Question

I was wondering what is the best practice re. passing (another class) amongst two instances of the same class (lets call this ‘Primary’). So, essentially in the constructor for the first, i can initialize the outside instance (lets call this ‘Shared’) – and then set it to a particular value whilst im processing this class in main().

So ‘Shared’, may be an int, say 999 by now.

Now what if i create another instance of the main class ‘Primary’? whats the best way to access the already initialized outside instance of ‘Shared’ – because if i don’t handle this correctly, the constructor for ‘Primary’, when called again will just go ahead and create one more instance of ‘Shared’, and thus i loose the value 999.. i can think of some messy solutions involving dynamic pointers and if statements (just) but i have a feeling there might be a simpler, cleaner solution?

Answer 1

As I understand it:

• You have a class A
• You have a class B
• For all members of class A there is a single instance of class B

You did not mention if any parameters from the A constructor are used to initialize B!
What happens to the parameters of the second A that are used for B?
So we will assume that B is default constructed.
We will also assume that you need the instance of B to be lazily evaluated otherwise you would just use a static member.

class A
{
    B&   bRef;
    public:
        A()
          :bRef(getLazyB())     // Get a reference to the only B for your object.
        {}                      

    private:
        static B& getLazyB()
        {
            static B instance; // Created on first use
            return instance;   // returned to all users.
        }
};