Conversion from child class to parent for templated function pointer - c++

Question

Here is a problem involving inheritance, template classes and function pointers. I'm developing a modular tool, here is a minimal depiction of it.
I have a generic base class that I need to be as general as possible (for reasons unspecified here):
// base class
class Base {
public:
virtual void assess( ) = 0;
};
Then I have a more specialized derived class, that I define as a template:
// derived template class
template <typename T>
class Derived : public Base
{
protected:
T mValue;
public:
void assess( );
T value( ) { return mValue; }
};
// function specialization here
template<>
inline void Derived<int>::assess( )
{
mValue = 3;
}
Some parts are specialized, like the assess() function, as shown above.
Then I have an extra layer of inheritance. The main idea is to have a general assess() function that involves a function pointer with a Base object as argument.
// class specialization for special evaluation through function pointers
template <typename T>
class DerivedFuncPtr : public Derived<T> {
protected:
T (*mFuncPtr)( Base& );
Base *mFuncVar;
public:
DerivedFuncPtr<T>( T (*f)(Base&), Base& variable )
{
mFuncPtr = f;
mFuncVar = variable;
}
void assess( )
{
mFuncVar->assess();
this->mValue = (*mFuncPtr)(*mFuncVar);
}
};
OK, the problem is how to use it. Main source looks like this:
int squared( Derived<int>& );
int squared( Derived<int>& d )
{
int val = d.value();
return val*val;
}
int main (int argc, const char * argv[])
{
Derived<int> object;
object.assess();
cout << object.value() << "\n" ;
DerivedFuncPtr<int> objectFP( squared, object ); // (*)
return 0;
}
I get an error at compilation on line (*)
Candidate constructor not viable: no known conversion from 'int (Derived<int> &)' to 'int (*)(Base &)' for 1st argument
Will I be forced to encapsulate the function pointer into a templated U (*)(Base&) class or am I missing something obvious here?
For the sake of esthetics and because I am really not familiar with most recent C++ evolutions, I'd prefer not to use boost libraries or C++11, but well, if it can't be helped...
Thank you.

Answer 1

DerivedFuncPtr<T>( T (*f)(Base&), Base& variable )
The first argument to your constructor is a pointer to a function that takes a Base& as an argument (and returns a T). That Base& argument to the function pointer doesn't match the argument to squared, which takes a Derived<int>&.
One way to fix this is to make DerivedFuncPtr a class template that take two template arguments, one the typename T you already have, and the other, some other typename that will replace the uses of Base in your data members and constructor.
BTW, you also get an error on mFuncVar = variable; in that constructor. mFuncVar is a pointer, variable is a reference.