Re: Object Factory and pre-main registering of classes

Paavo Helde <>
Mon, 11 May 2009 02:37:35 -0500
John <> kirjutas:


I am trying to create an object factory, AFactory, that allows new
object types (of base class A) to be added without changing the


3. If I compile files separately, link the factory and related classes
into a library and then link the main routine with the library

       g++ -c A*.cpp
       ar rvs liba.a A*.o
       g++ main.cpp -L. -la

the classes do not register themselves before the main function. A
workaround I found was to put the following in AFactory::GetInstance()

  // FIXME: This forces the maps to register. There should be some


  // to force the registration without creating this reverse


  if (m_factory.m_callback.empty())
     CreateCallBack fnc;
     fnc = AType1::CreateA;
     // Add all : fnc = ATypeX::CreateA;

I don't want to do this because it creates a reverse dependency that
requires modification of the factory to add a new class.

After re-reading your post I realize this is indeed too tight coupling,
meaning that the factory code must know all A* types. What I would do,
would be to add something like this into AFactory code:

// AFactory::GetFactory():
if (!initialized) {
     initialized=true; // to avoid infinite recursion
     // ...

Now in each library (so far you have only one), define the corresponding
registering function like this:

void RegisterClassesInLibraryA() {
     AFactory::GetFactory().RegisterA("a1", &AType1::CreateA);
     AFactory::GetFactory().RegisterA("a2", &AType2::CreateA);
     // ...

So the detailed knowledge about your classes is limited to the library
itself. All you have to expose from the library is a single registering
function with a trivial signature.


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