The static keyword and its various uses in C++

Question

The keyword static is one which has several meanings in C++ that I find very confusing and I can never bend my mind around how its actually supposed to work.

Answer 1

I'm not a C programmer so I can't give you information on the uses of static in a C program properly, but when it comes to Object Oriented programming static basically declares a variable, or a function or a class to be the same throughout the life of the program. Take for example.

class A
{
public:
    A();
    ~A();
    void somePublicMethod();
private:
    void somePrivateMethod();
};

When you instantiate this class in your Main you do something like this.

int main()
{
   A a1;
   //do something on a1
   A a2;
   //do something on a2
}

These two class instances are completely different from each other and operate independently from one another. But if you were to recreate the class A like this.

class A
{
public:
    A();
    ~A();
    void somePublicMethod();
    static int x;
private:
    void somePrivateMethod();
};

Lets go back to the main again.

int main()
{
   A a1;
   a1.x = 1;
   //do something on a1
   A a2;
   a2.x++;
   //do something on a2
}

Then a1 and a2 would share the same copy of int x whereby any operations on x in a1 would directly influence the operations of x in a2. So if I was to do this

int main()
{
   A a1;
   a1.x = 1;
   //do something on a1
   cout << a1.x << endl; //this would be 1
   A a2;
   a2.x++;
   cout << a2.x << endl; //this would be 2 
   //do something on a2
}

Both instances of the class A share static variables and functions. Hope this answers your question. My limited knowledge of C allows me to say that defining a function or variable as static means it is only visible to the file that the function or variable is defined as static in. But this would be better answered by a C guy and not me. C++ allows both C and C++ ways of declaring your variables as static because its completely backwards compatible with C.