Question

I am a C programmer, but had learnt C++ @school longtime back. Now I am trying to write code in C++ but getting compiler error. Please check and tell me whats wrong with my code.

typedef class _filter_session
{
private:
    static int session_count;  /* Number of sessions count -- Static */    
public:
    _filter_session();         /* Constructor */
    ~_filter_session();        /* Destructor */
}FILTER_SESSION;


_filter_session::_filter_session(void)
{
    (this->session_count)++;
    return;
}


_filter_session::~_filter_session(void)
{
    (this->session_count)--;
    return;
}

The error that I am getting is

“error LNK2001: unresolved external symbol “private: static int _filter_session::session_count” (?session_count@_filter_session@@0HA)”

I am using Visual Studio 2005 by the way.

Plz plz help me.

Regards,

Microkernel

Answer 1

static variables need to be defined outside of the class body somewhere. The declaration inside the class body is just a declaration.

E.g. at global scope:

int _filter_session::session_count;

You need to ensure that this definition occurs only once in the program so usually you would place it in a source file (.cc or .cpp) and not a header file which is included in more than once translation unit.

For portability you should avoid class names that start with an _. There is also little need to typedef your class name. class Name { //... introduces a type name in C++, you wouldn’t have to use class Name to refer to the type.