Inheritance from a templated class.

Hi all,

I'm working on a C++ class targetting BeagleBone Black/Green using bare metal C/C++ and Starterware.  Since there are multiple peripherals in the AM335x core, I want to make sure that there only exists one instance of each (software)-peripheral corresponding to each (hardware)-peripheral.

I wrote the following templated class : (I excluded the copyright notice for compactness)

#ifndef SYSTEM_ENUMERATEDSINGLETON_H_
#define SYSTEM_ENUMERATEDSINGLETON_H_

#include <map>

using namespace std;

namespace BMB
{

	template<class Type, class Enumerator> class EnumeratedSingleton
	{
		public:
			static Type* getInstance(const Enumerator& key)
			{
				static map<Enumerator, Type*> s_pInstances;

				if (NULL == s_pInstances[key])
				{
					return s_pInstances[key] = new Type(key);
				}
				return s_pInstances[key];
			}

			EnumeratedSingleton(const Enumerator& key)
			:	m_ID(key)
			{

			}

		protected:
			Enumerator	m_ID;

		private:

	};

} /* namespace BMB */

#endif /* SYSTEM_ENUMERATEDSINGLETON_H_ */

For testing purposes I wrote the following (useless atm) class:

#ifndef PERIPHERALS_GPIO_H_
#define PERIPHERALS_GPIO_H_

#include "../System/EnumeratedSingleton.h"

namespace BMB
{
	enum GPIOID
	{
		GPIO0 = 0,
		GPIO1,
		GPIO2,
		GPIO3
	};

	class GPIO : public EnumeratedSingleton<GPIO, GPIOID>
	{
		public:
			virtual ~GPIO();

		protected:
			GPIO(GPIOID id);

		private:
	};

} /* namespace BMB */

#endif /* PERIPHERALS_GPIO_H_ */

The constructor of class GPIO is protected, so that it is inaccessible from outside the class.  Clients must use GPIO::getInstance(...) to obtain access to a peripheral.

There are compilation errors regarding the accessibility of the constructor....

I can solve this by making class GPIO friend with it's base class :

.
.
.
		GPIO3
	};

	class GPIO : public EnumeratedSingleton<GPIO, GPIOID>
	{
		friend class EnumeratedSingleton<GPIO, GPIOID>;

		public:
			virtual ~GPIO();
.
.
.

Can anyone explain why to make a derived class friend to it's base class?

Edit:

this is the main function calling getInstance.  Both pGpio1 and pGpio1Bis points to the same instance ;)

#include "./Peripherals/GPIO.h"
#include "./Peripherals/GPIOPin.h"
#include "./Peripherals/UART.h"

using namespace BMB;

void main(void)
{
	GPIO* pGpio1 = GPIO::getInstance(GPIO1);
	GPIO* pGpio2 = GPIO::getInstance(GPIO2);
	GPIO* pGpio1Bis = GPIO::getInstance(GPIO1);

	return;
}

(end Edit)

Thanks,

Paul