How to ensure a specific class only can create an instance of another class?

Question

How can I restrict the instantiation of a class from only within a specific class?

I don't want to restrict it within a single file, so anonymous namespace is not an option for me.

Please note that I want to make the declaration of the to-be-restricted class visible to the entire world, just that only one specific candidate from the entire world can only instantiate it.

How can I achieve this?

Answer 1

Use friends! Making a class Foo a friend of a class Bar means that Foo can access Bar's private members. If you make the constructor(s) private, only Foo will be able to create an instance of Bar.

struct restricted_t {
    friend struct allowed_t; // allow 'allowed_t' to access my private members
private:
    // nobody can construct me (except for my friends of course)
    restricted_t() = default;
};

struct allowed_t {
    restricted_t yes; // ok
};

struct not_allowed_t {
    restricted_t no; // not ok
};

int main() {
    allowed_t a; // ok, can access constructor
    not_allowed_t b; // error, constructor is private
}