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

Cpp代码

1. class A {
2. public:
3. virtual void foo() {}
4. protected:
5. int i;
6. };

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

(2) 没有虚函数的基类

Cpp代码

1. class A {
2. public:
3. void foo() {}
4. protected:
5. int i;
6. };

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

(3) 虚继承，基类有vptr

Cpp代码

1. class Base {
2. public:
3. virtual ~Base() {}
4. virtual void foo() {}
5. virtual void bar() {}
6. protected:
7. int i;
8. };
9. class Derived : public virtual Base {
10. public:
11. ~Derived() {}
12. void bar() {}
13. protected:
14. int j;
15. };

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

Cpp代码

1. class Base {
2. public:
3. ~Base() {}
4. void foo() {}
5. void bar() {}
6. protected:
7. int i;
8. };
9. class Derived : public Base {
10. public:
11. ~Derived() {}
12. void bar() {}
13. protected:
14. int j;
15. };

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

Cpp代码

1. class Base {
2. public:
3. ~Base() {}
4. void foo() {}
5. void bar() {}
6. protected:
7. int i;
8. };
9. class Derived : public Base {
10. public:
11. ~Derived() {}
12. virtual void bar() {}
13. protected:
14. int j;
15. };

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

Cpp代码

1. class Base1 {
2. public:
3. virtual ~Base1() {}
4. virtual void foo() {}
5. virtual Base1 *clone() const {}
6. protected:
7. int i;
8. };
9. class Base2 {
10. public:
11. virtual ~Base2() {}
12. virtual void bar() {}
13. virtual Base2 *clone() const {}
14. protected:
15. int j;
16. };
17. class Derived : public Base1, public Base2 {
18. public:
19. virtual ~Derived() {}
20. virtual Derived *clone() const {}
21. protected:
22. int k;
23. };

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; };

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