C++虚函数与多继承

虚函数

C++用虚函数实现运行时多态，虚函数的实现是由两个部分组成的，虚函数指针与虚函数表。

虚函数指针（vptr）是指向虚函数表的指针，在一个被实例化的对象中，它总是被存放在该对象的地址首位。而虚函数表（vtbl）用于保存类包含的虚函数指针列表：

class Base {
public:
    virtual void f() {
        std::cout << "Base::f()" << std::endl;
    }

private:
    int n;
};

int main() {
    Base* base = new Base;

    return 0;
}

单继承下虚函数的内存布局

在单继承中，子类覆盖父类的虚函数，其实只需要在子类对应的虚函数表中覆盖对应的虚函数指针即可：

class Base {
public:
    virtual void f() {
        std::cout << "Base::f()" << std::endl;
    }

private:
    int n;
};

class Derived : public Base {
public:
    void f() override {
        std::cout << "Derived::f()" << std::endl;
    }

    virtual void k() {
        std::cout << "Derived::k()" << std::endl;
    }

private:
    int h;
};

int main() {
    Derived* derived= new Derived;

    return 0;
}

多继承下虚函数的内存布局

class Base1 {
public:
    virtual void f() {
        std::cout << "Base1::f()" << std::endl;
    }

private:
    int n;
};

class Base2 {
public:
    virtual void g() {
        std::cout << "Base2::g()" << std::endl;
    }

private:
    int m;
};

class Derived : public Base1, public Base2 {
public:
    void f() override {
        std::cout << "Derived::f()" << std::endl;
    }

    void g() override  {
        std::cout << "Derived::g()" << std::endl;
    }

    virtual void k() {
        std::cout << "Derived::k()" << std::endl;
    }

private:
    int h;
};

int main() {
    Derived* derived= new Derived;

    return 0;
}