setting static member variable inside a static method [duplicate]

Question

I am beginner to C++ and have a doubt about static member variables and member functions.

I have implemented a class as follows -

class Foo
{
private:
    static int myVariable;
public:
    static void setMyVariable()
    {
        myVariable = 100;
    }

    static void resetMyVariable()
    {
        myVariable = 0;
    }
};

There are following considerations when I wrote a code like that -

I want only one instance of class Foo. Thats why I made all member variables and functions as static.
I don't want the outside code to touch myVariable

I have put this class in a header file and included in my main file. When I do this, I get an error undefined reference to Foo::myVariable

I want to know if I can write a code which can satisfy above requirements?

Thanks !

If you only want one instance you should take a look at the singleton pattern (now flame me haters ;) ) — Sebastian Hoffmann
– Sebastian Hoffmann, Commented Dec 31, 2013 at 10:54
You want a set of free functions and a static global variable in a separate C++ file, neither a singleton or a class with only static members — Davidbrcz
– Davidbrcz, Commented Dec 31, 2013 at 11:02
Singletons can be an evil thing. But if he wants only one instance of the class, then he probably wants non-static member variables with a Singleton pattern in preference to a whole batch of static stuff. — RichardPlunkett
– RichardPlunkett, Commented Dec 31, 2013 at 11:06

doctorlove · Accepted Answer · 2013-12-31 10:25:27Z

2

You need to define static class variables somewhere: e.g. in your main C++ file,

int Foo::myVariable;

Note that technically, by making everything static, you may have no instances of Foo.

answered Dec 31, 2013 at 10:25

doctorlove

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2 Comments

Raj Over a year ago

thanks for the answer. I understood now what you mean and with other previous comments. Since now I have to define this variable in main, does it mean that it is exposed outside and no longer private.

Angew is no longer proud of SO Over a year ago

@Raj No, it's not exposed. Access control still applies when referring to the variable.

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