【More Effective C++】条款35：将非尾端类设计为抽象类

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考虑以下继承场景：

• 通过指针的赋值会出现部分赋值的情况：只修改了Animal的数据成员，Lizard数据没有被修改

class Animal {
public:Animal(int data):data(data) {}Animal& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}int getData() const { return data; }
public:int data;
};class Lizard : public Animal {
public: Lizard(int data): Animal(data), l_data(data) {}// Lizard& operator=(const Lizard& rhs) {//     if (&rhs == this) return *this;//     this->data = rhs.data;//     return *this;// }
public:int l_data;
};class Chicken : public Animal {
public: Chicken(int data): Animal(data), c_data(data) {}// Chicken& operator=(const Chicken& rhs) {//     if (&rhs == this) return *this;//     this->data = rhs.data;//     return *this;// }
public:int c_data;
};int main() {Animal a1(1);Animal a2(2);std::cout << a1.getData() << std::endl;a1 = a2;std::cout << a1.getData() << std::endl;return 0;
}

可以将拷贝赋值函数改为虚函数：

• 但是又引入了异性赋值的情况，让人难以理解

class Animal {
public:Animal(int data):data(data) {}virtual Animal& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}int getData() const { return data; }
public:int data;
};class Lizard : public Animal {
public: Lizard(int data): Animal(data), l_data(data) {}virtual Lizard& operator=(const Animal& rhs) {if (&liz_rhs == this) return *this;this->data = rhs.data;return *this;}
public:int l_data;
};class Chicken : public Animal {
public: Chicken(int data): Animal(data), c_data(data) {}virtual Chicken& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}
public:int c_data;
};int main() {Lizard liz(1);Chicken chick(2);Animal *pAnimal11 = &liz;Animal *pAnimal22 = &chick;*pAnimal11 = *pAnimal22;return 0;
}

解决办法是在拷贝赋值函数中进行类型转换：

• 无法转型的情况抛出异常

class Animal {
public:Animal(int data):data(data) {}virtual Animal& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}int getData() const { return data; }
public:int data;
};class Lizard : public Animal {
public: Lizard(int data): Animal(data), l_data(data) {}virtual Lizard& operator=(const Animal& rhs) {const Lizard& liz_rhs = dynamic_cast<const Lizard&>(rhs);if (&liz_rhs == this) return *this;this->data = rhs.data;return *this;}
public:int l_data;
};class Chicken : public Animal {
public: Chicken(int data): Animal(data), c_data(data) {}virtual Chicken& operator=(const Animal& rhs) {const Chicken& liz_rhs = dynamic_cast<const Chicken&>(rhs);if (&rhs == this) return *this;this->data = rhs.data;return *this;}
public:int c_data;
};

为了解决简单对象的赋值情况需要赋值转型成本，可以将拷贝函数进行重载

class Animal {
public:Animal(int data):data(data) {}virtual Animal& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}int getData() const { return data; }
public:int data;
};class Lizard : public Animal {
public: Lizard(int data): Animal(data), l_data(data) {}Lizard& operator=(const Animal& rhs) {if (&liz_rhs == this) return *this;this->data = rhs.data;return *this;}virtual Lizard& operator=(const Animal& rhs) {return operator=(dynamic_cast<const Lizard&>(rhs));}
public:int l_data;
};class Chicken : public Animal {
public: Chicken(int data): Animal(data), c_data(data) {}Chicken& operator=(const Animal& rhs) {if (&rhs == this) return *this;this->data = rhs.data;return *this;}virtual Chicken& operator=(const Animal& rhs) {return operator=(dynamic_cast<const Chicken&>(rhs));}
public:int c_data;
};

未完待续。。。

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