Re: Solving a template/struct issue

On Fri, 13 Jul 2007 06:06:49 +0000, Alexander Adam wrote:


As someone else pointed out, I think the term is "heterogeneous list"


Actually, I think Victor Bazarov hit it best:


It would really help if you could explain what you are trying to
achieve... I always get the sneaking suspicion that lists of different
objects implies a design mis-think of some sort. Something along the
lines of: if the objects really are different why would they be on the
same list? Which implies that either (i) they shouldn't be on the same
list, or (ii) they are really not that different.

A common example of the latter might be where different types of objects
have different ways of "doing some particular thing". Eg.:

#include <iostream>
#include <list>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

struct node_base // abstract
{
  virtual void do_something() const =0; // derived class implements
  virtual ~node_base() {} // ensure derived class dtor called
};

struct type1_node : node_base
{
  int a;
  string s;
  type1_node(const int a_, const string& s_) : a(a_), s(s_) {}
  void do_something() const
  {
    cout << "type 1: a = " << a << "\ts = " << s << endl;
  }
};

struct type2_node : node_base
{
  vector<double> v;
  type2_node(const double* const from, const double* const to) : v(from,to) {}
  void do_something() const
  {
    cout << "type 2: v = ";
    for (size_t i=0;i<v.size();++i) cout << '\t' << v[i];
    cout << endl;
  }
};

void dealloc(node_base* p)
{
  delete p;
}

int main()
{
  typedef list<node_base*> list_type; // smart pointer would be less clunky

  list_type my_list;

  // add some stuff to the list
  my_list.push_back(new type1_node(3,"hallo"));
  const double a[] = {3.0,-2.1,7.2};
  my_list.push_back(new type2_node(a,a+3));

  // each list member does its thing
  for_each(my_list.begin(), my_list.end(), mem_fun(&node_base::do_something));

  // avoid memory leak (wouldn't need this with smart pointer)
  for_each(my_list.begin(), my_list.end(), dealloc);
}

--
Lionel B