Using static modifier with template classes

// Class to store variable information.
template <class PType> class variable
{
  // Counter to ensure maximum limit of variables not breached.
  static int counter;
public:

  // Variable name and value.
  char *name;
  PType value;

  // Constructors.
  variable(void);
  variable(char *n, PType v);
};
//int variable<PType>::counter;

template <class T> class key
{
public:
      static T x;
};
template <class T> T key<T> ::x; // template definition
void main()
{
      key<int>::x = 0;
}

template <class PType> class variable
{
  static int counter;
public:

  // Variable name and value.
  char *name;
  PType value;

  // Constructors.
  variable(void);
  variable(char *n, PType v);
};

template<class PType> int variable<PType>::counter;

int main()
{
    variable<double>::counter=0;
...
...
...
}