Entity/Component Systems in C++, How do I discover types and construct components?

Question

I'm working on an entity component system in C++ that I hope to follow the style of Artemis (http://piemaster.net/2011/07/entity-component-artemis/) in that components are mostly data bags and it's the Systems that contain the logic. I'm hoping to take advantage of the data-centric-ness of this approach and build some nice content tools.

However, one hump I'm coming across is how to take some identifier string or GUID from a data file and use that to construct component for an Entity. Obviously I could just have one big parse function:

Component* ParseComponentType(const std::string &typeName)
{
    if (typeName == "RenderComponent") {
        return new RenderComponent();
    }

    else if (typeName == "TransformComponent") {
        return new TransformComponent();
    }

    else {
        return NULL:
    }
}

But that's really ugly. I intend to be adding and modifying components frequently, and hopefully building some sort of ScriptedComponentComponent, such that you could implement a component and system in Lua for the purposes of prototyping. I'd like to be able to write a class inheriting from some BaseComponent class, maybe toss in a couple of macros to make everything work, and then have the class available for instantiation at runtime.

In C# and Java this would be pretty straightforward, since you get nice reflection APIs to look up classes and constructors. But, I'm doing this in C++ because I want to increase my proficiency in that language.

So How is this accomplished in C++? I've read about enabling RTTI, but it seems most people are wary about that, especially in a situation where I only need it for a subset of object types. If a custom RTTI system is what I need there, where can I go to start learning to write one?

Answer 1

A comment:
The Artemis implementation is interesting. I came up with a similar solution, except I called my components "Attributes" and "Behaviors". This approach of separating types of components has worked very nicely for me.

Regarding the solution:
The code is easy to use, but the implementation might be hard to follow if you're not experienced with C++. So...

The desired interface

What I did is to have a central repository of all components. Each component type is mapped to a certain string (which represents the component name). This is how you use the system:

// Every time you write a new component class you have to register it.
// For that you use the `COMPONENT_REGISTER` macro.
class RenderingComponent : public Component
{
    // Bla, bla
};
COMPONENT_REGISTER(RenderingComponent, "RenderingComponent")

int main()
{
    // To then create an instance of a registered component all you have
    // to do is call the `create` function like so...
    Component* comp = component::create("RenderingComponent");

    // I found that if you have a special `create` function that returns a
    // pointer, it's best to have a corresponding `destroy` function
    // instead of using `delete` directly.
    component::destroy(comp);
}

The implementation

The implementation is not that bad, but it's still pretty complex; it requires some knowledge of templates and function pointers.

Note: Joe Wreschnig has made some good points in the comments, mainly on how my previous implementation made too many assumptions about how good the compiler is at optimizing code; the issue was not detrimental, imo, but it did bug me as well. I also noticed that the former COMPONENT_REGISTER macro did not work with templates.

I've changed the code and now all of those problems should be fixed. The macro works with templates and the issues that Joe raised have been addressed: now it's much easier for compilers to optimize away unnecessary code.

component/component.h

#ifndef COMPONENT_COMPONENT_H
#define COMPONENT_COMPONENT_H

// Standard libraries
#include <string>

// Custom libraries
#include "detail.h"


class Component
{
    // ...
};


namespace component
{
    Component* create(const std::string& name);
    void destroy(const Component* comp);
}

#define COMPONENT_REGISTER(TYPE, NAME)                                        \
    namespace component {                                                     \
    namespace detail {                                                        \
    namespace                                                                 \
    {                                                                         \
        template<class T>                                                     \
        class ComponentRegistration;                                          \
                                                                              \
        template<>                                                            \
        class ComponentRegistration<TYPE>                                     \
        {                                                                     \
            static const ::component::detail::RegistryEntry<TYPE>& reg;       \
        };                                                                    \
                                                                              \
        const ::component::detail::RegistryEntry<TYPE>&                       \
            ComponentRegistration<TYPE>::reg =                                \
                ::component::detail::RegistryEntry<TYPE>::Instance(NAME);     \
    }}}


#endif // COMPONENT_COMPONENT_H

component/detail.h

#ifndef COMPONENT_DETAIL_H
#define COMPONENT_DETAIL_H

// Standard libraries
#include <map>
#include <string>
#include <utility>

class Component;

namespace component
{
    namespace detail
    {
        typedef Component* (*CreateComponentFunc)();
        typedef std::map<std::string, CreateComponentFunc> ComponentRegistry;

        inline ComponentRegistry& getComponentRegistry()
        {
            static ComponentRegistry reg;
            return reg;
        }

        template<class T>
        Component* createComponent() {
            return new T;
        }

        template<class T>
        struct RegistryEntry
        {
          public:
            static RegistryEntry<T>& Instance(const std::string& name)
            {
                // Because I use a singleton here, even though `COMPONENT_REGISTER`
                // is expanded in multiple translation units, the constructor
                // will only be executed once. Only this cheap `Instance` function
                // (which most likely gets inlined) is executed multiple times.

                static RegistryEntry<T> inst(name);
                return inst;
            }

          private:
            RegistryEntry(const std::string& name)
            {
                ComponentRegistry& reg = getComponentRegistry();
                CreateComponentFunc func = createComponent<T>;

                std::pair<ComponentRegistry::iterator, bool> ret =
                    reg.insert(ComponentRegistry::value_type(name, func));

                if (ret.second == false) {
                    // This means there already is a component registered to
                    // this name. You should handle this error as you see fit.
                }
            }

            RegistryEntry(const RegistryEntry<T>&) = delete; // C++11 feature
            RegistryEntry& operator=(const RegistryEntry<T>&) = delete;
        };

    } // namespace detail

} // namespace component

#endif // COMPONENT_DETAIL_H

component/component.cpp

// Matching header
#include "component.h"

// Standard libraries
#include <string>

// Custom libraries
#include "detail.h"


Component* component::create(const std::string& name)
{
    detail::ComponentRegistry& reg = detail::getComponentRegistry();
    detail::ComponentRegistry::iterator it = reg.find(name);

    if (it == reg.end()) {
        // This happens when there is no component registered to this
        // name. Here I return a null pointer, but you can handle this
        // error differently if it suits you better.
        return nullptr;
    }

    detail::CreateComponentFunc func = it->second;
    return func();
}

void component::destroy(const Component* comp)
{
    delete comp;
}

Extending with Lua

I should note that with a bit of work (it's not very hard), this can be used to seamlessly work with components defined in either C++ or Lua, without ever having to think about it.

Answer 2

It seems like what you want is a factory.

http://en.wikipedia.org/wiki/Factory_method_pattern

What you can do is have your various components register with the factory what name they correspond to, and then you have some map of string identifier to constructor method signature to generate your components.

Answer 3

I worked with Paul Manta's design from the chosen answer for a while and eventually came to this more generic and concise factory implementation below that I'm willing to share for anyone coming to this question in the future. In this example, every factory object derives from the Object base class:

struct Object {
    virtual ~Object(){}
};

The static Factory class is as follows:

struct Factory {
    // the template used by the macro
    template<class ObjectType>
    struct RegisterObject {
        // passing a vector of strings allows many id's to map to the same sub-type
        RegisterObject(std::vector<std::string> names){
            for (auto name : names){
                objmap[name] = instantiate<ObjectType>;
            }
        }
    };

    // Factory method for creating objects
    static Object* createObject(const std::string& name){
        auto it = objmap.find(name);
        if (it == objmap.end()){
            return nullptr;
        } else {
            return it->second();
        }
    }

    private:
    // ensures the Factory cannot be instantiated
    Factory() = delete;

    // the map from string id's to instantiator functions
    static std::map<std::string, Object*(*)(void)> objmap;

    // templated sub-type instantiator function
    // requires that the sub-type has a parameter-less constructor
    template<class ObjectType>
    static Object* instantiate(){
        return new ObjectType();
    }
};
// pesky outside-class initialization of static member (grumble grumble)
std::map<std::string, Object*(*)(void)> Factory::objmap;

The macro for registering a sub-type of Object is as follows:

#define RegisterObject(type, ...) \
namespace { \
    ::Factory::RegisterObject<type> register_object_##type({##__VA_ARGS__}); \
}

Now usage is as follows:

struct SpecialObject : Object {
    void beSpecial(){}
};
RegisterObject(SpecialObject, "SpecialObject", "Special", "SpecObj");

...

int main(){
    Object* obj1 = Factory::createObject("SpecialObject");
    Object* obj2 = Factory::createObject("SpecObj");
    ...
    if (obj1){
        delete obj1;
    }
    if (obj2){
        delete obj2;
    }
    return 0;
}

The capacity for many string id's per sub-type was useful in my application, but the restriction to a single id per sub-type would be fairly straightforward.

I hope this has been useful!

Answer 4

Building off of @TimStraubinger's answer, I built a factory class using C++14 standards which can store derived members with an arbitrary number of arguments. My example, unlike Tim's, only takes one name/key per function. Like Tim's, every class being stored is derived from a Base class, mine being called Base.

Base.h

#ifndef BASE_H
#define BASE_H

class Base{
    public:
        virtual ~Base(){}
};

#endif

EX_Factory.h

#ifndef EX_COMPONENT_H
#define EX_COMPONENT_H

#include <string>
#include <map>
#include "Base.h"

struct EX_Factory{
    template<class U, typename... Args>
    static void registerC(const std::string &name){
        registry<Args...>[name] = &create<U>;
    }
    template<typename... Args>
    static Base * createObject(const std::string &key, Args... args){
        auto it = registry<Args...>.find(key);
        if(it == registry<Args...>.end()) return nullptr;
        return it->second(args...);
    }
    private:
        EX_Factory() = delete;
        template<typename... Args>
        static std::map<std::string, Base*(*)(Args...)> registry;

        template<class U, typename... Args>
        static Base* create(Args... args){
            return new U(args...);
        }
};

template<typename... Args>
std::map<std::string, Base*(*)(Args...)> EX_Factory::registry; // Static member declaration.


#endif

main.cpp

#include "EX_Factory.h"
#include <iostream>

using namespace std;

struct derived_1 : public Base{
    derived_1(int i, int j, float f){
        cout << "Derived 1:\t" << i * j + f << endl;
    }
};
struct derived_2 : public Base{
    derived_2(int i, int j){
        cout << "Derived 2:\t" << i + j << endl;
    }
};

int main(){
    EX_Factory::registerC<derived_1, int, int, float>("derived_1"); // Need to include arguments
                                                                    //  when registering classes.
    EX_Factory::registerC<derived_2, int, int>("derived_2");
    derived_1 * d1 = static_cast<derived_1*>(EX_Factory::createObject<int, int, float>("derived_1", 8, 8, 3.0));
    derived_2 * d2 = static_cast<derived_2*>(EX_Factory::createObject<int, int>("derived_2", 3, 3));
    delete d1;
    delete d2;
    return 0;
}

Output

Derived 1:  67
Derived 2:  6

I hope this helps people needing to use a Factory design which does not require an identity constructor to work. It was fun designing, so I hope it helps people needing more flexibility in their Factory designs.