How to assign a base class function pointer in derived class constructor?

[jesoj]

Hi All,

Can anybody help me to assign a base class function pointer in derived class constructor? I have got a compilation error while doing so. Please see the code below:

class Base
{
public:
	void (* fnptr) (void) ;
	void myvurtualfn()
	{
	}  	
};
class Derived:public Base
{
public:
	
	void myvurtualfn1()
	{
	}
	Derived()
	{	
                fnptr =this->myvurtualfn1(); //error C2440: '=' : cannot convert from 'void' to 'void (__cdecl *)(void)'
	}

};

Thanks in Adv,
Je

[Narue]

>fnptr =this->myvurtualfn1(); //error C2440: '=' : cannot convert from 'void' to 'void (__cdecl *)(void)'

1. You're not taking the address of the member function, you're calling it.
2. Pointers to member functions are not compatible with pointers to functions.

This might help with the syntax issues.

[jesoj]

Hi Naure,
Thanks v much for your help.

I have modified the code as given below. Now the assignment of base class member function pointer is OK. Now I have an another problem in calling the function. it gives compilation error as "error C2065: 'fnptr' : undeclared identifier" Is it possible to call like that? Or is it a syntax error?

class Base
{
public:
    void ( Base :: *fnptr)(void) ;
    void myvurtualfn(void){}  
    Base()
    {
        fnptr = &Base::myvurtualfn;
        (this->*fnptr)();
    }
};

class Derived:public Base
{
public:

    void myvurtualfn(){}
    Derived()
    {
            fnptr = &Base :: myvurtualfn;
    }
};
void myoldfn(Base &b)
{   
    (b.*fnptr)();//error C2065: 'fnptr' : undeclared identifier
}

Thanks in advance,
Jeso

[Narue]

The .* syntax only says what object you're applying the pointer to; you still need to do everything necessary to access the pointer itself:

(b.*b.fnptr)();

In your code, the compiler is looking for fnptr in the unadorned scopes, but fnptr is actually a member of Base, so you need to use the full name of b.fnptr .

Doing the double membership is awkward and can be confusing, so perhaps an intermediate local variable is a better solution:

void (Base::*p)() = b.fnptr;
(b.*p)();

[jesoj]

Hi Naure,
My problem has gone.
Thanks very much for your help.

Best regards,
Jeso