cpp——类——VBTBL

VBTBL

VBTBL，virtual base table，即虚基类表，如果子类虚继承父类，就会生成VBTBL，VBTBL包含类的新增virtual成员函数入口地址，包含VBTBL的类，包含字段vbptr指针，指向类的VBTBL，因此VBTBL被所有类实例对象所共享

子类会继承父类的VBTBL，也会继承父类的vbptr指针，并初始化为指向子类VBTBL

注：某些IDE中，比如xcode，如果虚基类无自定义字段(vptr除外)，则vptr和vbptr共享同一指针字段，VTBL和VBTBL也自然共享同一TBL，vs中如果虚基类无自定义字段(vptr除外)，vptr和vbptr不会共享同一指针字段，VTBL和VBTBL也自然不会共享同一TBL

单继承

子类不更新

class CAnimal{public:    virtual void feed()    {        cout << "CAnimal::feed()" << endl;    }        virtual void work()    {        cout << "CAnimal::work()" << endl;    }    private:    int mGroup;};class CDog : virtual public CAnimal{};typedef void (*PVM)();void vbtbl(){    CAnimal animal;    cout << "sizeof(animal) = " << sizeof(animal) << endl;    cout << "animal vptr addr = " << (long*)&animal << endl;    cout << "animal vtbl addr = " << (long*)*(long*)&animal << endl;    cout << "animal feed addr = " << (long*)*((long*)*(long*)&animal) << endl;    cout << "animal work addr = " << (long*)*((long*)*(long*)&animal + 1) << endl;    PVM animal_vm1 = (PVM)(long*)*(long*)*(long*)&animal;    PVM animal_vm2 = (PVM)(long*)*((long*)*(long*)&animal + 1);    animal_vm1();    animal_vm2();        CDog dog;    cout << "sizeof(dog) = " << sizeof(dog) << endl;    cout << "dog vptr addr = " << (long*)&dog << endl;    cout << "dog vtbl addr = " << (long*)*(long*)&dog << endl;    cout << "dog feed addr = " << (long*)*((long*)*(long*)&dog) << endl;    cout << "dog work addr = " << (long*)*((long*)*(long*)&dog + 1) << endl;    //PVM dog_vm1 = (PVM)(long*)*(long*)*(long*)&dog;    //PVM dog_vm2 = (PVM)(long*)*((long*)*(long*)&dog + 1);    //dog_vm1();    //dog_vm2();        cout << "dog vbptr addr = " << (long*)&dog + 1 << endl;    cout << "dog vbtbl addr = " << (long*)*((long*)&dog + 1) << endl;    cout << "dog feed addr = " << (long*)*((long*)*((long*)&dog + 1)) << endl;    cout << "dog work addr = " << (long*)*((long*)*((long*)&dog + 1) + 1) << endl;    PVM dog_vb_vm1 = (PVM)(long*)*(long*)*((long*)&dog + 1);    PVM dog_vb_vm2 = (PVM)(long*)*((long*)*((long*)&dog + 1) + 1);    dog_vb_vm1();    dog_vb_vm2();}

output：

sizeof(animal) = 16animal vptr addr = 0x7fff5fbff740animal vtbl addr = 0x1000020e0animal feed addr = 0x100001180animal work addr = 0x1000011d0CAnimal::feed()CAnimal::work()sizeof(dog) = 24dog vptr addr = 0x7fff5fbff718dog vtbl addr = 0x100002118dog feed addr = 0x0dog work addr = 0x0dog vbptr addr = 0x7fff5fbff720dog vbtbl addr = 0x100002138dog feed addr = 0x100001180dog work addr = 0x1000011d0CAnimal::feed()CAnimal::work()

总结：

• 如果包含vptr字段，vbptr字段位于vptr字段之后，否则vbptr字段为第一个字段
• VBTBL内容为父类VTBL

子类更新

class CAnimal{public:    virtual void feed()    {        cout << "CAnimal::feed()" << endl;    }        virtual void work()    {        cout << "CAnimal::work()" << endl;    }    private:    int mGroup;};class CDog : virtual public CAnimal{public:    virtual void watch()    {        cout << "CDog::watch()" << endl;    }        virtual void work()    {        cout << "CDog::work()" << endl;    }};typedef void (*PVM)();void vbtbl(){    CAnimal animal;    cout << "sizeof(animal) = " << sizeof(animal) << endl;    cout << "animal vptr addr = " << (long*)&animal << endl;    cout << "animal vtbl addr = " << (long*)*(long*)&animal << endl;    cout << "animal feed addr = " << (long*)*((long*)*(long*)&animal) << endl;    cout << "animal work addr = " << (long*)*((long*)*(long*)&animal + 1) << endl;    PVM animal_vm1 = (PVM)(long*)*(long*)*(long*)&animal;    PVM animal_vm2 = (PVM)(long*)*((long*)*(long*)&animal + 1);    animal_vm1();    animal_vm2();        CDog dog;    cout << "sizeof(dog) = " << sizeof(dog) << endl;    cout << "dog vptr addr = " << (long*)&dog << endl;    cout << "dog vtbl addr = " << (long*)*(long*)&dog << endl;    cout << "dog feed addr = " << (long*)*((long*)*(long*)&dog) << endl;    cout << "dog work addr = " << (long*)*((long*)*(long*)&dog + 1) << endl;    PVM dog_vm1 = (PVM)(long*)*(long*)*(long*)&dog;    PVM dog_vm2 = (PVM)(long*)*((long*)*(long*)&dog + 1);    dog_vm1();    dog_vm2();        cout << "dog vbptr addr = " << (long*)&dog + 1 << endl;    cout << "dog vbtbl addr = " << (long*)*((long*)&dog + 1) << endl;    cout << "dog feed addr = " << (long*)*((long*)*((long*)&dog + 1)) << endl;    cout << "dog work addr = " << (long*)*((long*)*((long*)&dog + 1) + 1) << endl;    PVM dog_vb_vm1 = (PVM)(long*)*(long*)*((long*)&dog + 1);    PVM dog_vb_vm2 = (PVM)(long*)*((long*)*((long*)&dog + 1) + 1);    dog_vb_vm1();    dog_vb_vm2();}

output：

sizeof(animal) = 16animal vptr addr = 0x7fff5fbff740animal vtbl addr = 0x1000020e0animal feed addr = 0x1000010a0animal work addr = 0x1000010f0CAnimal::feed()CAnimal::work()sizeof(dog) = 24dog vptr addr = 0x7fff5fbff718dog vtbl addr = 0x100002118dog feed addr = 0x100001140dog work addr = 0x100001190CDog::watch()CDog::work()dog vbptr addr = 0x7fff5fbff720dog vbtbl addr = 0x100002148dog feed addr = 0x1000010a0dog work addr = 0x1000011e0CAnimal::feed()CDog::work()

总结：

子类virtual成员函数(包括新增virtual成员函数，override virtual成员函数)构成VTBL，同时更新VBTBL

多继承

子类不更新

子类更新