pass-key-idiom.cc

class C {
public:
    class Key {
        friend std::shared_ptr<C> make_c();
        Key() {}
    };
 
    C(const Key &);
    //...
};
 
std::shared_ptr<C> make_c() {
    return std::make_shared<C>(C::Key());
}

Only allow access to methods for a single other method / class without exposing the entire private section by friending.