This is based on my previous two posts on Static Interfaces in C++ and Keep Track of and Enumerate All Sub-classes of a Particular Interface. The idea is that I want my code to be extensible in the feature without requiring any re-writing of the current code base. The code base operates on generic objects via their interfaces, so as long as newly-coded classes properly extend those interfaces, the program should know how to handle them. The problem is, how can we write the program in such a manner that a user interface can enumerate available options for implementations of a particular interface, and how can we instantiate those objects?
In Keep Track of and Enumerate All Sub-classes of a Particular Interface I showed how to maintain a registry of classes deriving from a given interface, which handles the first problem, but there is a limitation in that all of these classes must provide a factory method that takes no parameters (void input). I decided that, for my project, this was not acceptable and I needed a way to define the creation parameters as part of the factory methods, whereas the creation parameters may be different for particular interfaces.
In Static Interfaces in C++ I showed how we can enforce the requirement of a static method in derived classes with a particular signature using a template interface.
In this post I will combine the two so that we can create a registry of classes that inherit from a particular interface, and provide a static factory method for creating objects of that interface, using a particular creation method signature unique to that interface. The registry will pair class names with function pointers that match the specific signature of the interface the class is being registered for.
Disclaimer: I do not claim this is the "best" way to handle this issue. This is just what I came up with. It happens to be pretty involved and overly indirect, which means it's probably bad design. It is, however, an extremely interesting exercise in generic programming.
Prequil: the code will require these later so there they are:
/** * file RegistryTest.cpp * date: Aug 14, 2009 * brief: * * detail: */ #include #include #include #include using namespace std;
Ok, so lets begin. First let's define a couple of interfaces that we're interested in.
class InterfaceA{}; class InterfaceB{}; class InterfaceC{};
Now we create a template class whose sole purpose is to create a per-interface
typedef of the function signature that is necessary for instantiating and
object of that class. Is it really possible that all sub-objects can be
instantiated with the same parameters? If that's the case, shouldn't they all
be combined into a single class that just contains that information as private
members? Probably, but in my case these parameters are more like a "bare
minimum" for instantiation, and then many more parameters are set by the user.
It makes sense to me, I promise. If it doesn't to you, you don't have to use
this.
template< typename InterfaceType > class Factory { public: typedef InterfaceType*(*Creator)(void); };
Creator is now a typedef that aliases a function pointer that takes no
parameters. Wait, isn't that what we had before? Yes, but now we make a couple
of template specializations to define the different signatures for our
specific interfaces. These specializations would normally be in the file that
contained the interface declaration.
/// specializations can define other creators, this one requires an int template<> class Factory { public: typedef InterfaceB*(*Creator)(int); }; /// specializations can define other creators, this one requires an int, a /// bool, and a char template<> class Factory { public: typedef InterfaceC*(*Creator)(int,bool,char); };
Cool. Now we create a static interface that enforces it's derivative classes
to contain a static method called createNew which can be used to instantiate
a new object of that interface. We can use the typedef we just created to make
the function signature generic for this template (or specific to individual
instantiations of it).
template class IStaticFactory { public: IStaticFactory() { typename Factory::Creator check = ClassType::createNew; check = check; } };
Still following? Good. Now we define the registry class template, which maps the class name of a derived class to a function pointer with an interface- specific signature that serves as a static factory for objects of the derived class, returning a pointer to that object of the type of the interface. See my previous post for details on this class.
template class Registry { private: std::map< std::string, typename Factory::Creator > m_creatorMap; Registry(){} public: static Registry& getInstance(); bool registerClass( const std::string& name, typename Factory::Creator creator ); std::set getClassNames(); typename Factory::Creator Registry::getCreator( std::string className ); }; // A convient macro to compact the registration of a class #define RegisterWithInterface( CLASS, INTERFACE ) namespace { bool dummy_ ## CLASS = Registry::getInstance().registerClass( #CLASS, CLASS::createNew ); } template Registry& Registry::getInstance() { static Registry registry; return registry; } template bool Registry::registerClass( const std::string& name, typename Factory::Creator creator ) { m_creatorMap[name] = creator; return true; } template std::set Registry::getClassNames() { std::set keys; typename std::map< std::string, InterfaceType* (*)(void) >::iterator pair; for( pair = m_creatorMap.begin(); pair != m_creatorMap.end(); pair++) keys.insert( pair->first ); return keys; } template typename Factory::Creator Registry::getCreator( std::string className ) { return m_creatorMap[className]; }
The difference between this and the Registry in my previous post, is that this
time the registry uses the generic Factory<InterfaceType>::Creator typedef
to define the function pointer. This way, that pointer is forced to have the
specific signature. Sweet!
Now lets write some derived classes of those interfaces.
class DerivedA : public InterfaceA, public IStaticFactory { public: static InterfaceA* createNew(){ return (InterfaceA*)1; } }; RegisterWithInterface(DerivedA, InterfaceA); class DerivedB : public InterfaceB, public IStaticFactory { public: static InterfaceB* createNew(int a){ return (InterfaceB*)2; } }; RegisterWithInterface(DerivedB, InterfaceB); class DerivedC : public InterfaceC, public IStaticFactory { public: static InterfaceC* createNew(int a, bool b, char c){ return (InterfaceC*)3; } }; RegisterWithInterface(DerivedC, InterfaceC);
These classes are basically dummies, but inheriting from IStaticFactory… the
compiler will enforce that they contain the static method createNew with the
proper signature. Notice that InterfaceA uses the default template so the
static factory in DerivedA takes no parameters, while InterfaceB and
InterfaceC have specializations so the static factories in DerivedB and
DerivedC have their respective parameters. Since this is just an example,
the methods don't actually create new objects they just return pointers, but
in reality this is where we would use new DerivedA(…) and so on.
Well that's it. Pretty cool huh? The compiler will enforce all this stuff for us so we can actually say to ourselves when we write new implementations months from now "If it compiles, it will be compatible."
Lastly, here's a little test case to run
int main() { DerivedA a; DerivedB b; DerivedC c; InterfaceA* pA; InterfaceB* pB; InterfaceC* pC; Factory::Creator makesObjectOfA = Registry::getInstance().getCreator("DerivedA"); pA = (*makesObjectOfA)(); Factory::Creator makesObjectOfB = Registry::getInstance().getCreator("DerivedB"); pB = (*makesObjectOfB)(1); Factory::Creator makesObjectOfC = Registry::getInstance().getCreator("DerivedC"); pC = (*makesObjectOfC)(1,false,'a'); cout << "pA: " << pA << "n"; cout << "pB: " << pB << "n"; cout << "pC: " << pC << "n"; return 0; }
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