详解c++菱形继承和多态---下

菱形继承

#include<iostream>using namespace std;
class Animal
{
public:int m_Age;
};
class Sheep : public Animal {};
class Tuo : public Animal {};
class SheepTuo : public Sheep, public Tuo {};
void test()
{SheepTuo st;st.Sheep::m_Age = 18;st.Tuo::m_Age = 20;cout << "st.Sheep::m_Age: " << st.Sheep::m_Age << endl;cout << "st.Tuo::m_Age: " << st.Tuo::m_Age << endl;}
int main()
{test();return 0;
}

菱形继承数据存在二义性问题。

cl /d1 reportSingleClassLayoutSheepTuo test.cpp

虚继承 

class Sheep : virtual public Animal {};
class Tuo : virtual public Animal {};
class SheepTuo : public Sheep, public Tuo {};

多态

动态多态

满足条件

1.有继承关系

2.子类重写父类的虚函数

使用

父类的指针或引用执行子类对象。

重写

函数返回值类型，函数名，参数列表完全一致。

class Animal
{
public:virtual void speak(){cout << "Animal speak()" << endl;}
};
class Cat : public Animal
{
public:void speak(){cout << "Cat speak()" << endl;}
};
void doSpeak(Animal& animal)
{animal.speak();
}
void test()
{Cat cat;doSpeak(cat);
}

Animal类的大小为4，不加virtual大小为1（空类的大小为1）。

子类重写父类的虚函数，子类的虚函数表内部会替换为子类的虚函数地址。

class AbstractCalculator
{
public:virtual int getResult(){return 0;}int m_Num1;int m_Num2;
};
class AddCalculator :public AbstractCalculator
{
public:int getResult(){return m_Num1 + m_Num2;}
};
class SubCalculator :public AbstractCalculator
{
public:int getResult(){return m_Num1 - m_Num2;}
};
void test()
{AbstractCalculator* ac = new AddCalculator;ac->m_Num1 = 1;ac->m_Num2 = 2;cout << ac->getResult() << endl;
}

纯虚函数和抽象类

class AbstractDrinking
{
public:virtual void Boil() = 0;virtual void Brew() = 0;virtual void PourInCup() = 0;virtual void PutSomething() = 0;void makeDrink(){Boil();Brew();PourInCup();PutSomething();}
};
class Coffee :public AbstractDrinking
{virtual void Boil() { cout << "Boil Coffee" << endl; };virtual void Brew() { cout << "Brew Coffee" << endl; };virtual void PourInCup() { cout << "PourInCup Coffee" << endl; };virtual void PutSomething() { cout << "PutSomething Coffee" << endl; };
};
void doWork(AbstractDrinking* ad)
{ad->makeDrink();
}

虚析构和纯虚析构 

父类指针在析构的时候，不会调用子类中的析构函数，导致如果子类有堆区属性会出现内存泄漏。

class Animal
{
public:Animal(){cout << "Animal()" << endl;}virtual ~Animal(){cout << "~Animal()" << endl;}virtual void speak() = 0;
};
class Cat : public Animal
{
public:Cat(string name){cout << "Cat()" << endl;m_Name = new string(name);}void speak(){cout <<*m_Name<< "Cat speak()" << endl;}~Cat(){if (m_Name != nullptr){cout << "~Cat()" << endl;delete m_Name;m_Name = nullptr;}}string* m_Name;
};
void test()
{Animal* animal = new Cat("Tom");animal->speak();delete animal;
}

通过虚析构解决。

纯虚析构

virtual ~Animal() = 0;

Animal::~Animal()
{cout << "virtual ~Animal()" << endl;
}

需要有声明及实现。

总结

虚基表中存偏移量，用于计算X中虚基类XX对象的位置。

虚基表和虚表通常存在常量区。