I don't understand why I declare an empty internal structure and pass it in the constructor

Question

When I read the source code of Hypodermic, I found an empty structure declared in the constructor of its container. I don't understand why it did this?

class Container : public std::enable_shared_from_this< Container >
{
   private:
     struct PrivateKey {};
   
   public:
     Container(const PrivateKey&,
                  const std::shared_ptr< IRegistrationScope >& registrationScope,
                  const std::shared_ptr< IRuntimeRegistrationBuilder >& runtimeRegistrationBuilder)
            : m_registrationScope(registrationScope)
            , m_runtimeRegistrationBuilder(runtimeRegistrationBuilder)
     {
     }

     static std::shared_ptr<Container> create(const std::shared_ptr<IRegistrationScope>& registrationScope,
                                                   const std::shared_ptr<IRuntimeRegistrationBuilder>& runtimeRegistrationBuilder)
     {
        return std::make_shared<Container>(PrivateKey(), registrationScope, runtimeRegistrationBuilder);
     }

     // other code...
};

PrivateKey is defined in the above code, I don’t understand what problem it solves. It doesn't seem to do anything.

@underscore_d You can see the entire code through the link below：https://github.com/ybainier/Hypodermic/blob/master/Hypodermic/Container.h — candycat, Jul 13 '20 at 10:39
I don't see any reason for this, they could have just declared the ctor as private. — Sebastian Hoffmann, Jul 13 '20 at 10:44
Possibly used to solve some sort of member address alignment issue? — Adrian Mole, Jul 13 '20 at 10:47
The constructor cannot be declared as private, because the `std::make_shared` needs public access and cannot be made a friend (because of extra dependencies). So the Private Key Idiom is used by the class static factory function, which calls `std::make_shared`, and has access to the private key as a token to access the public constructor. — Eljay, Jul 13 '20 at 11:09
FYI: This answer to [SO: how to block usage of std::make_shared](https://stackoverflow.com/a/58016110/7478597) provides a use case for the constructor with argument of `private` type. — Scheff's Cat, Jul 13 '20 at 11:12

score 1 · Accepted Answer · answered Feb 08 '21 at 14:47

As @Eljay stated in a comment, the constructor has to be accessible when using std::make_shared.

The idea behind this factory method is to make sure no component will get an instance of Container that is not a std::shared_ptr thus enforcing the legitimacy of shared_from_this() inside Container methods.

If you need further explanation, feel free to reach me through GitHub.

I don't understand why I declare an empty internal structure and pass it in the constructor

1 Answers1