Chapter 13. Singleton objects

x::singleton implements support for classes that always have only a single class instance during the application's lifetime:

#include <x/singleton.H>

class myClassObj : virtual public x::obj {

// class definition


typedef x::ref<myClassObj> myClass;

typedef x::singleton<myClassObj> singleton_t;

// ...

myClass c=singleton_t::get();


The object gets constructed the first time get() gets invoked. Subsequent calls to get() returns a reference to the same object.

The parameter to the template class must be a reference-counted object that has a default constructor.


get() may not be invoked from constructors or destructors of other classes. Constructors and destructors can get called when a thread or an application is in the middle of initializing or terminating, and the underlying support for singleton objects is not available.

An x::singletonptr also implements a singleton object, but with a slightly different lifetime scope.

class global_dataObj : virtual public x::obj {

	// ...

typedef singletonptr<global_dataObj> global_data_t;

int main()
    global_data_t global_data{x::ref<global_dataObj>::create()};

    // ...

// ...

if (foo())
    global_data_t global_data;


An x::singleton object always gets declared in static or application-global scope, and its first get() constructs an object that persists for the duration of the application. The object gets destroyed during the global destruction phase. An x::singletonptr object always gets created in automatic scope, on the stack. The x::singletonptr itself is a subclass of an x::ptr, and gets used like one.

After an x::singletonptr gets constructed in automatic scope with an explicit object, until that instance goes out of scope and gets destroyed, all additional default-constructed instances of the x::singletonptr end up referencing the same object.

A default-constructed x::singletonptr gets constructed as a null x::ptr when no other instance of an explicitly-constructed x::singletonptr exists.

This is the intended design pattern for x::singletonptrs: