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<H2><A NAME="SECTION00053000000000000000">
Changing Attributes of a Mapping</A>
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<P>
Many classes of MappingMapping have attributes that provide values for parameter
used within the transformation. For instance, the ZoomMapZoomMap class has an
attribute called ``ZoomZoom'' that gives the scalar value by which each
coordinate is to be multiplied. These attribute values should be set when
the Mapping is created and should not be changed afterwards. Indeed, the
AST library will report an error if an attempt is made to change the
value of a Mapping attribute. This is because, once created, Mappings are
often later included within other objects such as FrameSets and CmpMaps.
This means that in general there could be many active references to a single
Mapping object within a program. Changing an attribute of the Mapping
via one particular reference (i.e pointer) would cause all the other
references to change too, with often undesirable or unpredictable
consequences. To avoid this, Mappings are considered <SPAN CLASS="textit">immutable</SPAN> in
most situations. The one exception is if the Mapping has not yet been
cloned or included in another ObjectObject (<SPAN CLASS="textit">i.e.</SPAN> it has a reference
couint of one) - changing the attributes of such a Mapping is allowed,
and will not generate an error.
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Note, the InvertInvert attribute of a Mapping is not subject to this rule and
can be changed at any time.
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