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Nullable.h
//
// Nullable.h
//
// $Id: //poco/1.4/Foundation/include/Poco/Nullable.h#1 $
//
// Library: Foundation
// Package: Core
// Module:  Nullable
//
// Definition of the Nullable template class.
//
// Copyright (c) 2010, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
// 
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//


#ifndef Foundation_Nullable_INCLUDED
#define Foundation_Nullable_INCLUDED


#include "Poco/Foundation.h"
#include "Poco/Exception.h"
#include <algorithm>


namespace Poco {


template <typename C>
00052 class Nullable
      /// Nullable is a simple wrapper class for value types
      /// that allows to introduce a "null" value or state
      /// to value objects.
      ///
      /// The class is useful for passing parameters to functions
      /// when parameters are optional and no default values 
      /// should be used.
      ///
      /// A Nullable can be default constructed. In this case, 
      /// the Nullable will have a Null value and isNull() will
      /// return true. Calling value() (without default value) on
      /// a Null object will throw a NullValueException.
      ///
      /// A Nullable can also be constructed from a value.
      /// It is possible to assign a value to a Nullable, and
      /// to reset a Nullable to contain a Null value by calling
      /// clear().
      ///
      /// For use with Nullable, the value type should support
      /// default construction.
{
public:
      Nullable(): 
            /// Creates an empty Nullable.
            _value(),
            _isNull(true)
      {
      }

      Nullable(const C& value): 
            /// Creates a Nullable with the given value.
            _value(value), 
            _isNull(false)
      {
      }
      
      Nullable(const Nullable& other):
            /// Creates a Nullable by copying another one.
            _value(other._value),
            _isNull(other._isNull)
      {
      }

00096       ~Nullable()
            /// Destroys the Nullable.
      {
      }

00101       Nullable& assign(const C& value)
            /// Assigns a value to the Nullable.
      {
            _value  = value;
            _isNull = false;
            return *this;
      }
      
00109       Nullable& assign(const Nullable& other)
            /// Assigns another Nullable.
      {
            Nullable tmp(other);
            swap(tmp);
            return *this;
      }
      
      Nullable& operator = (const C& value)
      {
            return assign(value);
      }

      Nullable& operator = (const Nullable& other)
      {
            return assign(other);
      }

      void swap(Nullable& other)
      {
            std::swap(_value, other._value);
            std::swap(_isNull, other._isNull);
      }

00133       const C& value() const
            /// Returns the Nullable's value.
            ///
            /// Throws a NullValueException if the Nullable is empty.
      {
            if (!_isNull)
                  return _value;
            else
                  throw NullValueException();
      }

00144       const C& value(const C& deflt) const
            /// Returns the Nullable's value, or the
            /// given default value if the Nullable is empty.
      {
            return _isNull ? deflt : _value;
      }

00151       bool isNull() const
            /// Returns true iff the Nullable is empty.
      {
            return _isNull;
      }
      
00157       void clear()
            /// Clears the Nullable.
      {
            _isNull = true;
      }

private:
      C _value;
      bool _isNull;
};


template <typename C>
inline void swap(Nullable<C>& n1, Nullable<C>& n2)
{
      n1.swap(n2);
}


} // namespace Poco


#endif // Foundation_Nullable_INCLUDED

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