Static initialization with private constructor

Question

In a class I have a static member that represents the singleton instance of that class:

class A {
public:
  static const std::shared_ptr<A> INSTANCE;
private:
  A();
};

In order to prevent more instances I made the constructor private. Now I have trouble to initialize the static var, because the initializer cannot access a private member. Here's the code I use in the .cpp file:

const std::shared_ptr<A> A::INSTANCE = std::make_shared<A>();

A factory method wouldn't help either, as it would have to be public as well. What else can I do to make this work? Note: I'd like to avoid the typical static get() method if possible.

Alan Stokes · Accepted Answer · 2016-04-24 15:22:10Z

5

You can't use make_shared, but you can just create the instance directly:

const std::shared_ptr<A> A::INSTANCE { new A };

answered Apr 24, 2016 at 15:22

Alan Stokes

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2 Comments

Mike Lischke Over a year ago

I'm baffled. Why can't make_shared() use that approach (or why has it trouble with the visibility while this variant has not)? Anyway, it works, so thanks :-)

Alan Stokes Over a year ago

make_shared is defined outside your class, so it can't access a private constructor. But the initializer for a static member is considered part of the class definition, so it is able to access private members.

orip · Accepted Answer · 2016-04-24 15:18:59Z

0

The initialization of a static member is unrelated to the constructor, so the global statement is indeed the right way to go. Is it not working for you?

EDIT: I just realized you're trying to avoid using a singleton access method for some reason. Sounds suspiciously like the Borg pattern. :) Unrelated to you r specific question but I'd advise you to reconsider.

answered Apr 24, 2016 at 15:18

orip

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