C / C++ / MFC

8-Feb-08 2:24

Hello everyone,

Why in the below code,

pb->foo();

will output Final other than output Base? I have tested that the output is Final.

My question is
1. pb points Final instance, and the foo in Final is not virtual method;
2. if function is virtual, we should invoke the foo based on the type of instance pointed to, if not virtual, we should invoke the foo based on the type which is the pionter type;
3. since foo in Final is not final, so the output should be invoking the foo based on the type of pointer (which is Base), then the output should be Base?

class Base {
public:
    virtual int foo() {cout << "Base" << endl; return 0;}
};
 
class Derived: public Base
{
public:
    int foo() {cout << "Derived" << endl;return 0;}
};
 
class Final: public Derived{
public:
  int foo() {cout << "Final" << endl;return 0;}
};
 
int main()
{
    Final f;
    Base* pb = reinterpret_cast<base*>(&f);
    pb->foo();
 
    return 0;
}
</base*>

regards,
George

David Crow8-Feb-08 2:44

David Crow

8-Feb-08 2:44

George_George wrote:
...and the foo in Final is not virtual method;

Why would you assert that? The virtual keyword is needed only in the base class's declaration of the method; any subsequent declarations in derived classes are virtual by default.

When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class's version of the function.

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

George_George8-Feb-08 21:35

8-Feb-08 21:35

Thanks DavidCrow,

My question is answered.

regards,
George

CPallini8-Feb-08 2:48

CPallini

8-Feb-08 2:48

actually pb->foo() outputs, without (a lot of) surprises

Final

at least on my system.
Smile | :)

George_George8-Feb-08 21:36

8-Feb-08 21:36

Thanks for your trial, CPallini!

Any comments to my original question?

regards,
George

BadKarma8-Feb-08 2:55

BadKarma

8-Feb-08 2:55

Hi,

Indeed, you didn't specify the virtual declarator to the foo function in final, but that isn't necessary since you declared it virtual in the base class. A function becomes virtual from the moment you declare it virtual to all its children.

Try the following

class Base {
public:
  // remove virtual here
    int foo() {cout << "Base" << endl; return 0;}
};
 
class Derived: public Base
{
public:
// place function virtual in derived
    virtual int foo() {cout << "Derived" << endl;return 0;}
};
 
class Final: public Derived{
public:
  int foo() {cout << "Final" << endl;return 0;}
};

int main()
{
  Final objF;

  objF.foo();

  Derived* pDer = dynamic_cast<Derived*>(&objF);
  if(pDer) pDer->foo();

  Base* pBase = dynamic_cast<Base*>(&objF);
  if(pBase) pBase->>foo();
}

The output of this will be:
Final
Final
Base

codito ergo sum

George_George8-Feb-08 21:39

8-Feb-08 21:39

Thanks BadKarma,

Why the statement output Base? I think pBase is pointed an object instance whose type is Final, and the foo in Derived class in virtual, it should output Final, right?

Base* pBase = dynamic_cast<base*>(&objF);
if(pBase) pBase->foo();

regards,
George

Rajkumar R8-Feb-08 3:40

8-Feb-08 3:40

George_George wrote:
2. if function is virtual, we should invoke the foo based on the type of instance pointed to, if not virtual, we should invoke the foo based on the type which is the pionter type;

Agree, and no confusion since pb is of Base type, for base foo is virtual hence the function called is through _vfptr, and _vfptr is same for both Base and Final, "Final" is the output,

u can check that by interchanging "virtual" keyword from Base to Final, in that case"since foo in Final is not final" is valid

George_George8-Feb-08 21:47

8-Feb-08 21:47

Thanks Rajkumar,

Two more comments.

1.

Rajkumar R wrote:
_vfptr is same for both Base and Final

What do you mean same? Suppose we create an instance of Base the content of __vfptr should be pointed to foo in Base, since foo is virtual; and if we create an instance of Final, the content of __vfptr is foo in Final, since foo is virtual declared in class Base. They are different. Why do you say they are the same.

2.

Rajkumar R wrote:
since foo in Final is not final"

I do not understand the statement "since foo in Final is not final". Could you describe in more details please?

regards,
George

Rajkumar R8-Feb-08 23:01

8-Feb-08 23:01

1. with _vfptr in that context i mean only one for Base * and Final * type for the instance of Final.

George_George wrote:
"since foo in Final is not final".

this is the quote from your first post, I mean if the foo of final is virtual and foo of base is not virtual calling with Base type will call foo of base.

George_George9-Feb-08 20:57

9-Feb-08 20:57

Thanks Rajkumar,

I doubt this. Suppose the foo in Base is not virtual and foo in Final is virtual. When we have an instance of Final, then converts to Base*, then use the Base* to invoke foo, output will be Base other than Final?

Rajkumar R wrote:
I mean if the foo of final is virtual and foo of base is not virtual calling with Base type will call foo of base.

regards,
George

Rajkumar R10-Feb-08 22:29

10-Feb-08 22:29

George_George wrote:
I doubt this.

see this [^]

George_George10-Feb-08 22:33

10-Feb-08 22:33

Great Rajkumar,

You mentioned before,

--------------------
1. with _vfptr in that context i mean only one for Base * and Final * type for the instance of Final.
--------------------

You mean if foo in Base and Final are both virtual, and if we create an instance of Final and make a Final pointer points to the instance, and also make a Base pointer points to the same instance, then pointer of type Base and pointer of type Final points to the same vtable, right?

regards,
George

Rajkumar R10-Feb-08 23:04

10-Feb-08 23:04

yes

George_George11-Feb-08 22:03

Embedding HTML code into VC

11-Feb-08 22:03

Thanks Rajkumar,

My question is answered.

regards,
George

Gita.Bairavi8-Feb-08 1:37

Gita.Bairavi

8-Feb-08 1:37

Hi,
I m trying to load a bitmap image(jpeg/gif) in OnPaint()..
But there s a loss of color identified(already posted qn..)
got an idea to embedd a HTML snippet into my code with ..
How to do it in a proper way??

thanks in advance..
Roll eyes | :rolleyes:

Gita

Re: Embedding HTML code into VC

Rajkumar R8-Feb-08 3:56