C++ Inserting a class into a map container

Question

I have a map in C++ and I wish to input my class as the value, and a string as the key. When I try to, I get an error 'Scene_Branding' : illegal use of this type as an expression I get an illegal use of this type as an expression, and I can't seem to find out why. Here is some code.

 string CurrentScene = "Scene_Branding";
 map<string, Scene> Scenes;
 Scenes.insert(std::make_pair("Scene_Branding", Scene_Branding));  //<-- Illegal Error parameter 2

and here is Scene Branding header..

#ifndef Scene_Branding_H
#define Scene_Branding_H

#include "Scene.h"
#include <iostream>
#include <string>


class Scene_Branding : Scene
{
public:
 Scene_Branding();
 ~Scene_Branding();
 void Draw();
};

#endif

and here is Scene header..

#ifndef Scene_H
#define Scene_H

#include <iostream>
#include <string>

class Scene
{
public:
 Scene();
 ~Scene();
 virtual void Draw();

};

#endif

and here is there cpp files.

Scene cpp.

#include "Scene.h"

Scene::Scene()
{

}
Scene::~Scene()
{

}
void Scene::Draw(){
 std::cout << "Hey";
}

Scene_Branding cpp

#include "Scene_Branding.h"

Scene_Branding::Scene_Branding()
{

}

Scene_Branding::~Scene_Branding()
{

}

void Scene_Branding::Draw()
{
 std::cout << "Drawing from Scene_branding";
}

Answer 1

First, don't store objects themselves in the map, store pointers to your objects.

Second, you need to give an instance of Scene_Branding to std::make_pair, not the class itself.

EDIT: Here's how you go about storing pointers:

 string CurrentScene = "Scene_Branding";
 map<string, Scene*> Scenes;
 Scenes.insert(std::make_pair("Scene_Branding", new Scene_Branding()));

But, since you asked this type of question, i recommend you read a good c++ book for further grasping of concepts like pointers.

Answer 2

Try:

Scenes.insert(std::make_pair("Scene_Branding", Scene_Branding()));

Answer 3

I think you don't want to do that.

1. there is no runtime type-mapping in C++, you store objects, not types.
2. you cannot store polymorphic types in STL containers, use boost::ptr_map instead if it is your wish

So, the "new" code:

class Scene
{
public:
  virtual ~Scene();                  // Virtual Destructor, it's a base class
  virtual Scene* clone() const = 0;  // Polymorphic construction
private:
  // whatever you wish
};

class Scene_Branding: public Scene
{
public:
  virtual Scene_Branding* clone() const { return new Scene_Branding(); }
};

And the new way to store those:

const std::string SceneBrandingKey = "Scene_Branding";

typedef boost::ptr_map<std::string, Scene> scenes_type;

scenes_type Scenes;
Scenes.insert(SceneBrandingKey, new Scene_Branding());

And you can use it thusly:

Scenes["Scene_Branding"].process(); // Note: use '.' not '->'

The nice thing about Boost Pointer Container is that it's been meant for polymorphic types, with exception safety and all, and yet mimics the behavior / interface of the STL so that you are not lost :)