static members and LNK error in C++

Question

Hi, 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:

"fatal error LNK1120: 1 unresolved externals"

"error LNK2001: unresolved external symbol "protected: static class Collection A::collection"

Any help will be appreciated. Thanks.

"a.h"

class A
{
protected:
 static Collection<A*> collection;
};

"a.cpp"

A::A() {
 A::collection.push_back(this);
}

Answer 1

You need to add

Collection<A*> A::collection;

to your a.cpp file.

Answer 2

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 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;
  }
};