virtual member template functions workaround

[Matthieu Paindavoine]

Hello,

I am coding a < composite / visitor / combinator > sandwich, and I
bumped again what seems a tough cookie: virtual member template
functions are not allowed.

#include <iostream>

class A
{
public:
template < class T >
void Op(T& _t)
{
std::cout << "A::Op " << _t.vis() << std::endl;
}

virtual void V_Op()
{
std::cout << "A::V_Op" << std::endl;
}
};

class B : public A
{
public:
template < class T >
void Op(T& _t)
{
std::cout << "B::Op " << _t.vis() << std::endl;
}

virtual void V_Op()
{
std::cout << "B::V_Op" << std::endl;
}
};

class X
{
public:
void vis()
{
std::cout << "X::vis" << std::endl;
}
};

int main()
{
A* a = new A();
A* b = new B();
X x;
a->Op(x);
a->V_Op();
b->Op(x);
b->V_Op();
}

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
So I would love to stick a virtual in front of the void Op(T& _t), but
I can't

Is there a workaround, or this is dead-end an indication I took a
wrong term some time ago?

Thank you for your suggestions

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[Andreas Huber]

Matthieu Paindavoine wrote:
> Hello,
>
> I am coding a < composite / visitor / combinator > sandwich, and I
> bumped again what seems a tough cookie: virtual member template
> functions are not allowed.

[snip]

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&frame=right&th=982ee5addad67d25&seekm=b16s9h%24hj8%241%40news-reader10.wanadoo.fr#link9
(virtual template method / design problem)

Regards,

Andreas

[Torsten Robitzki]

It's not really clear to me what you want to achieve with this but the
obvious solution to me is to factor out the code that is common to all
types T.

class A
{
public:
template <class T>
void Op(T& _t)
{

std::cout<<common_code()<< _t.vis() << std::endl;
}
private:
virtual const char* common_code() const
{
return "A::Op ";
}
};

Last week on de.comp.lang.iso-c++ a similar problem arose and we found a
solution for a fixed number of types T by not making the virtual
function a template and to generate the class with a type list and a
class that describe the function body in a static function.

regards
Torsten