[Boost-users] [flyweights] private constructor, friend access

Subject: [Boost-users] [flyweights] private constructor, friend access
From: r_at_[hidden]
Date: 2015-05-14 12:02:39

Dear all,

On a project of mine, I started to use Boost.Flyweight. I would like 
users of my class to only be able to instantiate objects of my class by 
using the Boost.Flyweight mechanism. For this, I make my class 
constructor private. However, when doing so the corresponding flyweight 
factory also cannot instantiate my class. What classes do I need to 
`friend` in order to make this work?

Here's a small self-contained example of the problem:

class X {
public:
     int data() const { return m_data; }

     struct key_extractor
     {
         int operator()(const X& x) const { return x.data(); }
     }

     typedef boost::flyweights::flyweight
     <
         boost::flyweights::keyvalue
         <
             int,
             X,
             key_extractor
         >
     >
     flyweight;

private:
     int m_data;
     X(int x) : m_data(x) {}
     // friend declarations should go here
};

int main()
{
     auto my_x = X::flyweight(42);
     return 0;
}

Kind regards,

Raoul Wols