Sto solo provando qualcosa - Volevo creare un factory generico che restituisca shared_ptr
a un tipo.C++ Factory che utilizza i problemi dei modelli variadic
Ho una classe derivata che utilizza metodi statici per restituire un shared_ptr
alla classe base. L'idea è che mi piacerebbe poter registrare questi metodi con un factory generico, ma non è in grado di determinare quale metodo registrare al momento della compilazione. Forse c'è un modo per ottenere questo usando SFINAE, ma sto appena cominciando a capire le sue complessità.
Ci scusiamo per l'esempio di codice piuttosto lungo, disponibili anche presso http://coliru.stacked-crooked.com/a/331e08de86004592
attivazione di più di uno dei metodi di fabbrica in 'DerivedA' causeranno un errore di compilazione.
#include <iostream>
#include <string>
#include <vector>
#include <unordered_map>
#include <memory>
// Factory which returns a shared_ptr of type T.
template<class T, class Tag, class... Args>
class NameFactory
{
public:
typedef std::function<std::shared_ptr<T>(Args...)> Function;
static NameFactory& instance();
void registerType(const std::string& type, const Function& createFunction);
std::shared_ptr<T> createObject(const std::string& type, Args&&... arguments);
private:
NameFactory() {}
std::unordered_map<std::string, Function> m_functionMap;
};
template<class T, class Tag, class... Args>
NameFactory<T, Tag, Args...>& NameFactory<T, Tag, Args...>::instance()
{
static NameFactory<T, Tag, Args...> m_instance;
return m_instance;
}
template<class T, class Tag, class... Args>
void NameFactory<T, Tag, Args...>::registerType(const std::string& type, const Function& createFunction)
{
m_functionMap[type] = createFunction;
}
template<class T, class Tag, class... Args>
std::shared_ptr<T> NameFactory<T, Tag, Args...>::createObject(const std::string& type, Args&&... arguments)
{
auto iter(m_functionMap.find(type));
if (iter != m_functionMap.end())
{
return (iter->second)(std::forward<Args>(arguments)...);
}
throw std::logic_error("Cannot find constructor for type '" + type + "'");
}
template<class T, class Tag, class... Args>
class NameFactoryRegistration
{
public:
typedef NameFactory<T, Tag, Args...> Factory;
NameFactoryRegistration(const std::string& type, const typename Factory::Function& createFunction)
{
Factory::instance().registerType(type, createFunction);
}
private:
};
class MyBase
{
public:
typedef std::shared_ptr<MyBase> SPtr;
};
class DerivedA : public MyBase
{
public:
static SPtr create()
{
return SPtr(new DerivedA);
}
// Enabling this factory method (and/or the two args method below causes an 'unresolved overloaded function type' error
//static SPtr create(const std::string& s)
//{
// return SPtr(new DerivedA(s));
//}
//static SPtr create(const std::string& s, double d)
//{
// return SPtr(new DerivedA(s,d));
//}
private:
DerivedA()
{
std::cout << "DerivedA - no args" << std::endl;
}
DerivedA(const std::string& s)
{
std::cout << "DerivedA - one arg: " << s << std::endl;
}
DerivedA(const std::string& s, double d)
{
std::cout << "DerivedA - two args: " << s << " : " << d << std::endl;
}
};
// Tags to help differentiate the factories
struct NoArgsReg;
struct SingleArgReg;
struct TwoArgReg;
typedef NameFactory<MyBase, NoArgsReg> NoArgsFactory;
typedef NameFactoryRegistration<MyBase, NoArgsReg> NoArgsRegistration;
typedef NameFactory<MyBase, SingleArgReg, const std::string&> SingleArgFactory;
typedef NameFactoryRegistration<MyBase, SingleArgReg, const std::string&> SingleArgRegistration;
typedef NameFactory<MyBase, TwoArgReg, const std::string&, double> TwoArgsFactory;
typedef NameFactoryRegistration<MyBase, TwoArgReg, const std::string&, double> TwoArgsRegistration;
// Register the factory methods into the NameFactory
NoArgsRegistration dAReg0("A", DerivedA::create);
//SingleArgRegistration dAReg1("A", DerivedA::create);
//TwoArgsRegistration dAReg2("A", DerivedA::create);
int main()
{
auto object0(NoArgsFactory::instance().createObject("A"));
// Not registered,
//auto object1(SingleArgFactory::instance().createObject("A","testString"));
//auto object2(TwoArgsFactory::instance().createObject("A","testString",3.142));
return 0;
}
Fantastico, grazie Barry - molto utile sapere! Non avevo pensato di usare i puntatori di funzione, avevo considerato std :: function come un "magic bullet" che poteva sostituirli completamente. – DaveM