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 <iostream>
#include <vector>
#include <string>

#include <boost/fusion/container/list/cons.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/view/iterator_range.hpp>

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<Child1, cons<Child3, cons<Child100> > > MyChildClasses;

typedef vector<Base*> Children;
typedef vector<string> Names;

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

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

  template <class T>
  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<result_of::begin<MyChildClasses>::type, 0>::type,
      result_of::advance_c<result_of::begin<MyChildClasses>::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;
}

Answer 2

You can abuse the preprocessor and set up some static class members that register your classes with a factory via a hash_map like Ben describes. If you have visual studio, look at how DECLARE_DYNCREATE is implemented in MFC. I've done something similar to implement a class factory. Non-standard for sure but since C++ does not offer any kind of support for this type of mechanism any solution is probably going be non-standard.

Edit

I said in a comment earlier I was working on documenting a scaled down version of something I had done. The scaled down version is still rather large so I posted it here. If there is enough interest I can copy/paste it on this site. Let me know.

Answer 3

C++ does not provide a method for dynamic construction of class instances like this. However, you may be able to use code generation to generate the "brute force" code (like you showed above) from a list of classes. Then, #include the generated code in your convert_string_to_instance method.

You can also set up your project build system to rebuild the generated code anytime the list of classes changes.

Answer 4

It sounds like you might be using subclasses for things that should be encoded as fields.

Instead of coding the different behaviour in 100 classes, consider building a look-up table with rules/constants/function-pointers that allow you to implement the proper behaviour from one class.

For example, instead of:

class SmallRedSquare  : public Shape {...};
class SmallBlueSquare : public Shape {...};
class SmallBlueCircle : public Shape {...};
class SmallRedCircle  : public Shape {...};
class BigRedSquare    : public Shape {...};
class BigBlueSquare   : public Shape {...};
class BigBlueCircle   : public Shape {...};
class BigRedCircle    : public Shape {...};

try:

struct ShapeInfo
{
   std::string type;
   Size size;
   Color color;
   Form form;
};

class Shape
{
public:
    Shape(std::string type) : info_(lookupInfoTable(type)) {}

    void draw()
    {
        // Use info_ to draw shape properly.
    }

private:
    ShapeInfo* lookupInfoTable(std::string type) {info_ = ...;}

    ShapeInfo* info_;
    static ShapeInfo infoTable_[];
};

const ShapeInfo Shape::infoTable_[] =
{
    {"SmallRedSquare",  small,  red, &drawSquare},
    {"SmallBlueSquare", small, blue, &drawSquare},
    {"SmallRedCircle",  small,  red, &drawCircle},
    {"SmallBlueCircle", small, blue, &drawCircle},
    {"BigRedSquare",      big,  red, &drawSquare},
    {"BigBlueSquare",     big, blue, &drawSquare},
    {"BigBlueCircle",     big,  red, &drawCircle},
    {"BigRedCircle",      big, blue, &drawCircle}
}

int main()
{
    Shape s1("SmallRedCircle");
    Shape s2("BigBlueSquare");
    s1.draw();
    s2.draw();
}

This idea might not be applicable to your problem, but I figure it couldn't hurt to present it anyway. :-)

My idea is like the Replace Subclass with Fields refactoring, but I go a bit further.

Answer 5

This is the skeleton of a horrible, horrible way to do it:

class Factory {
  public:
    virtual Base * make() = 0;
};

template<typename T> class TemplateFactory : public Factory {
  public:
    virtual Base * make() {
      return dynamic_cast<Base *>(new T());
    }
};

map<string, Factory *> factories;

#define REGISTER(classname) factories[ #classname ] = new TemplateFactory<classname>()

Then call REGISTER(classname); for every relevant derived class of Base, and use factories["classname"]->make() to get a new object of type classname. Obvious flaws with the above code as written include massive potential for memory leaks, and the general awfulness of combining macros and templates.

Answer 6

I asked a question entitled automatic registration of object creator function with a macro that has the following example program that runs:

#include <map>
#include <string>
#include <iostream>

struct Object{ virtual ~Object() {} }; // base type for all objects

struct ObjectFactory {
  static Object* create(const std::string& id) { // creates an object from a string
    const Creators_t::const_iterator iter = static_creators().find(id);
    return iter == static_creators().end() ? 0 : (*iter->second)(); // if found, execute the creator function pointer
  }

 private:
  typedef Object* Creator_t(); // function pointer to create Object
  typedef std::map<std::string, Creator_t*> Creators_t; // map from id to creator
  static Creators_t& static_creators() { static Creators_t s_creators; return s_creators; } // static instance of map
  template<class T = int> struct Register {
    static Object* create() { return new T(); };
    static Creator_t* init_creator(const std::string& id) { return static_creators()[id] = create; }
    static Creator_t* creator;
  };
};

#define REGISTER_TYPE(T, STR) template<> ObjectFactory::Creator_t* ObjectFactory::Register<T>::creator = ObjectFactory::Register<T>::init_creator(STR)

namespace A { struct DerivedA : public Object { DerivedA() { std::cout << "A::DerivedA constructor\n"; } }; }
REGISTER_TYPE(A::DerivedA, "A");

namespace B { struct DerivedB : public Object { DerivedB() { std::cout << "B::DerivedB constructor\n"; } }; }
REGISTER_TYPE(B::DerivedB, "Bee");

namespace C { struct DerivedC : public Object { DerivedC() { std::cout << "C::DerivedC constructor\n"; } }; }
REGISTER_TYPE(C::DerivedC, "sea");

namespace D { struct DerivedD : public Object { DerivedD() { std::cout << "D::DerivedD constructor\n"; } }; }
REGISTER_TYPE(D::DerivedD, "DEE");

int main(void)
{
  delete ObjectFactory::create("A");
  delete ObjectFactory::create("Bee");
  delete ObjectFactory::create("sea");
  delete ObjectFactory::create("DEE");
  return 0;
}

compile and run output is:

> g++ example2.cpp && ./a.out
A::DerivedA constructor
B::DerivedB constructor
C::DerivedC constructor
D::DerivedD constructor