Re: Design question: alternative to inheritance.

Supersedes: <visitor-20080717054545@ram.dialup.fu-berlin.de>

ram@zedat.fu-berlin.de (Stefan Ram) writes:


  The following example for the visitor pattern might deviate
  from the visitor pattern published elsewhere, because I have
  only used two types, ?A? and ?B?, and I have ?reinvented? it
  from my recollection, without verification with the literature.

  But at least it shows a combination of two dynamic-static
  adaptors to get a ?bidynamic-bistatic adaptor?:

#include <iostream>
#include <ostream>

struct A;
struct B;

struct S
{ static void static_process( ::A const * const a, ::A const * const a_ )
  { ::std::cout << "::S::static_process( A, A )\n"; }
  static void static_process( ::A const * const a, ::B const * const b )
  { ::std::cout << "::S::static_process( A, B )\n"; }
  static void static_process( ::B const * const b, ::A const * const a )
  { ::std::cout << "::S::static_process( B, A )\n"; }
  static void static_process( ::B const * const b, ::B const * const b_ )
  { ::std::cout << "::S::static_process( B, B )\n"; }};

struct X
{ virtual void process( ::A const * const a )const = 0;
  virtual void process( ::B const * const b )const = 0;
  virtual void process( ::X const * const x )const = 0; };

struct A : public ::X
{ void process( ::A const * const a )const{ ::S::static_process( a, this ); }
  void process( ::B const * const b )const{ ::S::static_process( b, this ); }
  void process( ::X const * const x1 )const{ x1->process( this ); }};

struct B : public ::X
{ void process( ::A const * const a )const{ ::S::static_process( a, this ); }
  void process( ::B const * const b )const{ ::S::static_process( b, this ); }
  void process( ::X const * const x1 )const{ x1->process( this ); }};

void process( ::X const * const x0, ::X const * const x1 )
{ x0->process( x1 ); }

int main()
{ ::A const a;
  ::B const b;
  ::X const * x0;
  ::X const * x1;
  x0 = &a; x1 = &a; process( x0, x1 );
  x0 = &a; x1 = &b; process( x0, x1 );
  x0 = &b; x1 = &a; process( x0, x1 );
  x0 = &b; x1 = &b; process( x0, x1 ); }

::S::static_process( A, A )
::S::static_process( A, B )
::S::static_process( B, A )
::S::static_process( B, B )

  (The last four lines are the output.)