Static members and LNK error in C++

Question

I have a class that has a static member, which I want to use in the class constructor, but the code doesn\'t compile, and I\'m left with these errors:

fat

Answer 1

In your .cpp you need to add:

Collection<A*> A::collection;

The .h only declared that there would be a copy somewhere. You need to provide that copy in the .cpp.

Answer 2

You need to add

Collection<A*> A::collection;

to your a.cpp file.

Answer 3

alternatively, if you don't want to put that line in a cpp file, you can use a static method which returns a reference to a static instance... i.e.

class A
{
public:
  static Collection<A*>& collection()
  {
    static Collection<A*> singleInstance;
    return singleInstance;
  }
};