11. Extended singleton code

class Singleton {
public:
    static Singleton* get_singleton();

protected:
    static void register_instance( string name, Singleton* ptr );
    Singleton();

private:
    static Singleton* m_ptr;
    static map< string, Singleton* > m_registry;

};//class Singleton

Singleton* Singleton::m_ptr = NULL;

Singleton* Singleton::get_singleton()
{
    if ( m_ptr == NULL ) {
        string name = getenv( "SINGLETON" );
        m_ptr = m_registry[ name ];
    }
    return m_ptr;
}

void Singleton::register_instance( string name, Singleton* ptr )
{
    m_registry[name] = ptr;
}

Subclasses must register themselves with the base class.

Might do this using constructor for new class together with single static object.

This does introduce possible startup sequence problems if using multiple compilation units.

To avoid this, one needs an explicit switch in get_singleton( ), but still no new client code.