/** 
 * @file lllazyvalue.h
 * @brief generic functor/value abstraction for lazy evaluation of a value
 * parsing construction parameters from xml and LLSD
 *
 * $LicenseInfo:firstyear=2008&license=viewergpl$
 * 
 * Copyright (c) 2008-2009, Linden Research, Inc.
 * 
 * Second Life Viewer Source Code
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 * 
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 */

#ifndef LL_LAZY_VALUE_H
#define LL_LAZY_VALUE_H

#include <boost/function.hpp>

// Holds on to a value of type T *or* calls a functor to generate a value of type T
template<typename T>
class LLLazyValue
{
public:
	typedef typename boost::add_reference<typename boost::add_const<T>::type>::type	T_const_ref;
	typedef typename boost::function<T_const_ref (void)>							function_type;

public:
	LLLazyValue(const function_type& value) 
	:	mValueGetter(value)
	{} 
	LLLazyValue(T_const_ref value)
	:	mValue(value)
	{}
	LLLazyValue()
	:	mValue()
	{}

	void set(const LLLazyValue& val)
	{
		mValueGetter = val.mValueGetter;
	}

	void set(T_const_ref val)
	{
		mValue = val;
		mValueGetter = NULL;
	}

	T_const_ref get() const
	{
		if (!mValueGetter.empty())
		{
			return mValueGetter();
		}
		return mValue;
	}

	bool isUsingFunction() const
	{
		return mValueGetter != NULL;
	}

private:
	function_type	mValueGetter;
	T				mValue;
};

#endif // LL_LAZY_VALUE_H