在继承中如果出现：

这种情况，B和C都继承了A，D继承了B、C

在D中访问A的成员会出现：

 

这样的警告

是因为在继承时A出现两条分支：ABD、ACD

编译器不知道访问的A中的元素是经过B继承还是C继承 

所以B、C在继承A时要用到虚继承virtual

即：

class B : virtual public A

class C : virtual public A

#include <iostream>

using namespace std;

class A
{
private:
	int m_a;
public:
	A(int a);
	void print();
};

A::A(int a)
{
	m_a = a;
}

void A::print()
{
	cout << "m_a = " << m_a << endl;
}

class B : virtual public A
{
private:
	int m_b;
public:
	B(int a, int b);
	void print();
};

B::B(int a, int b) : A(a)
{
	m_b = b;
}

void B::print()
{
	cout << "m_b = " << m_b << endl;
}

class C : virtual public A
{
private:
	int m_c;
public:
	C(int a,int c);
	void print();
};

C::C(int a, int c) : A(a)
{
	m_c = c;
}

void C::print()
{
	cout << "m_c = " << m_c << endl;
}

class D : public B, public C
{
private:
	int m_d;
public:
	D(int a, int b, int c, int d);
	void print();
};

D::D(int a, int b, int c, int d) : A(a), B(a, b), C(a, c)
{
	m_d = d;
}

void D::print()
{
	A::print();
	B::print();
	C::print();
	cout << "m_d = " << m_d << endl;
}

int main()
{
	D d(1, 2, 3, 4);
	d.print();
	return 0;
}

运行结果：