The singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist. If one does exist it returns a reference to the object that already exists. To make sure that the object cannot be instantiated any other way the constructor is made either private or protected.
The singleton pattern must be carefully constructed in multi-threaded applications. If both threads execute the creation method at the same time when a singleton does not yet exist, they both do a check for an instance of the singleton and then both create a new one.
The classic solution to this problem is to use mutual exclusion on the data that indicates that the object is already instantiated.
A Java programming language solution is as follows (see the Q&A link in external links):
public class Singleton {
// Private constructor suppresses
// default public constructor
private Singleton() {}
public static Singleton getInstance() {
return INSTANCE;
}
}
private static final Singleton INSTANCE = new Singleton();
A possible C++ solution where the singleton is a static local object:
template
public:
class OnlyOne : public Singleton
External Links
static T& Instance()
{
if(!m_instance)
{
static T theSingleInstance;
m_instance = &theSingleInstance;
}
return *m_instance;
}
private:
static T* m_instance;
}; //..rest of interface defined here
};