declare template friend function of template class

Question

I have a class template Obj and a function template make_obj. Obj has a private single constructor defined, which takes a reference to its templated type to bind to.

template <typename T> class Obj {   private:     T& t;     Obj(T& t)         : t{t}     { } };  template <typename T> Obj<T> make_obj(T& t) {      return {t}; } 

What I want is to declare the make_obj function a friend so that it can create Obj's, but no one else can (except via the copy ctor).

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I have tried several friend declaration including

friend Obj make_obj(T&); 

and

template <typename T1, typename T2> friend Obj<T1> make_obj(T2&); 

The latter being a less than desirable attempt at making all template instantiations of make_obj friends of the Obj class. However in both of these cases I get the same error:

error: calling a private constructor of class 'Obj<char const[6]>'     return {t};            ^  note: in instantiation of function template specialization       'make_obj<const char *>' requested here     auto s = make_obj("hello");              ^ 

trying to do make_obj("hello"); for example purposes.

How can I allow only make_obj access to Obj's value contructor?

Answer 1

You need a few forward declarations:

template <typename T> class Obj;  template <typename T> Obj<T> make_obj(T t);  template <typename T> class Obj { private:     T & t;     Obj (T & t) : t(t) { }     Obj() = delete;      friend Obj make_obj<T>(T t); };  template <typename T> Obj<T> make_obj(T t) {      return Obj<T>(t); } 

live example

And BTW: I don't think you really want T & t; for your class' member variable. Probably T t; is a better choice ;)

Answer 2

With the automatic return type syntax, you only need to forward declare the function and everything works. Here is an example

template <typename T> auto make_obj(T t);  template <typename T> class Obj { private:     T & t;     Obj (T & t) : t(t) { }     Obj() = delete;      friend auto make_obj<T>(T t); };  template <typename T> auto make_obj(T t) {     return Obj<T>{t}; }  int main() {     make_obj(1);     return 0; } 

https://ideone.com/3k86gx