bug for static functions in template class

Here is myc ode, I could compile it, but could not link. Any body has any idea? Thanks.

===================
#include <iostream.h>

template < class T>
class Base
{
public:
Base();
static int getNum();
static void setNum(int);
private:
static int m_num;
};

static int m_num = 0;

template <class T>
int Base<T>::getNum()
{
return m_num;
}

template <class T>
void Base<T>::setNum(int num)
{
m_num=num;
}

void main()
{
Base<int> A;
A.setNum(10);
Base<double> B;
cout<< "Base<T>::m_mum is " << B.getNum() <<endl;
}

Two things:
* Your declaration of the static member variable is incorrect:

Code:

---

template <class T> int Base<T>::m_num = 0;

---

* Where is your constructor?

Code:

---

template <class T>
Base<T>::Base()
{
}

---

Templates are sneaky fellows. If they are not defined in the .h file you will have problems with the template definition. Template definitions (storage allocation) doesn't occur until you create an object.

If you have your template implementation is in a .cpp it will compile with the <T> type you're using but create only the declaration of the type and move on. Again templates only allocate storage when instantiated. If the template definition is in the .h file the storage will be allocated when instantiated. But there will be no storage allocated if its in a .cpp because its already compiled and it created only the declaration prior to the instance. If you have Bruce Eckel's Thinking in C++ look at page 400 for a better explanation.

thanks

dan

Actually I think that it all is in one file. I think Esss got it right. It looks like linker saw the declaration for the constructor in the class declaration, but it could not find the definition for the constructor.

Always make sure to define what you declare (at least with functions; with variables it won't offend the compiler not to define them), otherwise, the linker will look for the definition of that function, and will be unsuccessful, so it will generate an error.