[OT] general C++ question

[Christoph Höger]

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)?

[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.

[Christoph Höger]

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.

[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"));

[Michael Schwendt]

On Thu, 02 Sep 2010 15:23:54 +0200, Christoph wrote:
> The intention is to extend a visitor pattern with a templated class that
> overloads exactly one method.

Since my reply sent yesterday is truncated for unknown reasons, let me try
to reconstruct the missing part:

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

The declaration of GenericVisitor::visit hides the other visit members
found in the base class FakeVisitor. You would need to pull it into scope
either as above or with a "using" declaration (using FakeVisitor::visit)
in GenericVisitor.
That is independent from what you're trying to achieve with Boost, btw.