c为C ++盐我对虚函数有疑问。 [英] <C++> I have question about virtual functions.

问题描述

#include <iostream>
#include <cstring>
#include <cstdlib>

using namespace std;

class Fir
{
public:
	virtual void mfunc() { cout << "First func" << endl; }
};

class Sec : public Fir
{
public:
	virtual void mfunc() { cout << "Second func" << endl; }
};

class Thi : public Sec
{
public:
	virtual void mfunc() { cout << "Third func" << endl; }
};

int main()
{
	Thi * tptr = new Thi();
	Sec * sptr = tptr;
	Fir * fptr = sptr;

	fptr->mfunc();
	sptr->mfunc();
	tptr->mfunc();
	delete tptr;
	return 0;
}

我的尝试：



当我执行上面的代码时，Third func会显示三次。我想知道为什么fptr-> mfunc（）的结果是第三个函数而不是第二个函数，即使它指向一个Sec对象。

What I have tried:

When I execute the above code, "Third func" is displayed three times. I am wondering why the result of " fptr->mfunc()" is " Third func" instead of "Second func", even though it points to a Sec object.

推荐答案

虚拟方法按原样使用;或者你覆盖他们;你没有做过;你只是在每个班级中创建了一个替换虚拟方法。



在这种情况下尝试向下投射毫无意义。

Virtual methods are used "as is"; or you "override" them; you have done neither; you have simply created a "replacement" virtual method in each class.

Trying to "down cast" in this situation makes no sense.

这就是虚函数的目的：允许多态。

考虑一下，例如下面的代码

That's the very purpose of the virtual functions: to allow polymorphism.
Consider, for instance the following code

void draw_shapes( Shape * sh[], size_t count)
{
  for ( size_t n = 0; n < count; ++n)
    sh[n]->draw_myself();
}

数组中的每个项目（每个 Shape ）将根据其真实性质绘制自己：例如， Circle 和 Rectangle 将生成完全不同的图纸。



[更新]

every item of the array (every Shape) will draw itself depending on its real nature: a Circle and a Rectangle, for instance, will produce quite different drawings.

[Update]

引用：

有仍然有点令人困惑的部分。当前类的继承关系是Fir< -Sec< -Thi。如果调用虚函数，是否最后覆盖了thi类的mFunc（）函数，以便三次输出相同的结果？

There is still a little confusing part. The inheritance relationship of the current classes is "Fir <- Sec <- Thi". If the virtual function is called, is the thi class's mFunc () function last overridden so that the same result is output all three times?

每个对象即使指针声明指定了基类，它也会执行自己的 mfunc 代码。尝试：

Every object performs its own mfunc code, even if the pointer declaration specifies the base class. Try:

int main()
{
  Fir * p[] = { new Fir(), new Sec(), new Thi()};

  for ( size_t n = 0; n < 3; ++n)
    p[n]->mfunc();

  for ( size_t n = 0; n < 3; ++n)
    delete p[n];
  
  return 0;
}

[/ Update]

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