13. 多重继承

1.构造顺序：按继承顺序，先继承的先构造，同样遵循先构造父类，再构造子类的规则。

2.析构顺序，与构造顺序相反。

3.内存分布：按继承顺序，先继承的分布在前，同样遵循先构造父类，再构造子类的规则。

class Sofa {
public:
	Sofa() { cout << "Sofa()" << endl; }
	void Sit() { cout << "Sofa Sit" << endl; }
	int length = 0xaaaaaaaa;
	int color = 1;
};

class Bed {
public:
	Bed() { cout << "Bed()" << endl; }
	void Sleep() { cout << "Bed Sleep" << endl; }
	int width = 0xbbbbbbbb;
	int color = 2;
};

class SofaBed : public Sofa, public Bed  {
public:
	SofaBed() { cout << "SofaBed()" << endl; }
	int height = 0xdddddddd;
};

int  main() {
	SofaBed sb;
	sb.Sit();
	sb.Sleep();

	Sofa *pSofa = &sb;
	pSofa->length = 1;

	Bed *pBed = &sb;//安全，指针会定位到Bed数据开始的地址
	pBed->width = 2;

	//sb.color = 3;// error C2385: 对“color”的访问不明确
	sb.Sofa::color = 3;//指明是哪个作用域
	
	return 0;
}

　　

菱形继承（钻石继承）

问题：两个父类分别从祖宗类继承了一份数据，子类从两个父类继承，则子类有两份祖宗类的数据。

虚继承：解决菱形继承的问题，继承时添加virtual关键字，使相同的祖宗类成员在内存中只有一份。

虚继承时构造顺序：先构造祖宗类，再按继承顺序构造父类，最后构造孙类。

虚继承时析构顺序：与构造顺序相反。

注意：避免使用菱形继承，结构复杂且效率低下。

class Furniture {
public:
	int color = 1;
};

class Sofa : virtual public Furniture {//虚继承，继承时添加virtual关键字
public:
	Sofa() { cout << "Sofa()" << endl; }
	void Sit() { cout << "Sofa Sit" << endl; }
	int length = 0xaaaaaaaa;
};

class Bed : virtual public Furniture {//虚继承，继承时添加virtual关键字
public:
	Bed() { cout << "Bed()" << endl; }
	void Sleep() { cout << "Bed Sleep" << endl; }
	int width = 0xbbbbbbbb;
};

class SofaBed : public Sofa, public Bed  {
public:
	SofaBed() { cout << "SofaBed()" << endl; }
	int height = 0xdddddddd;
};

int  main() {
	SofaBed sb;
	sb.Sit();
	sb.Sleep();

	Sofa *pSofa = &sb;
	pSofa->length = 1;

	Bed *pBed = &sb;//安全，指针会定位到Bed数据开始的地址
	pBed->width = 2;

	sb.color = 3;//由于虚继承，SofaBed中只有一份color了
	
	return 0;
}