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Singleton Design Pattern in C++

Singleton design pattern

  1. Define a private static attribute in the “single instance” class
  2. Define a public static accessor function in the class
  3. Do “lazy initialization” (creation on demand) in the accessor function
  4. Define all constructors to be protected or private
  5. Clients may only use the accessor function to manipulate the Singleton
  6. Inheritance can be supported, but static functions may not be overridden. The base class must be declared a friend of the derived class (in order to access the protected constructor).
#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;

class Number
{
  public:
    // 2. Define a public static accessor func
    static Number *instance(); 
    static void setType(string t)
    {
      type = t;
      delete inst;
      inst = 0;
    }
    virtual void setValue(int in)
    {
      value = in;
    }
    virtual int getValue()
    {
      return value;
    }
  protected:
    int value;
    // 4. Define all ctors to be protected
    Number()
    {
        cout << ":ctor: ";
    } 
  // 1. Define a private static attribute
  private:
    static string type;
    static Number *inst; 
};

string Number::type = "decimal";
Number *Number::inst = 0;

class Octal: public Number
{
  // 6. Inheritance can be supported
  public:
    friend class Number;
    void setValue(int in)
    {
      char buf[10];
      sprintf(buf, "%o", in);
      sscanf(buf, "%d", &value);
    }
  protected:
    Octal(){}
};

Number *Number::instance()
{
  if (!inst)
    // 3. Do "lazy initialization" in the accessor function
    if (type == "octal")
      inst = new Octal();
    else
      inst = new Number();
  return inst;
}

int main()
{
  // Number  myInstance; - error: cannot access protected constructor
  // 5. Clients may only use the accessor function to manipulate the Singleton
  Number::instance()->setValue(42);
  cout << "value is " << Number::instance()->getValue() << endl;
  Number::setType("octal");
  Number::instance()->setValue(64);
  cout << "value is " << Number::instance()->getValue() << endl;
}

Output

:ctor: value is 42
:ctor: value is 100

Code examples

More info, diagrams and examples of the Singleton design pattern you can find on our new resource Refactoring.Guru.