C++ Class Static variable problem - C programmer new to C++

Question

Hi guys,

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

Just use session_count++. A static variable is not tied to any instance of a class, and hence it cannot be accessed through the this-pointer. All instances of your class share one instance of session_count. In fact, session_count can be accessed even if no instances of your class exist.

Edit Ok, my answer does not solve the problem, but Charles Bailey`s does.

Answer 2

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.

Answer 3

Not to do with your problem, but in C++ there is no need to typedef classes and structs like this:

typedef class _filter_session
{
  ...
}FILTER_SESSION;

You can and should simply say:

class filter_session
{
  ...
};

Also, c;lass names should not normally be in uppercase, as people will confuse them with macros. and you rarely need to use the this-> construct - your code certainly does not.

Answer 4

You need to initialize the static variable. This code actually compiles:

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

int _filter_session::session_count = 0;


_filter_session::_filter_session(void)
{
    session_count++;
    return;
}


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

int main(int argc, const char **argv)
{
  return 0;
}

Note, I commented FILTER_SESSION to compile on g++/Linux, and also added a main and removed the this-> (as another member mentions, the variable is not a property of the object, but of the class. Think it as a namespaced global variable)