Dynamically creating an instance of a class from a string containing the class name in C++

Question

Lets say I have a base class with 100 children:

class Base { 
  virtual void feed();
  ...   
};
class Child1 : public Base {
  void feed();  //specific proc         


        

Answer 1

Behold the mighty Boost.

The one thing you have to do in order to use my solution is to add a new member to all your classes, and that is a static const string that contains the name of the class. There are probably other ways to do it too, but that's what I have right now.

#include 
#include 
#include 

#include 
#include 
#include 

using namespace std;
using boost::fusion::cons;


class Base { virtual void feed(){ } };

class Child1 : public Base{
  void feed(){ }

public:
  static const string name_;
};
const string Child1::name_ = "Child1";

class Child3 : public Base{
  void feed(){ }

public:
  static const string name_;
};
const string Child3::name_ = "Child3";

//...
class Child100 : public Base{

  void feed(){ }

public:
  static const string name_;
};
const string Child100::name_ = "Child100";

// This is probably the ugliest part, but I think it's worth it.
typedef cons > > MyChildClasses;

typedef vector Children;
typedef vector Names;

struct CreateObjects{      // a.k.a convert_string_to_instance() in your example.

  CreateObjects(Children& children, string name) : children_(&children), name_(name){ }

  template 
  void operator()(T& cs) const{

    if( name_ == cs.name_ ){
      cout << "Created " << name_ << " object." << endl;
      (*children_).push_back(new T);
    }else{
      cout << name_ << " does NOT match " << cs.name_ << endl;
    }
  }

  Children* children_;
  string name_;
};

int main(int argc, char* argv[]){

  MyChildClasses myClasses;

  Children children;
  Names names;
  names.push_back("Child1");
  names.push_back("Child100");
  names.push_back("Child1");
  names.push_back("Child100");

  // Extra test.
  // string input;
  // cout << "Enter a name of a child class" << endl;
  // cin >> input;
  // names.push_back(input);

  using namespace boost::fusion;
  using boost::fusion::begin;
  using boost::fusion::for_each;

  for(Names::iterator namesIt = names.begin(); namesIt != names.end(); ++namesIt){

    // You have to know how many types there are in the cons at compile time.
    // In this case I have 3; Child1, Child3, and Child100
    boost::fusion::iterator_range<
      result_of::advance_c::type, 0>::type,
      result_of::advance_c::type, 3>::type
      > it(advance_c<0 >(begin(myClasses)),
       advance_c<3>(begin(myClasses)));
    for_each(it, CreateObjects(children, *namesIt));
  }

  cout << children.size() << " objects created." << endl;
  return 0;
}