QgsExpressionNodeColumnRef class

An expression node which takes it value from a feature's field.

Reference

Base classes

class QgsExpressionNode: Abstract base class for all nodes that can appear in an expression.

Constructors, destructors, conversion operators

QgsExpressionNodeColumnRef(const QString& name): Constructor for QgsExpressionNodeColumnRef, referencing the column with the specified name.

Public functions

auto clone() const -> QgsExpressionNode* override: Generate a clone of this node.
auto dump() const -> QString override: Dump this node into a serialized (part) of an expression.
auto evalNode(QgsExpression* parent, const QgsExpressionContext* context) -> QVariant override: Abstract virtual eval method Errors are reported to the parent.
auto isStatic(QgsExpression* parent, const QgsExpressionContext* context) const -> bool override: Returns true if this node can be evaluated for a static value.
auto name() const -> QString: The name of the column.
auto needsGeometry() const -> bool override: Abstract virtual method which returns if the geometry is required to evaluate this expression.
auto nodes() const -> QList<const QgsExpressionNode*> override: Returns a list of all nodes which are used in this expression.
auto nodeType() const -> QgsExpressionNode::NodeType override: Gets the type of this node.
auto prepareNode(QgsExpression* parent, const QgsExpressionContext* context) -> bool override: Abstract virtual preparation method Errors are reported to the parent.
auto referencedColumns() const -> QSet<QString> override: Abstract virtual method which returns a list of columns required to evaluate this node.
auto referencedFunctions() const -> QSet<QString> override: Returns a set of all functions which are used in this expression.
auto referencedVariables() const -> QSet<QString> override: Returns a set of all variables which are used in this expression.

Function documentation

QgsExpressionNode* QgsExpressionNodeColumnRef::clone() const override

Generate a clone of this node.

Returns	a deep copy of this node.

Ownership is transferred to the caller.

QString QgsExpressionNodeColumnRef::dump() const override

Dump this node into a serialized (part) of an expression.

The returned expression does not necessarily literally match the original expression, it's just guaranteed to behave the same way.

QVariant QgsExpressionNodeColumnRef::evalNode(QgsExpression* parent, const QgsExpressionContext* context) override

Abstract virtual eval method Errors are reported to the parent.

bool QgsExpressionNodeColumnRef::isStatic(QgsExpression* parent, const QgsExpressionContext* context) const override

Returns true if this node can be evaluated for a static value.

This is used during the prepare() step and in case it returns true, the value of this node will already be evaluated and the result cached (and therefore not re-evaluated in subsequent calls to eval()). In case this returns true, prepareNode() will never be called.

bool QgsExpressionNodeColumnRef::needsGeometry() const override

Abstract virtual method which returns if the geometry is required to evaluate this expression.

Returns	true if a geometry is required to evaluate this expression

This needs to call needsGeometry() recursively on any child nodes.

QList<const QgsExpressionNode*> QgsExpressionNodeColumnRef::nodes() const override

Returns a list of all nodes which are used in this expression.

QgsExpressionNode::NodeType QgsExpressionNodeColumnRef::nodeType() const override

Gets the type of this node.

Returns	The type of this node

bool QgsExpressionNodeColumnRef::prepareNode(QgsExpression* parent, const QgsExpressionContext* context) override

Abstract virtual preparation method Errors are reported to the parent.

QSet<QString> QgsExpressionNodeColumnRef::referencedColumns() const override

Abstract virtual method which returns a list of columns required to evaluate this node.

Returns	A list of columns required to evaluate this expression

When reimplementing this, you need to return any column that is required to evaluate this node and in addition recursively collect all the columns required to evaluate child nodes.