#include <string>
#include <iostream>
 
template <class T, class Factory, Factory f> class add_ctor {
    public:
        add_ctor() : obj(f()()) {
        }
        T* operator->() {
            return obj;
        }
    private:
        T* obj;
};
 
class A {
    public:
        std::string v;
        static A createOne();
    private:
        A();
};
A A::createOne() {
    A a;
    a.v = "Hello!";
    return a;
}
 
int main() {
    // this should work:
    add_ctor<A, decltype(&A::createOne), &A::createOne> added_a;
    std::cout << added_a->v << "\n";
 
    // this should error:
    //A non_added_a;
    //std::cout << non_added_a.v << "\n";
    return 0;
}

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