[OT] general C++ question

[Michael Schwendt]

On Thu, 02 Sep 2010 15:23:54 +0200, Christoph wrote:

> Am Donnerstag, den 02.09.2010, 12:54 +0200 schrieb Michael Schwendt:
> > On Thu, 02 Sep 2010 12:15:27 +0200, Christoph wrote:
> > 
> > > Hi all,
> > > 
> > > I just learned about closures in the Boost library and wanted to write
> > > some shorter code. To test, I used the following snippet:
> > > 
> > > #include <iostream>
> > > #include <string>
> > > 
> > > #include <boost/function.hpp>
> > > #include <boost/bind.hpp>
> > > 
> > > class FakeVisitor {
> > > public:
> > >   virtual void visit(int e) {}
> > >   virtual void visit(std::string e) = 0;
> > > };
> > > 
> > > template <typename T> class GenericVisitor : public FakeVisitor {
> > > public:
> > >   boost::function<void (T e)> f;
> > >   virtual void visit(T e) { f(e); }
> > > };
> > > 
> > > void print(int a, int i) {
> > >   std::cout << a << ":" << i << std::endl;
> > > }
> > > 
> > > int main(int argc, char** argv) {
> > >   GenericVisitor<int> v;
> > >   v.f = boost::bind(&print, 0, _1);
> > > 
> > >   v.visit((int) 1);
> > >   v.visit(std::string("hallo"));
> > >   //boost::bind(print, 1, _1)(2);
> > > }
> > > 
> > > Apparently, it does not work. Does anyone know why GenericVisitor<int>
> > > does not inherit visit(std::string)?
> > 
> > It does, but it's still declared a pure virtual function, i.e.
> > FakeVisitor (and GenericVisitor, too) is an abstract class that
> > cannot be instantiated. You would need to declare an implementation
> > of the visit(std::string) virtual method in GenericVisitor to make
> > it a non-abstract type.
> 
> Ah, sorry, wrong snippet. It does not work with 
> 
> virtual void visit(std::string e) {}
> 
> in FakeVisitor.
> 
> The intention is to extend a visitor pattern with a templated class that
> overloads exactly one method.

v.FakeVisitor::visit(std::string("hallo"));