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：