unresolved external, template class prob

When compiling this code I get an unresolved external symbol "public: static class Outer<int>::Inner Outer<int>::inner" with VC6 and "undefined reference to `Outer<int>::inner' " with g++. Perhaps Im missing something obvious but I cant figure it out. Any ideas?

Code:

---

template <class T>
class Outer {
  class Inner
  {
    public:
      void f() {}
  };
  public:
    static Inner inner;
};

int main()
{
  Outer<int>::inner.f();
  return 0;
}

---

I don't see anything obvious either. Perhaps Inner needs to be a templated class aswell.

Because you declare a static member variable but you never define it. This line:

static Inner inner;

does NOT allocate space for an object of class Outer::Inner. It only says to the compiler "I promise that in some source file, I will define Outer<T>::inner.

One issue here is that you have templatized this class; static members and templates mix poorly because of the convoluted way you must both declare and later define the static member. The correct code is:

Code:

---

template <class T>
class Outer {
  class Inner
  {
    public:
      void f() {}
  };
  public:
    static Inner inner;
};

int main()
{
  Outer<int>::inner.f();
  return 0;
}

Outer<int>::Inner Outer<int>::inner;

---

The bad news is that you need to explicitly allocate that static variable, and you can't templatize it. So you should try to do without a static data member if you can possibly avoid it.

It's the same reason that:

Code:

---


class C{
public:
  static int c;
};

int main(){
  C::c = 5; // causes a link error, C::c doesn't exist.
}

---

is a linker error (you declare a static int to exist but don't actually make it happen), while:

Code:

---

class C{
public:
  static int c;
};

int C::c;

int main(){
  C::c = 5;
}

---

is OK.

Thanks Cat!

code involving templates are always giving me the biggest headaches :(