Feature request #9546
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There is an increasing number of additional features in expression engine that make sense only in particular context. These features are available either directly in QgsExpression API (e.g. $rownum, $scale) or through "special columns" (e.g. $map, $page, ...). Additionally, with special columns, their value is shared between all QgsExpression instances, which may eventually lead to problems with concurrency (especially with multi-threading).