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C++ Cookbook
book

C++ Cookbook

by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell
November 2005
Beginner to intermediate content levelBeginner to intermediate
594 pages
16h 23m
English
O'Reilly Media, Inc.
Content preview from 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 ...

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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page