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Intent |
This pattern describes the network of inheritance relationships that is
needed when a given hierarchy must be adapted, in its entirety, to
another class.
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Motivation |
If a class hierarchy is to be reusable, it cannot depend upon detailed
implementations. For example, one might have a class hierarchy of
payroll objects such as: Employee, SalariedEmployee, HourlyEmployee,
etc. If these classes focus only upon the algorithms necessary to
implement their particular abstractions, then they are highly reusable.
However, if they were to incorporate the methods for reading and
writing such objects on a particular database engine, then they would
not be reusable in applications that did not have access to, or need
that particular engine. Thus, we would like to keep any knowledge of
database engines out of these objects.
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Solution |
To do this, a new set of classes needs to be created that inherits the
ability to read and write themselves using the particular database
engine required by the application, and the methods that model the
payroll abstraction. This keeps the payroll objects separate and,
reusable.
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Notes |
The structure diagram shows the use of virtual inheritance within the
structure of this pattern. This is a C++-ism, but has correspondents in
other languages. The use of virtual inheritance is necessary in this
pattern to prevent the repeated inheritance of the base classes. The
desire is that there be only one copy of all the base objects in any of
the derived objects.
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Applicability |
This pattern is best used to insure the isolation of concepts so that
reusable class hierarchies do not become polluted with concepts that
are application specific.
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[Source: Robert Martin, "Oh! What a relief it is!",
C++ Report, Feb 96, p76]
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