Initializing private static variable in class

Question

I have a class in header file:

class Employee
    {
        //Private data members
    private:
        string firstName;
        string lastName;
        char gender;

        //number of employees
        const static int numEmployees = 0;

    public: 
    ....
    };

The dumb thing is in "GUIDELINE" from instructor said that declare numEmployees as a static integer value of 0 in private member of class

Problem is I can't update numEmployees variable since it's const, for example when you declare Constructor in public: .. you can not increase numEmployees = numEmployees + 1.

If you don't declare numEmployees as const, just do static int numEmployees; visual studio 2010 give error said that only const will be declared in class.

Any idea how to declare numEmployees? Thank you!

Answer 1

Since numEmployees is going to change, it should not be const. Non-const static variables have to be initialized outside of the class declaration, for example in the source file, like so:

int Employee::numEmployees = 0;

that being said, numEmployees being a member of the Employee class is probably not the best idea.

And I would make gender an enum, not a char.

Answer 2

In C++, static variables needs to be declared in the class declaration but also defined in an implementation module.

// --- .h interface file
class MyClass
{
    public:
        static int my_static_variable;
    ...
};

// --- .cpp implementation file
#include "myclass.h"
int MyClass::my_static_variable = 0;

There is no real technical argument for this limitation, but it's how the language is defined.

If for some reason you really need to circumvent it, you can use function-level statics:

class MyClass
{
    public:
        // Note: returning a reference to int!
        static int& my_static_variable()
        {
            static int n = 0;
            return n;
        }
};

However, to access the variable, you will need in this case to formally call a method:

MyClass::my_static_variable() = 0;
MyClass::my_static_variable() ++;
MyClass::my_static_variable() *= 2;

I said "formally" because for such a simple function declared inline, the machine code generated by a decent compiler will be the same as the code needed to handle just a variable.

Answer 3

Initialize numEmployees outside of the class as

Employee::numEmployees = 0

and declare it a public member in the class as

static int numEmployees;

You can also declare it private without intialization, as a static member is assigned zero by default.

Answer 4

You have declared it as const. How can you change a const parameter ?

So you can have it as static and initialize to some value you want in the constructor. By default the static variables are initialized to zero.