Re: [Boost-users] How to convert a vector of base classes to template based code?

10 Jul 2008


      Do you mean you want your vector to be heterogenous, i.e. containing
different types?
If so, you can use boost::variant:

typedef boost::variant<square, circle, triangle> shape_type; //
square, circle, triangle may be unrelated types providing show_me()
method
std::vector<shape_type> shapes;
struct drawer : boost::static_visitor<>
{
  template<typename shape> void operator()(shape &sh) const
  {
    sh.show_me();
  }
};

int main()
{
  shapes.push_back(triangle());
  shapes.push_back(circle());
  BOOST_FOREACH(std::vector<shape_type>::reference shape, shapes)
    boost::apply_visitor(drawer(), shape);
}


2008/7/10, Peng Yu <pengyu.ut@gmail.com>:
...
Hi,
The following program uses virtual functions. Essentially I need to go
through all the pairs in 'v'.
Since I have already know what are in v at compile time. I would like
to convert the dynamic polymorphism to template. Therefore, the
runtime would be better. I think this should be done with the help of
boost::mpl. But I'm not sure how to do it. Would you please help me?
Thanks,
Peng
#include <iostream>
#include <vector>
#include <boost/shared_ptr.hpp>
struct shape {
 virtual void show_me() const = 0;
 virtual ~shape() { }
};
struct square : public shape {
 void show_me() const {
  std::cout << "square";
 }
};
struct circle : public shape {
 void show_me() const {
  std::cout << "circle";
 }
};
struct triangle : public shape {
 void show_me() const {
  std::cout << "triangle";
 }
};
int main() {
 std::vector<boost::shared_ptr<shape> > v;
 {
  boost::shared_ptr<shape> s(new square);
  v.push_back(s);
 }
 {
  boost::shared_ptr<shape> s(new circle);
  v.push_back(s);
 }
 {
  boost::shared_ptr<shape> s(new square);
  v.push_back(s);
 }
 {
  boost::shared_ptr<shape> s(new triangle);
  v.push_back(s);
 }
for(std::vector<boost::shared_ptr<shape> >::const_iterator it =
v.begin(); it != v.end(); ++ it) {
  for(std::vector<boost::shared_ptr<shape> >::const_iterator it2 =
it + 1; it2 != v.end(); ++ it2) {
    (*it)->show_me();
    std::cout << " vs. ";
    (*it2)->show_me();
    std::cout << std::endl;
  }
 }
}
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Re: [Boost-users] How to convert a vector of base classes to template based code?

Igor R