C / C++ / MFC

31-Jan-08 7:07

If I have one class derived from another, an overloaded virtual function in the base class, and an implementation of one of the overloaded functions in the derived class, the compiler will only let me call one of the overloaded functions from a pointer/reference of an instance of the derived class. I am confused as why it won't acknowledge the other function. Can anyone help me understand why this is?

class CBaseClass
{
public:
CBaseClass(void) {}
virtual ~CBaseClass(void) {}
virtual void DoSomething(int a) {}
virtual void DoSomething(int a, int b) {}
};

class CDerivedClass : public CBaseClass
{
public:
CDerivedClass(void) {}
virtual ~CDerivedClass(void) {}
virtual void DoSomething(int a) {}
};

int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
CDerivedClass testing;
testing.DoSomething(5,5); // Get an error on this line

// error C2660: 'CDerivedClass::DoSomething' : function does not take 2 arguments
}

David Crow31-Jan-08 7:22

31-Jan-08 7:22

You either need to implement a two-argument DoSomething() function, or make testing a CBaseClass object.

"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

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bob1697231-Jan-08 7:34

31-Jan-08 7:34

Thanks for the advice on how to cope with it. You don't by any chance know what is responsible, topic wise, for this behavior?

Is this because of the concept of "Hiding"? I'm not sure what topic I need to google for to better understand it.

Thanks

David Crow31-Jan-08 7:50

31-Jan-08 7:50

bob16972 wrote:
You don't by any chance know what is responsible, topic wise, for this behavior?

CDerivedClass does not have a two-argument DoSomething() method, hence the compiler error.

bob1697231-Jan-08 7:58

31-Jan-08 7:58

I guess I'm missing the point. I apologize if I'm missing something right under my nose.

If your saying a derived class needs to implement all the virtual methods that are in the base, then I guess I'm not understanding why this works...

class CBaseClass
{
public:
CBaseClass(void) {}
virtual ~CBaseClass(void) {}
virtual void DoSomething(int a) {}
virtual void DoSomething(int a, int b) {}
};

class CDerivedClass : public CBaseClass
{
public:
CDerivedClass(void) {}
virtual ~CDerivedClass(void) {}
};

int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
CDerivedClass testing;
testing.DoSomething(5,5); // Get an error on this line
}

David Crow31-Jan-08 8:44

31-Jan-08 8:44

bob16972 wrote:
If your saying a derived class needs to implement all the virtual methods that are in the base,

They don't have to be implemented, but when they are, their types must match.

CPallini31-Jan-08 9:24

CPallini

31-Jan-08 9:24

BTW, the following is allowed

int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
  CBaseClass *pCDerivedClass = new CDerivedClass();
  pCDerivedClass->DoSomething(5,5);
}

Please note: error check and cleanup left to the reader...

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

bob1697231-Jan-08 9:59

31-Jan-08 9:59

Noted.

I had become complacent with being able to call any function implemented in the base class regardless of whether it was virtual using a pointer from any level in the derived hierarchy when there were no overloaded functions in the base class. This kinda throws me curve ball but I'm learning.

Thanks for the info.

Mark Salsbery31-Jan-08 9:31

Mark Salsbery

31-Jan-08 9:31

hmm...what happened to inheritance?

CDerivedClass is a CBaseClass.

Logically, this doesn't seem like it should be an error.
Until you convince me otherwise Smile | :)

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

led mike31-Jan-08 8:36

led mike

31-Jan-08 8:36

bob16972 wrote:
Thanks for the advice on how to cope with it. You don't by any chance know what is responsible, topic wise, for this behavior?

Is this because of the concept of "Hiding"? I'm not sure what topic I need to google for to better understand it.

Polymorphism[^]

Therefore, what the virtual keyword does is to allow a member of a derived class with the same name as one in the base class to be appropriately called from a pointer, and more precisely when the type of the pointer is a pointer to the base class but is pointing to an object of the derived class

led mike

bob1697231-Jan-08 9:15

31-Jan-08 9:15

thanks. I guess the part that is confusing me is why a pointer, at any level, will invoke the base implementations when no function is overloaded (and not implemented in a derived class) but a pointer at the base class is required when functions are overloaded and an implementation for at least one overloaded function is not provided in the derived class.

Brian R19-Mar-08 9:14

Brian R

19-Mar-08 9:14

You can add a using statement to promote the visibility of the base class functions.

class CDerivedClass : public CBaseClass
{
public:
CDerivedClass(void) {}
using CBaseClass::DoSomething;
virtual ~CDerivedClass(void) {}
virtual void DoSomething(int a) {}
};

The eliminates the "hiding" problem.

bob1697212-Apr-08 19:57

12-Apr-08 19:57

That seems to fix the issue. Smile | :)

Thanks for the help. Now I need to go back and undo the workaround code and the half baked solution I put in.

OOP Classes / Windows

egerving31-Jan-08 6:20

egerving

31-Jan-08 6:20

? I understand the basics of OOP and classes. But all the general info always talks about the box, the cat, the dog, the car etc. No one ever talks about user interface, database etc. Can someone shed some light on these subjects? Seems to me that for a compete program one would have to develop various class modules so to speak.
Thanks.

David Crow31-Jan-08 7:12

31-Jan-08 7:12

egerving wrote:
No one ever talks about user interface, database etc. Can someone shed some light on these subjects?

You might want to look at MFC. It has several classes for UI components (e.g, CListBox, CEdit) and databases (e.g., CDatabase). While those are specific examples, it shows that most anything can be thought of an as object.

Maximilien31-Jan-08 8:16

Maximilien

31-Jan-08 8:16

A cat is a box with four legs.
A car is a box with four wheels.

Maximilien Lincourt
Your Head A Splode - Strong Bad

Mark Salsbery31-Jan-08 8:29

Mark Salsbery

31-Jan-08 8:29

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

CPallini31-Jan-08 8:59

CPallini

31-Jan-08 8:59

That is because OOP primary objective was zoo simulation.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]

Maximilien31-Jan-08 9:02

Maximilien

31-Jan-08 9:02

zoop ?

Maximilien Lincourt
Your Head A Splode - Strong Bad

CPallini31-Jan-08 9:05

CPallini

31-Jan-08 9:05

ZOOP for the sake of precision!
Big Grin | :-D

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

[my articles]