Extending library visitor for user-defined types with macros (example)

01_README.md

While the previous gist documented my attempt at finding a working solution, this one provides a small working example.

Here, the library code declares classes A and B both inheriting form a base class Base. A Visitor is introduced, and both A and B have a apply method that calls the appropriate visitor method.

The client introduces a new class C that also inherits from base, and which is able to use a Visitor-derived class on any Base* pointer, be it a A, a B or a C under the hood.

client.cc

// includes

#include <iostream>



// custom user type predeclaration

class DerivedC;

#define USER_TYPES (DerivedC)
#include "library.hh"
#include "library_static.hh"



// custom user type

class DerivedC : public Base
{
  public:
    void apply(const Visitor& v) const;
};

void DerivedC::apply(const Visitor& v) const
{
  v(*this);
}



// custom visitor

class PrintVisitor : public Visitor
{
  public:
    void operator()(const DerivedA&) const;
    void operator()(const DerivedB&) const;
    void operator()(const DerivedC&) const;
};

void PrintVisitor::operator()(const DerivedA&) const { std::cout << "A"; }
void PrintVisitor::operator()(const DerivedB&) const { std::cout << "B"; }
void PrintVisitor::operator()(const DerivedC&) const { std::cout << "C"; }



// main

int main()
{
  PrintVisitor v;
  DerivedA a;
  DerivedB b;
  DerivedC c;
  Base const* pa = &a;
  Base const* pb = &b;
  Base const* pc = &c;

  pa->apply(v);
  pb->apply(v);
  pc->apply(v);

  std::cout << std::endl;
}

library.cc

// includes

#include "library.hh"



// destructors (and thus vtables)

Base::~Base()
{
}

DerivedA::~DerivedA()
{
}

DerivedB::~DerivedB()
{
}

library.hh

#pragma once



// predeclarations

class Visitor;



// class hierarchy

class Base
{
  public:
    virtual ~Base();
    virtual void apply(const Visitor& v) const = 0;
};

class DerivedA : public Base
{
  public:
    virtual ~DerivedA();
    virtual void apply(const Visitor& v) const;
};

class DerivedB : public Base
{
  public:
    virtual ~DerivedB();
    virtual void apply(const Visitor& v) const;
};

library_static.hh

// must be included once in the client
// in a source file, not a header



// includes

#include <boost/preprocessor.hpp>
#include "library.hh"



// boost_pp magic

#define USER_TYPE_DECL(R, _, TYPE) \
  virtual void operator() (TYPE const&) const = 0;



// visitor

class Visitor
{
  public:
    virtual ~Visitor();

#ifdef USER_TYPES
    BOOST_PP_SEQ_FOR_EACH(USER_TYPE_DECL, _, USER_TYPES)
#endif
    virtual void operator() (DerivedA const&) const = 0;
    virtual void operator() (DerivedB const&) const = 0;
};

Visitor::~Visitor()
{
}



// apply implementation

void DerivedA::apply(const Visitor& v) const
{
  v(*this);
};

void DerivedB::apply(const Visitor& v) const
{
  v(*this);
};

Makefile

CC = clang++ -Wweak-vtables -Wall -Wextra -Werror

client: library.a client.o
	$(CC) client.o -L. -lrary -o $@

client.o: client.cc library.hh Makefile
	$(CC) -c $< -o $@

library.o: library.cc library.hh Makefile
	$(CC) -c $< -o $@

library.a: library.o
	ar -scr $@ $<

clean:
	rm -fv client library.a library.o client.o