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// $Id: Nullable.hh 2641 2007-09-02 21:31:02Z flaterco $
// Nullable: mix-in to make a data type nullable.
/*
Copyright (C) 2006 David Flater.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
// Lecture on inheritance, Part 2. (For Part 1, see Amplitude.hh.)
// There are two ways to implement nullability, only one of which
// results in valid inheritance if you make a nullable subclass of a
// non-nullable data type like this:
// class PlainOldNumber { ... }
// class NullableNumber: public PlainOldNumber, public Nullable { ... }
// The "soft" or "lax" implementation enables clients to check for
// nullness, but if they don't, nothing prevents them from using and
// propagating garbage values of the underlying data type. While the
// absence of integrity checking is disturbing, the resulting
// subclasses preserve substitutability.
// The "hard" or "strict" implementation prevents clients from using
// or propagating garbage values. This breaks substitutability
// because an operation that succeeds on every value in the domain of
// the underlying data type may fail on a null value. If the
// operation was written for the underlying data type, it won't even
// know to check for null.
// The hard implementation is not a problem if it is used like this
// with no non-nullable parent class:
// class NullableNumber: public Nullable { ... }
class Nullable {
public:
// No reason to use Nullable alone, hence no public constructor.
// Since there is little chance that I would ever delete a Nullable
// (as opposed to the correct subclass), it is not worth the
// performance hit to force Timestamp to carry around virtual
// function overhead.
// virtual ~Nullable();
const bool isNull () const;
void makeNull ();
protected:
Nullable (); // Makes a null.
Nullable (bool isNull); // Init to null if true.
bool _isNull;
};
// Cleanup2006 Done
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