<C++> I have question about virtual functions.

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C++

#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;
}

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.

Posted 24-Jul-18 10:00am

서형박

Updated 24-Jul-18 11:21am

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2 solutions

Solution 1

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.

Posted 24-Jul-18 10:47am

Gerry Schmitz

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CPallini · Accepted Answer · 2018-07-24T11:21:00

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

C++

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

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]

Quote:
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?

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

C++

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]