System.h
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	/*******************************************************************************
	*
	* File: System.h
	*
	* Contents: Declaration of System base-class.
	*
	* Copyright 2017
	*
	* Author: David Juhasz (david.juhasz@tuwien.ac.at)
	*
	******************************************************************************/

	#ifndef ROSA_CORE_SYSTEM_H
	#define ROSA_CORE_SYSTEM_H

	#include <atomic>
	#include <memory>
	#include <string>

	namespace rosa {

	// Forward declarations.
	class Unit;

	// Base-class for actual agent-systems, the class provides facitlities to keep
	// track of Units of the system.
	// NOTE: This class and any subclasses are supposed to provide thread-safe
	// interfaces.
	class System {
	public:
	// Signature of creator functions for Units.
	using UnitCreator = Unit ()(const size_t, const std::string&, System&);

	// Returns an object implementing the interface defined by the class.
	static std::unique_ptr<System> createSystem(const std::string &Name) noexcept;

	protected:
	// Protected ctor, only subclasses can instantiate.
	System(const std::string &Name) noexcept;

	// No copy and move.
	System(const System&) = delete;
	System(System&&) = delete;
	System &operator=(const System&) = delete;
	System &operator=(System&&) = delete;

	public:
	// Dtor. Only a cleared System can be destroyed.
	// PRE: SystemCleaned
	virtual ~System(void);

	protected:

	// Sets SystemCleaned flag. Can be called only once and the System must not
	// have any live Units.
	// PRE: !SystemCleaned && empty()
	// POST: SystemCleaned
	void markCleaned(void) noexcept;

	// Creates a Unit instance with the given UnitCreator, using the given Name
	// if any, then registers the new Unit instance by calling the virtual
	// member function registerUnit.
	// NOTE: This function handles the member field CountUnits.
	// PRE: !SystemCleaned
	Unit &createUnit(UnitCreator C,
	const std::string &Name = std::string()) noexcept;

	// Registers the given Unit instance to the System.
	// PRE: !isUnitRegistered(U)
	// POST: isUnitRegistered(U)
	virtual void registerUnit(Unit &U) noexcept = 0;

	// Unregisters and destroys the given Unit.
	// PRE: isUnitRegistered(U)
	// POST: !isUnitRegistered(U) && 'U is destroyed'
	virtual void destroyUnit(Unit &U) noexcept = 0;

	// Returns if the given Unit is registered in the System.
	virtual bool isUnitRegistered(const Unit &U) const noexcept = 0;

	public:
	// The textual name of the System.
	const std::string Name;

	private:
	// Number of Units constructed in the system.
	// NOTE: Should never be decremented!
	std::atomic<size_t> CountUnits;

	// Indicates that the System has been cleaned and is ready for destruction.
	// The field is initialized as 'false' and can be set by the member function
	// markCleaned.
	// Subclasses must set the flag upon destroying the System instance, which
	// indicates to the destructor of the base-class that all the managed
	// resources has been properly released.
	std::atomic<bool> SystemCleaned;

	public:

	// Returns the number of Units constructed in the System so far,
	// including those being already destroyed.
	size_t numberOfConstructedUnits(void) const noexcept;

	// Returns the number of live Units, that have been constructed and not
	// destroyed yet.
	virtual size_t numberOfLiveUnits(void) const noexcept = 0;

	// Returns if the System has no live Units.
	virtual bool empty(void) const noexcept = 0;
	};

	} // End namespace rosa

	#endif // ROSA_CORE_SYSTEM_H

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