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:

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);
}

possible duplicate of [Why won't my C++ program link when my class has static members?](http://stackoverflow.com/questions/1976983/why-wont-my-c-program-link-when-my-class-has-static-members) — Mark Ingram, Oct 28 '10 at 15:13

score 5 · Answer 1 · answered Oct 28 '10 at 13:48

5

You need to add

Collection<A*> A::collection;

to your a.cpp file.

answered Oct 28 '10 at 13:48

harper

score 2 · Answer 2 · answered Oct 28 '10 at 13:50

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.

answered Oct 28 '10 at 13:50

Mark Ransom

score 1 · Answer 3 · answered Oct 28 '10 at 14:14

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;
  }
};

3 Answers3