更改VTable条目不会重定向功能吗？

Question

I have 3 classes (Cat, HouseCat:Cat, Lion:Cat). What I'm trying to do is change HouseCat's VTable to make HouseCat eat Meat instead of cat food.

Classes I Use:

class Cat
{
public:
    int age = 2;
    virtual void eat() {
        cout << "Meat" << this->age << endl;
    };

    virtual void sound() {
        cout << "Meow!" << this->age << endl;
    };
};


class HouseCat : public Cat
{
public:
    virtual void eat() {
        cout << "Cat Food" << this->age << endl;
    };
};

class Lion : public Cat
{
public:
    virtual void sound() {
        cout << "ROAR!" << this->age << endl;
    };
};

I'm trying to edit those classes' VTable entries by a VTable struct I created.

static void __memcpy(void * set, void * data, int size){
    DWORD old;
    VirtualProtect(set, size, PAGE_EXECUTE_READWRITE, &old);
    char*dest = (char*)set;
    char*src = (char*)data;
    for (int i = 0; i < size; i++)dest[i] = src[i];
    VirtualProtect(set, size, old, &old);
}

struct VTable{

    static VTable read(void * object){
        VTable  vt = *(VTable*)(object);
        int i = 0;
        while ((DWORD)vt.functions[i] != 0x0)
            i++;
        vt.size = i;
        return vt;
    }
    void ** functions;
    int size;

    void redirectFunction(int i, void * redirect){
        __memcpy(&functions[i], &redirect, 4);
    }
};

I confirmed that VTable[0] = eat(), so i decided to try making a change on the Vtable like this :

int main(int argc, char* argv[])
{

    Lion lion = Lion();
    Cat base = Cat();
    HouseCat home = HouseCat();



    VTable lionVTable = VTable::read(&lion);
    VTable baseVTable = VTable::read(&base);
    VTable homeVTable = VTable::read(&home);
    cout << "-------------- BEFORE EDIT -----------------" << endl
    << "Base:" << endl
    << (baseVTable.functions[0]) << endl
    << (baseVTable.functions[1]) << endl
    << "HomeCat:" << endl
    << (homeVTable.functions[0]) << endl
    << (homeVTable.functions[1]) << endl
    << "Lion:" << endl
    << (lionVTable.functions[0]) << endl
    << (lionVTable.functions[1]) << endl;

    homeVTable.redirectFunction(0, lionVTable.functions[0]);


    cout << "-------------- AFTER EDIT -----------------" << endl
    << "Base:" << endl
    << (baseVTable.functions[0]) << endl
    << (baseVTable.functions[1]) << endl
    << "HomeCat:" << endl
    << (homeVTable.functions[0]) << endl
    << (homeVTable.functions[1]) << endl
    << "Lion:" << endl
    << (lionVTable.functions[0]) << endl
    << (lionVTable.functions[1]) << endl;

    pause();




    cout << "---Base---" << endl << endl;
    base.eat();
    base.sound();
    cout << "---Lion---" << endl << endl;
    lion.eat();
    lion.sound();
    cout << "---Home---" << endl << endl;
    home.eat();
    home.sound();
    cout << "---End---" << endl;



    pause();
    return 0;

}

It outputed;

-------------- BEFORE EDIT ----------------
Base:
0031106E
0031121C
HomeCat:
00311285
0031121C
Lion:
0031106E
003113F2
-------------- AFTER EDIT -----------------
Base:
0031106E
0031121C
HomeCat:
0031106E
0031121C
Lion:
0031106E
003113F2

You can see that HomeCat[0] changed from 0x311285->0x31106E

VMT.exe+11285 - E9 B6350000           - jmp VirtualMethodTable test.HouseCat::eat
[Cat Food]
->
VMT.exe+1106E - E9 ED450000           - jmp VirtualMethodTable test.Cat::eat
[Meat]

The problem is the output of the functions didnt change at all.

---Base---

Meat2

Meow!2

---Lion---

Meat2

ROAR!2

---Home---

Cat Food2

Meow!2

---End---

I'm using Visual Studio 2013. Release/Debug didnt make a difference either.

Did i do something wrong in my code or is it somekind of compiler stuff I'm missing?

Answer 1

I agree that this is a horrible hacky thing to do... however, to get it working I'd try changing your lion / base / home variables to be pointers to objects. Right now since they are not pointers, the compiler may be automatically calling the correct function without using the vtable (since it knows exactly what type the object is).