多继承出现的菱形继承问题

问题：存在多继承特性的语言，在进行类的多继承时就会出现菱形继承的问题。

1、菱形继承案例：

例如下述CPP代码：

c 复制代码

class GrandParent_A
{
public:
	std::string a;
	GrandParent_A()
	{
		a = "GrandParent_A";
		std::cout << "GrandParent_A Constructed" << std::endl;
	}
	//static void Print()
	//{
	//	std::cout << "is GrandParent_A Function" << std::endl;
	//}
	void Print()
	{
		std::cout << "is GrandParent_A Function" << std::endl;
	}
	virtual void PrintVirtual()
	{
		std::cout << "is GrandParent_A virtual Function" << std::endl;
	}

	virtual void PrintPureVirtual() = 0;
};

class Parent_B :public GrandParent_A
{
public:
	std::string b;
	Parent_B()
	{
		b = "Parent_B";
		std::cout << "Parent_B Constructed" << std::endl;
	}
	virtual void PrintVirtual()
	{
		std::cout << "is Parent_B virtual Function" << std::endl;
	}
	void PrintPureVirtual() override {
		std::cout << "is Parent_B Purevirtual Function" << std::endl;

	}
};

class Parent_C :public GrandParent_A
{
public:
	std::string c;
	Parent_C()
	{
		c = "Parent_C";
		std::cout << "Parent_C Constructed" << std::endl;
	}
	virtual void PrintVirtual()
	{
		std::cout << "is Parent_C virtual Function" << std::endl;
	}
	void PrintPureVirtual() override {
		std::cout << "is Parent_C Purevirtual Function" << std::endl;
	}
};

class GrandSon_D :public Parent_B, public Parent_C
{
public:
	std::string d;
	GrandSon_D() {
		d = "GrandSon_D";
		std::cout << "GrandSon_D Constructed" << std::endl;
	}
	//virtual	void PrintVirtual()
	//{
	//	std::cout << "is GrandSon_D override Function" << std::endl;
	//}

};

类Parent_B，类Parent_C 分别继承类GrandParent_A，然后GrandSon_D同时继承了Parent_B，Parent_C。这就形成了菱形继承，这时候发现GrandParent_A会进行两次构造将内容复制两份。

如下图，查看输出发现GrandParent_A进行了两次构造，

GrandParent_A进行了两次构造

再查看内存地址发现存在两个GrandParent_A字符串内容。

所以这是后想输出GrandParent_A的成员会出现 "不明确"警告。

c 复制代码

	GrandSon_D d;
	//std::cout << d.a << std::endl; //"GrandSon Da"不明确
	std::cout << d.Parent_B::a << std::endl;
	std::cout << d.Parent_C::a << std::endl;
	//d.Print(); //"GrandSonD::Print"不明确
	d.Parent_C::Print();
	d.Parent_B::Print();
	//d.PrintVirtual(); //"GrandSonD::PrintVirtual不明确
	d.Parent_C::PrintVirtual();
	d.Parent_B::PrintVirtual();
	//d.PrintPureVirtual(); //"GrandSonD:PrintPureVirtual"不明确
	d.Parent_C::PrintPureVirtual();
	d.Parent_B::PrintPureVirtual();

2、使用虚继承解决菱形继承问题：

对中间基类进行虚继承（继承时加上Virutal关键字），如下：

c 复制代码

#pragma region 虚继承
class Parent_VirtualB : virtual public GrandParent_A
{
public:
	std::string b;
	Parent_VirtualB()
	{
		b = "Parent_VirtualB";
		std::cout << "Parent_VirtualB Constructed" << std::endl;
	}
	//virtual void PrintVirtual()
	//{
	//	std::cout << "is Parent_VirtualB virtual Function" << std::endl;
	//}
	void PrintPureVirtual() override {
		std::cout << "is Parent_VirtualB Purevirtual Function" << std::endl;

	}
};

class Parent_VirutalC :virtual public GrandParent_A
{
public:
	std::string c;
	Parent_VirutalC()
	{
		c = "Parent_VirutalC";
		std::cout << "Parent_VirutalC Constructed" << std::endl;
	}
	//virtual void PrintVirtual()
	//{
	//	std::cout << "is Parent_VirutalC virtual Function" << std::endl;
	//}
	void PrintPureVirtual() override {
		std::cout << "is Parent_VirutalC Purevirtual Function" << std::endl;
	}
};

class GranSon_VirtualD :public Parent_VirtualB, public Parent_VirutalC
{

public:
	std::string d;
	GranSon_VirtualD() {
		d = "GranSon_VirtualD";
		std::cout << "GranSon_VirtualD Constructed" << std::endl;
	}
	void PrintPureVirtual() override {
		std::cout << "is GranSon_VirtualD Purevirtual Function" << std::endl;
	}
};
#pragma endregion

这时候就只会出现一次构造，内存里也只会有一份GrandParent_A内容，也避免了1里不明确的问题出现。

3、为了避免多继承问题出现，要么多使用virtual，要么使用接口多实现来代替。

c 复制代码

#pragma region 接口实现
class IGrandP_A
{
public:
	virtual void Print() = 0;
	IGrandP_A()
	{
		std::cout << "IGrandP_A Constructed" << std::endl;
	}
};

class IParent_B :public IGrandP_A
{
public:
	//virtual void Print() = 0;
	virtual void Print()
	{
		std::cout << "is IParent_B function" << std::endl;
	}
	IParent_B()
	{
		std::cout << "IParent_B Constructed" << std::endl;

	}
};

class IParent_C :public IGrandP_A
{
public:
	//virtual void Print() = 0;
	virtual void Print()
	{
		std::cout << "is IParent_C function" << std::endl;
	}
	IParent_C()
	{
		std::cout << "IParent_C Constructed" << std::endl;

	}
};

class GrandSon_ID :public IParent_B, public IParent_C
{
public:
	void Print()
	{
		std::cout << "is GrandSon_ID function" << std::endl;
	}
	GrandSon_ID()
	{
		std::cout << "GrandSon_ID Constructed" << std::endl;

	}
};

#pragma endregion

因为接口没有实现，子类实现是就属于子类了。而且没有字段。

其他没有多继承的语言如C#可能就是考虑到这个问题来，使用接口来避免。

参考链接：

arduino 复制代码

https://zhuanlan.zhihu.com/p/365223471

arduino 复制代码

https://zhuanlan.zhihu.com/p/342271992

arduino 复制代码

https://zhuanlan.zhihu.com/p/651947545