How to make std::make_unique a friend of my class

Question

I want to declare std::make_unique function as a friend of my class. The reason is that I want to declare my constructor protected and provide an alternative method of creating the object using unique_ptr. Here is a sample code:

#include <memory>  template <typename T> class A { public:     // Somehow I want to declare make_unique as a friend     friend std::unique_ptr<A<T>> std::make_unique<A<T>>();       static std::unique_ptr<A> CreateA(T x)     {         //return std::unique_ptr<A>(new A(x)); // works         return std::make_unique<A>(x);         // doesn't work     }  protected:     A(T x) { (void)x; } };  int main() {     std::unique_ptr<A<int>> a = A<int>::CreateA(5);     (void)a;     return 0; } 

Right now I get this error:

Start In file included from prog.cc:1: /usr/local/libcxx-head/include/c++/v1/memory:3152:32: error: calling a protected constructor of class 'A<int>' return unique_ptr<_Tp>(new _Tp(_VSTD::forward<_Args>(__args)...));                            ^ prog.cc:13:21: note: in instantiation of function template specialization 'std::__1::make_unique<A<int>, int &>' requested here     return std::make_unique<A>(x);     // doesn't work                 ^ prog.cc:22:41: note: in instantiation of member function 'A<int>::CreateA' requested here std::unique_ptr<A<int>> a = A<int>::CreateA(5);                                     ^ prog.cc:17:5: note: declared protected here A(T x) { (void)x; } ^ 1 error generated. 1 Finish 

What is the correct way to declare std::make_unique as a friend of my class?

Answer 1

make_unique perfect forwards the arguments you pass to it; in your example you're passing an lvalue (x) to the function, so it'll deduce the argument type as int&. Your friend function declaration needs to be

friend std::unique_ptr<A> std::make_unique<A>(T&); 

Similarly, if you were to move(x) within CreateA, the friend declaration would need to be

friend std::unique_ptr<A> std::make_unique<A>(T&&); 

This will get the code to compile, but is in no way a guarantee that it'll compile on another implementation because for all you know, make_unique forwards its arguments to another internal helper function that actually instantiates your class, in which case the helper would need to be a friend.