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Static member variable

Static member variable is similar to global variable , but it is applied to class.

Static member variables only exist once in a program regardless of how many class objects are defined! 

One way to think about it is that all objects of a class share the static variables. 

(The static member variable is defined in private area in this situation.)

for example :

 

class Something
{
private:
    static int s_nIDGenerator;
    int m_nID;
public:
    Something() { m_nID = s_nIDGenerator++; }
 
    int GetID() const { return m_nID; }
};
 
int Something::s_nIDGenerator = 1;
 
int main()
{
    Something cFirst;
    Something cSecond;
    Something cThird;
 
    using namespace std;
    cout << cFirst.GetID() << endl;
    cout << cSecond.GetID() << endl;
    cout << cThird.GetID() << endl;
    return 0;
}


We can also define the static member variable in the public area , 
it means that the variable exists independently , 
it can be accessed directly using the class name and the scope operator.

for example :

class Something
{
public:
    static int s_nValue;
};
 
int Something::s_nValue = 1;
 
int main()
{
    Something cFirst;
    cFirst.s_nValue = 2;
 
    Something cSecond;
    std::cout << cSecond.s_nValue;
 
    return 0;
}


ref : http://www.learncpp.com/cpp-tutorial/811-static-member-variables/
 
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