Inspired by answer Johannes Schaub's answer I tried to implement a reflective factory. The idea is that you can create objects by passing the name of the class to a method which creates the concerning object on the heap and returns a pointer to a common base class. Using dynamic_cast and RTTI the objects can be casted back to their original type if necessary. One should be able to use the reflective factory as shown below:
// Base is the base class of all objects to create.
class Factory: public AbstractFactory<Base>
{
public:
Factory()
{
m_map["DerivedA"] = &AbstractFactory::createInstance<DerivedA>;
m_map["DerivedB"] = &AbstractFactory::createInstance<DerivedB>;
}
};
int main()
{
Factory factory;
Base *a = factory.Create("DerivedA");
DerivedA *aa = dynamic_cast<DerivedA*>(a);
// etc..
}
I got this working so far. However there are two main problems I have with the code below. It is ugly and if I make the methods of the abstract factory protected it complaints that it cannot access the createInstance() method. This is something I do not understand. By definition the derived class should be able to access the protected methods of the base class. I tested the code in VS 2008. Further notes: I know the base class is not really abstract, because it does not contain a pure virtual function. I know about std::function however so far I haven't used it. Maybe I'll do in the future.
#include <iostream>
#include <map>
#include <typeinfo>
#include <string>
struct Base
{
virtual std::string SayHello() = 0;
};
struct DerivedA: public Base
{
virtual std::string SayHello()
{
return "Hello from DerivedA";
}
};
struct DerivedB: public Base
{
virtual std::string SayHello()
{
return "Hello form DerivedB";
}
};
/**
* @brief Reflective Factory class. Creates objects of classes which derive from
* a common base class.
*
*/
template<class BASE_T>
struct AbstractFactory
{
// Macro to call ptrs to member functions as recommended in the C++ FAQ lite.
// http://www.parashift.com/c++-faq-lite/pointers-to-members.html
#define CALL_MEMBER_FN(object, ptrToMember) ((object).*(ptrToMember))
// Recall funcion ptr syntax for members: ReturnType (class::*) (Arguments)
// using a typedef makes it easier..
typedef BASE_T* (AbstractFactory::*func_ptr_type) ();
// retType^ ClassName^ AliasName^ Arguments^
typedef std::map<std::string, func_ptr_type> map_type;
template<typename DERIVED_T>
BASE_T * createInstance()
{ return new DERIVED_T; }
map_type m_map;
/**
* @brief Creates an object from a class with the name given as string.
*/
BASE_T * Create(std::string className)
{
// Note the last () at the end.
return CALL_MEMBER_FN(*this, m_map[className])();
}
#undef CALL_MEMBER_FN
};
class Factory: public AbstractFactory<Base>
{
public:
Factory()
{
m_map["DerivedA"] = &AbstractFactory::createInstance<DerivedA>;
m_map["DerivedB"] = &AbstractFactory::createInstance<DerivedB>;
}
};
int main()
{
Factory factory;
Base *a = factory.Create("DerivedA");
DerivedA *aa = dynamic_cast<DerivedA*>(a);
std::cout << typeid(a).name() << std::endl;
std::cout << typeid(*a).name() << std::endl;
std::cout << typeid(aa).name() << std::endl;
std::cout << aa->SayHello() << std::endl;
std::cin.get();
return 0;
}
Update
The exact error I get using VS 2008 is (in German, sorry was not my choice..)
1>------ Erstellen gestartet: Projekt: ReflectiveFactory, Konfiguration: Debug Win32 ------
1>Kompilieren...
1>main.cpp
1>.\main.cpp(82) : error C2248: "AbstractFactory<BASE_T>::createInstance": Kein Zugriff auf protected Member, dessen Deklaration in der AbstractFactory<BASE_T>-Klasse erfolgte.
1> with
1> [
1> BASE_T=Base
1> ]
1> .\main.cpp(55): Siehe Deklaration von 'AbstractFactory<BASE_T>::createInstance'
1> with
1> [
1> BASE_T=Base
1> ]
1>.\main.cpp(83) : error C2248: "AbstractFactory<BASE_T>::createInstance": Kein Zugriff auf protected Member, dessen Deklaration in der AbstractFactory<BASE_T>-Klasse erfolgte.
1> with
1> [
1> BASE_T=Base
1> ]
1> .\main.cpp(55): Siehe Deklaration von 'AbstractFactory<BASE_T>::createInstance'
1> with
1> [
1> BASE_T=Base
1> ]
