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Jul 20th, 2009

Problem in c++

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I have a function under Base class and at the definition tine of this base class function I need to call another function which is define under the Derived class. how to do this??
My class declaration is below

C++ Syntax (Toggle Plain Text)
class Base{
 
public:
 
void showdata(double);
 
};
 
class Derived : public Base{
 
public:
	inline double F(double x)
{
	return exp(-x) + x*x;
}
class Base{

public:

void showdata(double);

};

class Derived : public Base{

public:
	inline double F(double x)
{
	return exp(-x) + x*x;
}

Define the function below :

C++ Syntax (Toggle Plain Text)
void Base::showdata(double a)
{
	std::cout<<"The value of the Function : " << F(a);
}
void Base::showdata(double a)
{
	std::cout<<"The value of the Function : " << F(a);
}

and the main function :

C++ Syntax (Toggle Plain Text)
int main()
{
//Base obj;
Derived obj;
obj.F(1.0);
return 0;
}
int main()
{
//Base obj;
Derived obj;
obj.F(1.0);
return 0;
}

I'm getting an error :

Quote ...

error C3861: 'F': identifier not found

So how to solve this problem.
With Warm Regards
sdmahapatra

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sdmahapatra

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Jul 20th, 2009

Re: Problem in c++

You have to end your class declarations with a semicolon.

EDIT:: Oops! This post probably doesn't make much sense

Last edited by tux4life; Jul 20th, 2009 at 8:07 am.

tux4life

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Jul 20th, 2009

Re: Problem in c++

Do you specifically need to use the base class instead of the derived class? I was taught that you derive classes to add functionality to a base class. After which you use the derived class instead of the base class whenever you know that you will need the extra methods in the derived class. You can call the methods in the base class from the derived class using: Base::method().

necrolin

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Jul 20th, 2009

Re: Problem in c++

I know this is a correct error but I need to call a derived class function into the base class function.Is it possible any how?? or I have to go different way like template etc..

sdmahapatra

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Jul 20th, 2009

Re: Problem in c++

>I know this is a correct error but I need to call a derived class function into the base class function.Is it possible any how??
Can't you create an object of the derived class inside the base class, and then call one of it's methods?

tux4life

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Jul 20th, 2009

Re: Problem in c++

I don't understand your objective behing calling derive funciton in base at all, I understand C++ is multi paradigm language, but I can't see any logical reasoning behind this, could you please elloborate more.. so we can provide you better alternative than this, or correct any mistakes whatsoever

Laiq Ahmed

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Jul 20th, 2009

Re: Problem in c++

Click to Expand / Collapse Quote originally posted by Laiq Ahmed ...

I don't understand your objective behing calling derive funciton in base at all, I understand C++ is multi paradigm language, but I can't see any logical reasoning behind this, could you please elloborate more.. so we can provide you better alternative than this, or correct any mistakes whatsoever

Some people just get assignments which aren't logical.

tux4life

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Jul 20th, 2009

Re: Problem in c++

sdmahapatra,
Definition of showdata must be:

C++ Syntax (Toggle Plain Text)
void Base::showdata(double a)
{
	std::cout<<"The value of the Function : " << a;
}
void Base::showdata(double a)
{
	std::cout<<"The value of the Function : " << a;
}

and definition of main will be,

C++ Syntax (Toggle Plain Text)
int main()
{
//Base obj;
Derived obj;
obj.showdata(obj.F(1.0));
return 0;
}
int main()
{
//Base obj;
Derived obj;
obj.showdata(obj.F(1.0));
return 0;
}

adatapost

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Posting Genius

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Jul 20th, 2009

Re: Problem in c++

You really need to go down to the basics of inheritance - why we use it.
When you construct a base class, it is not really a base class but it is 'just a class'. It is when you use that class to derive a child class, the former becomes a base class.
The base class knows none about its derived class (because at the time of writing base class, the programmer didn't knew(or suppose to know) that there would be some derived class D which would be inherited from this class.
Such anomalous demands indicate a major flaw in design.
I agree with Laiq in that you should re-factor your design.
Think before you code or else you will be in deep _ _ _ _.

Last edited by siddhant3s; Jul 20th, 2009 at 10:08 am.

siddhant3s

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Jul 20th, 2009

Re: Problem in c++

Quote ...

I have a function under Base class and at the definition tine of this base class function I need to call another function which is define under the Derived class. how to do this??

Make the method virtual and protected in the base class, then override it as public in the derived class:

C++ Syntax (Toggle Plain Text)
class Base
{
public:
    void showdata(double a)
    {
        cout<<"The value of the Function : " << F(a);
    }
protected:
    virtual double F(double) { return 0; }
};
 
class Derived: public Base
{
public:
    double F(double x)
    {
        return exp(-x) + x*x;
    }
};
class Base
{
public:
    void showdata(double a)
    {
        cout<<"The value of the Function : " << F(a);
    }
protected:
    virtual double F(double) { return 0; }
};

class Derived: public Base
{
public:
    double F(double x)
    {
        return exp(-x) + x*x;
    }
};

That solves the problem of a base class needing to call a derived class method, but your example does not show why you need that behavior. What are you trying to do?

Tom Gunn

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