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 ...