深入理解C++的多态

一. 编译器对C++的多态是通过在Class里面插入一个vptr指针,同时生成一个virtual table, 让vptr指向这个virtual table来实现的，那么什么样的Class才有必要设置vptr指针呢？如下图所示:

下面分6种情况讨论

(1)有虚函数的基类

class A {
public:
        virtual void foo() {}
protected:
        int i;
};

(2) 没有虚函数的基类

class A {
public:
        void foo() {}
protected:
        int i;
};

(3) 虚继承，基类有vptr

class Base {
public:
        virtual ~Base() {}
        virtual void foo() {}
        virtual void bar() {}
protected:
        int i;
};
class Derived : public virtual Base {
public:
        ~Derived() {}
        void bar() {}
protected:
        int j;
};

(4)非虚继承，基类没有vptr，派生类没有虚函数

class Base {
public:
        ~Base() {}
        void foo() {}
        void bar() {}
protected:
        int i;
};
class Derived : public Base {
public:
        ~Derived() {}
        void bar() {}
protected:
        int j;
};

(5)非虚继承，基类没有vptr，派生类有虚函数

 

class Base {
public:
        ~Base() {}
        void foo() {}
        void bar() {}
protected:
        int i;
};
class Derived : public Base {
public:
        ~Derived() {}
        virtual void bar() {}
protected:
        int j;
};

(6) 非虚继承，基类有vptr

class Base1 {
public:
        virtual ~Base1() {}
        virtual void foo() {}
        virtual Base1 *clone() const {}
protected:
        int i;
};
class Base2 {
public:
        virtual ~Base2() {}
        virtual void bar() {}
        virtual Base2 *clone() const {}
protected:
        int j;
};
class Derived : public Base1, public Base2 {
public:
        virtual ~Derived() {}
        virtual Derived *clone() const {}
protected:
        int k;
};