Question

I want do make some classes, which will have private/protected constructors, and will have static method named create().
Let me show you the code:

template <class T>
struct ServicePtr
{
    std::shared_ptr<T> service;
};

template <class T>
struct ServicePtrDeleter
{
    void operator()(T* ref) const
    {
        if (ref->service.get())
        {
        }

        delete ref;
    }
};

template <typename T>
struct ServiceCreator
{
    static std::shared_ptr< ServicePtr<T> > create()
    {
        std::shared_ptr< ServicePtr<T> > servicePtr(new ServicePtr<T>);
        servicePtr->service.reset(new T);
        return servicePtr;
    }
};

class S:public IService,
public ServiceCreator<S>
{
    protected:
    S()
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
    }

    public:
    virtual ~S()
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
    }
};

int main()
{
    auto s=S::create();
    return 0;
}

Compiling with GCC 4.6.1, I am given the following error:

main.cc: In static member function ‘static std::shared_ptr > ServiceCreator::create() [with T = S]’:
main.cc:310:12: instantiated from here
main.cc:275:3: error: ‘S::S()’ is protected
main.cc:177:3: error: within this context
make: * [main.o] Error 1

Answer 1

The main problem is that ServiceCreator<T>::create() is going to try and instantiate S over here:

static std::shared_ptr< ServicePtr<T> > create()
{
    // ...
    servicePtr->service.reset(new T); // T = class S
    return servicePtr;
}

But of course it cannot do that because class S‘s constructor is protected! An easy way to fix this is to just make ServiceCreator a friend to S:

class S : public IService, public ServiceCreator<S>
{
  friend class ServiceCreator<S>;
  // ...
};