CRTP访问受保护的CTOR派生

Question

I've looked around on stackoverflow, and I don't see my precise issue, so I'm posting it. I am building this with cxx03, so cxx11+ features are not permitted.

I have the following simplified illustration:

template<typename T>
struct unprotect : public T {
    static T *create(void) {
        return new T;
    }
};

template<class T>
struct Base
{
private:
public:    
    static T * Create(void) {
        return unprotect<T>::create();
    }
    static void Destroy(T *p) {
        delete p;
    }
};

struct Derived : public Base<Derived>
{
protected:
    Derived(void){}
};

int main(int argc,const char** argv)
{
    Derived * d = Derived::Create();

    Derived::Destroy(d);
    return( 0 );
}

And this produces the following error in g++ v4.8:

24)~/workspace/tricks $ g++ src/tricks.cpp 
src/tricks.cpp: In instantiation of 'static T* unprotect<T>::create() [with T = Derived]':
src/tricks.cpp:14:37:   required from 'static T* Base<T>::Create() [with T = Derived]':
src/tricks.cpp:29:28:   required from here
src/tricks.cpp:24:5: error: 'Derived::Derived()' is protected
     Derived(void){}
     ^
src/tricks.cpp:4:20: error: within this context
         return new T;

I would have thought that, since unprotect inherits from T, the unprotect::create() would have access to the derived class' protected constructor.

And that's the rub, really: Using CRTP, I want Base to be able to add a create() method that has access to the CTOR for Derived, but does not require Derived to claim friendship with Base.

A bad approach would be to do something like the following:

template<typename T>
struct unprotect : public T {
    static T *create(void) {
        return new unprotect;
    }
};

This would get past the initial problem, but it would balk on (pure) virtuals, and very probably cause a nasty surprise or two.

Is there a way to make the original implementation work? Again, I do not want to add Base friendship to Derived.

Answer 1

Unfortunately in C++ you can't do what you're trying to do. You either have to write the public create function in Derived or allow its constructor to be public (what problem are you trying to solve with a protected constructor here?).

All that protected means is that a derived class's this pointer can be used to access protected members of the parent class. It doesn't grant a child class arbitrary access to arbitrary protected members of arbitrary parent class instances (including when the instance is just created by new ).