Override number of parameters of pure virtual functions - c++

Question

I have implemented the following interface:
template <typename T>
class Variable
{
public:
Variable (T v) : m_value (v) {}
virtual void Callback () = 0;
private:
T m_value;
};
A proper derived class would be defined like this:
class Derived : public Variable<int>
{
public:
Derived (int v) : Variable<int> (v) {}
void Callback () {}
};
However, I would like to derive classes where Callback accepts different parameters (eg: void Callback (int a, int b)).
Is there a way to do it?

Answer 1

This is a problem I ran in a number of times.
This is impossible, and for good reasons, but there are ways to achieve essentially the same thing. Personally, I now use:
struct Base
{
virtual void execute() = 0;
virtual ~Base {}
};
class Derived: public Base
{
public:
Derived(int a, int b): mA(a), mB(b), mR(0) {}
int getResult() const { return mR; }
virtual void execute() { mR = mA + mB; }
private:
int mA, mB, mR;
};
In action:
int main(int argc, char* argv[])
{
std::unique_ptr<Base> derived(new Derived(1,2));
derived->execute();
return 0;
} // main

Answer 2

Even if such a thing were possible, it no longer makes much sense to have it as a virtual function, as the derived instantiations couldn't be called polymorphically via a pointer to the base class.

Answer 3

don't think this will be possible, because you can never interface it back to Variable.
This is what i mean
int a=0; int b = 0;
Variable<int>* derived = new Derived();
derived->Callback(a, b); //this won't compile because Variable<int> does not have Callback with 2 vars.

Answer 4

I know this there is an accepted answer, but there is one (ugly) way to achieve what you want, although I would not recommend it:
template <typename T>
class Variable
{
public:
Variable (T v) : m_value (v) {}
virtual void Callback (const char *values, ...) = 0;
private:
T m_value;
};
class Derived : public Variable<int>
{
public:
Derived (int v) : Variable<int> (v) {}
virtual void Callback (const char *values, ...) {
}
};
Now, you can use:
int a=0;
double b = 0;
Variable<int>* derived = new Derived(3);
derived->Callback("");
derived->Callback("df", a, b);
You need the values argument in order to obtain the remaining arguments inside the method. You also need to know the argument types, and pass them like printf does.
This method is error prone, as you must match the argument types on values with the real argument types.

Answer 5

You will have to add an overload of Callback in the base class that accepts these parameters. It would also be possible to do bad things, like accept a void*, or pass in a raw pointer-to-bytes.
The only scenario in which it is valid to alter virtual function signature is when you override the return value to something polymorphic to the original return value, e.g. *this.