C++ :: Undefined Reference Error When Accessing Static Variable Inside Member Function

I am modifying a set of static variables inside of the class's member function. The static variables are private. An example of what I'm doing is as below,

utilities.h
-----------
class utilities {
private:
static int num_nodes;

public:
void parse_details(char* );

[Code] ....

I get a compilation error in the function void utilities::parse_details(char* filename)

which says: undefined reference to `utilities::num_nodes'

compiler: g++