类型兼容性的解决心方法

#include <iostream>
using namespace std;
class parent{
public:
	parent(int a)
	{
		this->va = a;
	}
	virtual void print(void)
	{
		cout << "基类的a" <<this->va<< endl;
	}
private:
	int va;
};
class child : public parent{
public:
	child(int a) : parent(100)
	{
		this->a = a;
	}
	virtual void print(void)
	{
		cout << "派生类的a" <<this->a<< endl;
	}
private:
	int a;
};
int main()
{
	child oop(10);
	parent oop1(20);
	parent* p = NULL;
	p = &oop;
	p = &oop1;
	parent &pt = oop;
	p->print();
	pt.print();
	system("pause");
	return 0;
}

* 父类指针可以直接指向子类对象

* 父类引用可以直接指向子类对象

上述结论不管是父类指针还是引用指向子类对像，调用的print都是父类的print，这种解决办法是在print的实现前面加上virtual关键字

实例：

#include <iostream>
using namespace std;
class parent{
public:
	parent(int a)
	{
		this->va = a;
	}
	virtual void print(void)
	{
		cout << "基类的a" <<this->va<< endl;
	}
private:
	int va;
};
class child : public parent{
public:
	child(int a) : parent(100)
	{
		this->a = a;
	}
	virtual void print(void)
	{
		cout << "派生类的a" <<this->a<< endl;
	}
private:
	int a;
};
void test1(parent* p)
{
	p->print();
}

void test2(parent & pt)
{
	pt.print();
}
int main()
{
	child oop(10);
	parent oop1(20);
	parent* p = NULL;
	p = &oop;
	p = &oop1;
	parent &pt = oop;
	p->print();
	pt.print();
	test1(&oop);
	test1(&oop1);
	test2(oop);
	test2(oop1);
	system("pause");
	return 0;
}

这样就可执行到子类的print了，这里的virtual也可以只加载父类的virtual前，这也是一种多态的调用案例