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



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

(2) 没有虚函数的基类

Cpp代码



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

(3) 虚继承，基类有vptr

Cpp代码



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;
};
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



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



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



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;
};
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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