virtual void Func5()
    {
        puts("Func4");
    }virtual void Func4()
    {
        puts("Func5");
    }virtual void Func6()
    {
        puts("Func6");
    }
};

class SubSub:Sub
{
public:
    virtual void Func7()
    {
        puts("Func7");
    }virtual void Func8()
    {
        puts("Func8");
    }virtual void Func9()
    {
        puts("Func9");
    }
};

class Override:Base
{
public:
    virtual void Func4()
    {
        puts("Func4");
    }
    virtual void Func5()
    {
        puts("Func5");
    }
    virtual void Func6()
    {
        puts("Func6");
    }
    virtual void Func2()
    {
        puts("Func2 Override");
    }virtual void Func1()
    {
        puts("Func1 Override");
    }
};

class Override2:Override
{
public:
    virtual void Func7()
    {
        puts("Func7");
    }virtual void Func8()
    {
        puts("Func8");
    }virtual void Func9()
    {
        puts("Func9");
    }

    virtual void Func1()
    {
        puts("Func1 Override2");
    }
    virtual void Func4()
    {
        puts("Func4 Override2");
    }
};

void test1()	//通过对象调用时，虚函数和普通成员函数都是直接CALL(E8 ...)
{
    Test t;
    t.func1();
    t.func2();
}

void test2()	//通过类指针调用时，虚函数变为间接CALL(FF ...)
{
    Test t;
    Test* pt = &t;
    pt->func1();
    pt->func2();
}

void test3()
{
    typedef void(*Proc)(void);
    Sub s;
    Sub* ps = &s;
    ps->Func4();

    for (size_t i = 0; i < 6; i++)
    {
        Proc vtFunc = (Proc)(*(int*)(*(int*)(&s) + 0x4 * i));	//对象的地址的前4字节为虚表的地址，虚表里保存虚函数的地址
        printf("0x%x\t", (int)vtFunc);	//单继承无重写，虚表按顺序来，父类到子类虚函数依次排列
        vtFunc();
    }
    
}

void test4()
{
    typedef void(*Proc)(void);
    Override s;
    Override* ps = &s;
    
    ps->Func1();
    
    for (size_t i = 0; i < 4; i++)
    {
        Proc vtFunc = (Proc)(*(int*)(*(int*)(&s) + 0x4 * i));
        printf("0x%x\t", (int)vtFunc);	//单继承有重写，虚表中的函数地址会被重写的函数替换掉
        vtFunc();
    }
}

void test5()
{
    typedef void(*Proc)(void);
    SubSub s;
    SubSub* ps = &s;

    ps->Func7();

    for (size_t i = 0; i < 9; i++)
    {
        Proc vtFunc = (Proc)(*(int*)(*(int*)(&s) + 0x4 * i));
        printf("0x%x\t", (int)vtFunc);	//多层继承无重写，虚表按顺序来，父类到子类再到子子类虚函数依次排列
        vtFunc();
    }

}

void test6()
{
    typedef void(*Proc)(void);
    Override2 s;
    Override2* ps = &s;

    ps->Func4();

    for (size_t i = 0; i < 9; i++)
    {
        Proc vtFunc = (Proc)(*(int*)(*(int*)(&s) + 0x4 * i));
        printf("0x%08x\t", (int)vtFunc);	//多层继承有重写，
        vtFunc();
    }

}

void CheckVt()
{
    Base b1;
    Base b2;
    Sub s;
    Base* pb1 = &b1;
    Base* pb2 = &b2;
    Sub* ps = &s;
    printf("0x%08x\n0x%08x\n", *(int*)pb1, *(int*)pb2);
    
}

int main()
{
    //test6();
    CheckVt();
    return 0;
}