C++ Cookbook

8.5. Ensuring a Single Copy of a Member Variable

Problem

You have a member variable that you want only one instance of, no matter how many instances of the class are created. This kind of member variable is generally called a static member or a class variable, as opposed to an instance variable, which is one that is instantiated with every object of a class.

Solution

Declare the member variable with the static keyword, then initialize it in a separate source file (not the header file where you declared it) as in Example 8-5.

Example 8-5. Using a static member variable

// Static.h
class OneStatic {
public:
    int getCount() {return count;}
    OneStatic();
protected:
    static int count;
};

// Static.cpp
#include "Static.h"

int OneStatic::count = 0;

OneStatic::OneStatic() {
   count++;
}

// StaticMain.cpp
#include <iostream>
#include "static.h"

using namespace std;

int main() {
   OneStatic a;
   OneStatic b;
   OneStatic c;

   cout << a.getCount() << endl;
   cout << b.getCount() << endl;
   cout << c.getCount() << endl;
}

Discussion

static is C++’s way of allowing only one copy of something. If you declare a member variable static, only one of it will ever be constructed, regardless of the number of objects of that class that are instantiated. Similarly, if you declare a variable static in a function, it is constructed at most once and retains its value from one function call to another. With member variables, you have to do a little extra work to make sure member variables are allocated properly, though. This ...