c++ automatic factory registration of derived types

Question

Like many before me, I\'m trying so get my derived types to automatically register with my factory. I read through many question and tried to focus on what I didn\'t find th

Answer 1

--- registration.h ---

#include <iostream>
#include <typeinfo>
#include <set>
#include <string>
using namespace std;

template<class T>
struct registrar {
        struct proxy { inline proxy();};
        static proxy p;
};

template<class T> typename registrar<T>::proxy registrar<T>::p;



struct factory {
        template <typename T> static T* create() {
               registrar<T>::p;
               return new T();
        }
};

set<string> & types();

template<typename T>
registrar<T>::proxy::proxy() { types().insert(typeid(T).name());}

--- registration.cpp ---

#include "registration.h"
#include <boost/foreach.hpp>

set<string> & types() {static set<string> types; return types;} 

int main() {
    BOOST_FOREACH(const string & s, types()) { cout<<s<<"\n";}
        factory::create<int>();
    factory::create<double>();
    factory::create<bool>();
        return 0;
}

--- registration_ext.cpp ---

#include "registration.h"

class myclass {};

void phony() {
    factory::create<double>();
    factory::create<myclass>();
}

Then compiled with:

$ g++ registration.cpp registration_ext.cpp -o registration

When run:

$ ./registration 
7myclass
b
d
i

So, it seems to have registered the classes before main is called.

Edit: I've noticed that this solution is implementation dependent. Under 3.6.2 the C++ standard says:

It is implementation-defined whether the dynamic initialization of a non-local variable with static storage duration is done before the first statement of main. If the initialization is deferred to some point in time after the first statement of main, it shall occur before the first odr-use (3.2) of any function or variable defined in the same translation unit as the variable to be initialized.

Answer 2

I use a singleton with a member for registration, basically:

template< typename KeyType, typename ProductCreatorType >
class Factory
{
    typedef boost::unordered_map< KeyType, ProductCreatorType > CreatorMap;
    ...
};

Using Loki I then have something along these lines:

 typedef Loki::SingletonHolder< Factory< StringHash, boost::function< boost::shared_ptr< SomeBase >( const SomeSource& ) > >, Loki::CreateStatic > SomeFactory;

Registration is usually done using a macro such as:

#define REGISTER_SOME_FACTORY( type ) static bool BOOST_PP_CAT( type, __regged ) = SomeFactory::Instance().RegisterCreator( BOOST_PP_STRINGIZE( type ), boost::bind( &boost::make_shared< type >, _1 ) );

This setup has a number of advantages:

• Works with for example boost::shared_ptr<>.
• Does not require maintaining a huge file for all the registration needs.
• Is very flexible with the creator, anything goes pretty much.
• The macro covers the most common use case, while leaving the door open for alternatives.

Invoking the macro in the .cpp file is then enough to get the type registered at start up during static initialization. This works dandy save for when the type registration is a part of a static library, in which case it won't be included in your binary. The only solutions which compiles the registration as a part of the library which I've seen work is to have one huge file that does the registration explicitly as a part of some sort of initialization routine. Instead what I do nowadays is to have a client folder with my lib which the user includes as a part of the binary build.

From your list of requirements I believe this satisfies everything save for using a registrator class.