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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

8.9. Creating a Singleton Class

Problem

You have a class that must only ever be instantiated once, and you need to provide a way for clients to access that class in such a way that the same, single object is returned each time. This is commonly referred to as a singleton pattern, or a singleton class.

Solution

Create a static member that is a pointer to the current class, restrict the use of constructors to create the class by making them private, and provide a public static member function that clients can use to access the single, static instance. Example 8-9 demonstrates how to do this.

Example 8-9. Creating a singleton class

#include <iostream>

using namespace std;

class Singleton {
public:
   // This is how clients can access the single instance
   static Singleton* getInstance();

   void setValue(int val) {value_ = val;}
   int  getValue()           {return(value_);}

protected:
   int value_;

private:
   static Singleton* inst_;   // The one, single instance
   Singleton() : value_(0) {} // private constructor
   Singleton(const Singleton&);
   Singleton& operator=(const Singleton&);
};

// Define the static Singleton pointer
Singleton* Singleton::inst_ = NULL;

Singleton* Singleton::getInstance() {
   if (inst_ == NULL) {
      inst_ = new Singleton();
   }
   return(inst_);
}

int main() {

   Singleton* p1 = Singleton::getInstance();

   p1->setValue(10);
    
   Singleton* p2 = Singleton::getInstance();

   cout << "Value = " << p2->getValue() << '\n';
}

Discussion

There are many situations where you want at most one instance of a class—this is why ...

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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page