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!

possible duplicate of [Initializing private static members](http://stackoverflow.com/questions/185844/initializing-private-static-members) — johnsyweb, May 20 '12 at 08:44

score 6 · Answer 1 · edited May 20 '12 at 08:47

6

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.

edited May 20 '12 at 08:47

Cody Gray - on strike

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answered May 20 '12 at 08:33

stijn

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The dump thing is in "GUIDELINE" from instructor said that declare numEmployees as a static integer value of 0 in private member of class – Ronaldinho Learn Coding May 20 '12 at 08:36
@stijn We don't tag questions homework because that's what they *sound* like. Questions should only be tagged [homework] if someone is looking for homework-style answers (e.g., those that *hint* at the solution without providing actual working code). It's the asker's choice to tag his questions as (s)he pleases. Please do not try and force people to add this tag. – Cody Gray - on strike May 20 '12 at 08:48
Never mind, I can accept that, "homework" make me feel good 'cause I dont have any hw for longtime. OK – Ronaldinho Learn Coding May 20 '12 at 08:55
@CodyGray thanks for pointing that out, I never actually read the whole story behind the homework tag. – stijn May 20 '12 at 13:13

score 2 · Answer 2 · edited May 20 '12 at 08:49

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.

score 1 · Accepted Answer · edited May 20 '12 at 08:51

1

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.

edited May 20 '12 at 08:51

Cody Gray - on strike

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answered May 20 '12 at 08:30

ravi

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I meant in class definition.put current declaration without const in public access. – ravi May 20 '12 at 08:34
The dump thing is in "GUIDELINE" from instructor said that declare numEmployees as a static integer value of 0 in private member of class – Ronaldinho Learn Coding May 20 '12 at 08:37
static member is initialized with 0 always – ravi May 20 '12 at 08:41

Jay D · Answer 4 · 2012-05-20T08:41:55.720

1

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.

edited May 20 '12 at 08:41

answered May 20 '12 at 08:32

Jay D

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The dump thing is in "GUIDELINE" from instructor said that declare numEmployees as a static integer value of 0 in private member of class – Ronaldinho Learn Coding May 20 '12 at 08:37

Initializing private static variable in class

4 Answers4