Class for parsing and evaluation of expressions (formerly called "search strings"). More...

#include <qgsexpression.h>

Inheritance diagram for QgsExpression:

Classes
class	Function
	A abstract base class for defining QgsExpression functions. More...

struct	Help

struct	HelpArg

struct	HelpExample

struct	HelpVariant

class	Interval

class	Node

class	NodeBinaryOperator

class	NodeColumnRef

class	NodeCondition

class	NodeFunction

class	NodeInOperator

class	NodeList

class	NodeLiteral

class	NodeUnaryOperator

class	StaticFunction
	c++ helper class for defining QgsExpression functions. More...

class	Visitor
	Support for visitor pattern - algorithms dealing with the expressions may be implemented without modifying the Node classes. More...

class	WhenThen

Public Types
enum	BinaryOperator { boOr, boAnd, boEQ, boNE, boLE, boGE, boLT, boGT, boRegexp, boLike, boNotLike, boILike, boNotILike, boIs, boIsNot, boPlus, boMinus, boMul, boDiv, boIntDiv, boMod, boPow, boConcat }
	list of binary operators More...

typedef QVariant(*	FcnEval) (const QVariantList &values, const QgsFeature f, QgsExpression parent)

typedef QVariant(*	FcnEvalContext) (const QVariantList &values, const QgsExpressionContext context, QgsExpression parent)
	Function definition for evaluation against an expression context. More...

enum	NodeType { ntUnaryOperator, ntBinaryOperator, ntInOperator, ntFunction, ntLiteral, ntColumnRef, ntCondition }

enum	SpatialOperator { soBbox, soIntersects, soContains, soCrosses, soEquals, soDisjoint, soOverlaps, soTouches, soWithin }

enum	UnaryOperator { uoNot, uoMinus }
	list of unary operators More...

typedef QList< WhenThen * >	WhenThenList

Public Member Functions
	QgsExpression (const QString &expr)
	Creates a new expression based on the provided string. More...

	QgsExpression (const QgsExpression &other)
	Create a copy of this expression. More...

	~QgsExpression ()

void	acceptVisitor (Visitor &v) const
	Entry function for the visitor pattern. More...

QgsUnitTypes::AreaUnit	areaUnits () const
	Returns the desired areal units for calculations involving geomCalculator(), eg "$area". More...

Q_DECL_DEPRECATED int	currentRowNumber ()
	Return the number used for $rownum special column. More...

QGis::UnitType	distanceUnits () const
	Returns the desired distance units for calculations involving geomCalculator(), eg "$length" and "$perimeter". More...

QString	dump () const
	Return an expression string, constructed from the internal abstract syntax tree. More...

QString	evalErrorString () const
	Returns evaluation error. More...

Q_DECL_DEPRECATED QVariant	evaluate (const QgsFeature *f)
	Evaluate the feature and return the result. More...

Q_DECL_DEPRECATED QVariant	evaluate (const QgsFeature &f)
	Evaluate the feature and return the result. More...

Q_DECL_DEPRECATED QVariant	evaluate (const QgsFeature *f, const QgsFields &fields)
	Evaluate the feature and return the result. More...

Q_DECL_DEPRECATED QVariant	evaluate (const QgsFeature &f, const QgsFields &fields)
	Evaluate the feature and return the result. More...

QVariant	evaluate ()
	Evaluate the feature and return the result. More...

QVariant	evaluate (const QgsExpressionContext *context)
	Evaluate the expression against the specified context and return the result. More...

QString	expression () const
	Return the original, unmodified expression string. More...

QgsDistanceArea *	geomCalculator ()
	Return calculator used for distance and area calculations (used by $length, $area and $perimeter functions only) More...

bool	hasEvalError () const
	Returns true if an error occurred when evaluating last input. More...

bool	hasParserError () const
	Returns true if an error occurred when parsing the input expression. More...

bool	isField () const
	Checks whether an expression consists only of a single field reference. More...

bool	needsGeometry () const
	Returns true if the expression uses feature geometry for some computation. More...

QgsExpression &	operator= (const QgsExpression &other)
	Create a copy of this expression. More...

QString	parserErrorString () const
	Returns parser error. More...

Q_DECL_DEPRECATED bool	prepare (const QgsFields &fields)
	Get the expression ready for evaluation - find out column indexes. More...

bool	prepare (const QgsExpressionContext *context)
	Get the expression ready for evaluation - find out column indexes. More...

QStringList	referencedColumns () const
	Get list of columns referenced by the expression. More...

const Node *	rootNode () const
	Returns root node of the expression. Root node is null is parsing has failed. More...

double	scale ()

void	setAreaUnits (QgsUnitTypes::AreaUnit unit)
	Sets the desired areal units for calculations involving geomCalculator(), eg "$area". More...

Q_DECL_DEPRECATED void	setCurrentRowNumber (int rowNumber)
	Set the number for $rownum special column. More...

void	setDistanceUnits (QGis::UnitType unit)
	Sets the desired distance units for calculations involving geomCalculator(), eg "$length" and "$perimeter". More...

void	setEvalErrorString (const QString &str)
	Set evaluation error (used internally by evaluation functions) More...

void	setGeomCalculator (const QgsDistanceArea &calc)
	Sets the geometry calculator used for distance and area calculations in expressions. More...

void	setScale (double scale)

Static Public Member Functions
static const QStringList &	BuiltinFunctions ()

static void	cleanRegisteredFunctions ()
	Deletes all registered functions whose ownership have been transferred to the expression engine. More...

static double	evaluateToDouble (const QString &text, const double fallbackValue)
	Attempts to evaluate a text string as an expression to a resultant double value. More...

static QString	formatPreviewString (const QVariant &value)
	Formats an expression result for friendly display to the user. More...

static int	functionCount ()
	Returns the number of functions defined in the parser. More...

static int	functionIndex (const QString &name)
	return index of the function in Functions array More...

static const QList< Function * > &	Functions ()

static QString	group (const QString &group)
	Returns the translated name for a function group. More...

static bool	hasSpecialColumn (const QString &name)
	Check whether a special column exists. More...

static QString	helptext (QString name)
	Returns the help text for a specified function. More...

static bool	isFunctionName (const QString &name)
	tells whether the identifier is a name of existing function More...

static Q_DECL_DEPRECATED bool	isValid (const QString &text, const QgsFields &fields, QString &errorMessage)

static bool	isValid (const QString &text, const QgsExpressionContext *context, QString &errorMessage)
	Tests whether a string is a valid expression. More...

static QString	quotedColumnRef (QString name)
	Returns a quoted column reference (in double quotes) More...

static QString	quotedString (QString text)
	Returns a quoted version of a string (in single quotes) More...

static QString	quotedValue (const QVariant &value)
	Returns a string representation of a literal value, including appropriate quotations where required. More...

static QString	quotedValue (const QVariant &value, QVariant::Type type)
	Returns a string representation of a literal value, including appropriate quotations where required. More...

static bool	registerFunction (Function *function, bool transferOwnership=false)
	Registers a function to the expression engine. More...

static Q_DECL_DEPRECATED QString	replaceExpressionText (const QString &action, const QgsFeature feat, QgsVectorLayer layer, const QMap< QString, QVariant > substitutionMap=nullptr, const QgsDistanceArea distanceArea=nullptr)
	This function currently replaces each expression between [% and %] in the string with the result of its evaluation on the feature passed as argument. More...

static QString	replaceExpressionText (const QString &action, const QgsExpressionContext context, const QMap< QString, QVariant > substitutionMap=nullptr, const QgsDistanceArea *distanceArea=nullptr)
	This function replaces each expression between [% and %] in the string with the result of its evaluation with the specified context. More...

static Q_DECL_DEPRECATED void	setSpecialColumn (const QString &name, const QVariant &value)
	Assign a special column. More...

static Q_DECL_DEPRECATED QVariant	specialColumn (const QString &name)
	Return the value of the given special column or a null QVariant if undefined. More...

static QList< Function * >	specialColumns ()
	Returns a list of special Column definitions. More...

static bool	unregisterFunction (const QString &name)
	Unregisters a function from the expression engine. More...

static Q_DECL_DEPRECATED void	unsetSpecialColumn (const QString &name)
	Unset a special column. More...

static QString	variableHelpText (const QString &variableName, bool showValue=true, const QVariant &value=QVariant())
	Returns the help text for a specified variable. More...

Static Public Attributes
static const char *	BinaryOperatorText []

static QStringList	gmBuiltinFunctions

static QList< Function * >	gmFunctions

static QList< Function * >	gmOwnedFunctions
	List of functions owned by the expression engine. More...

static const char *	UnaryOperatorText []

Protected Member Functions
	QgsExpression ()
	Used by QgsOgcUtils to create an empty. More...

void	detach ()
	Helper for implicit sharing. More...

void	initGeomCalculator ()

Static Protected Member Functions
static void	initFunctionHelp ()

static void	initVariableHelp ()

Protected Attributes
QgsExpressionPrivate *	d

Static Protected Attributes
static QHash< QString, Help >	gFunctionHelpTexts

static QHash< QString, QString >	gGroups

static QMap< QString, QString >	gmSpecialColumnGroups

static QMap< QString, QVariant >	gmSpecialColumns

static QHash< QString, QString >	gVariableHelpTexts

Friends
class	QgsOgcUtils

Detailed Description

Class for parsing and evaluation of expressions (formerly called "search strings").

The expressions try to follow both syntax and semantics of SQL expressions.

Usage:

QgsExpression exp("gid*2 > 10 and type not in ('D','F'));
if (exp.hasParserError())
{
  // show error message with parserErrorString() and exit
}
QVariant result = exp.evaluate(feature, fields);
if (exp.hasEvalError())
{
  // show error message with evalErrorString()
}
else
{
  // examine the result
}

Three Value Logic

Similarly to SQL, this class supports three-value logic: true/false/unknown. Unknown value may be a result of operations with missing data (NULL). Please note that NULL is different value than zero or an empty string. For example 3 > NULL returns unknown.

There is no special (three-value) 'boolean' type: true/false is represented as 1/0 integer, unknown value is represented the same way as NULL values: invalid QVariant.

Performance

For better performance with many evaluations you may first call prepare(fields) function to find out indices of columns and then repeatedly call evaluate(feature).

Type conversion

Operators and functions that expect arguments to be of a particular type automatically convert the arguments to that type, e.g. sin('2.1') will convert the argument to a double, length(123) will first convert the number to a string. Explicit conversion can be achieved with to_int, to_real, to_string functions. If implicit or explicit conversion is invalid, the evaluation returns an error. Comparison operators do numeric comparison in case both operators are numeric (int/double) or they can be converted to numeric types.

Implicit sharing

This class is implicitly shared, copying has a very low overhead. It is normally preferable to call QgsExpression( otherExpression ) instead of QgsExpression( otherExpression.expression() ). A deep copy will only be made when prepare() is called. For usage this means mainly, that you should normally keep an unprepared master copy of a QgsExpression and whenever using it with a particular QgsFeatureIterator copy it just before and prepare it using the same context as the iterator.

Implicit sharing was added in 2.14

Definition at line 107 of file qgsexpression.h.

Member Typedef Documentation

typedef QVariant( * QgsExpression::FcnEval) (const QVariantList &values, const QgsFeature *f, QgsExpression *parent)

Definition at line 427 of file qgsexpression.h.

typedef QVariant( * QgsExpression::FcnEvalContext) (const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent)

Function definition for evaluation against an expression context.

Definition at line 431 of file qgsexpression.h.

typedef QList<WhenThen*> QgsExpression::WhenThenList

Definition at line 1017 of file qgsexpression.h.

Member Enumeration Documentation

enum QgsExpression::BinaryOperator

list of binary operators

Note: if any change is made here, the definition of QgsExpression::BinaryOperatorText[] must be adapted.

Enumerator
boOr
boAnd
boEQ
boNE
boLE
boGE
boLT
boGT
boRegexp
boLike
boNotLike
boILike
boNotILike
boIs
boIsNot
boPlus
boMinus
boMul
boDiv
boIntDiv
boMod
boPow
boConcat

Definition at line 374 of file qgsexpression.h.

enum QgsExpression::NodeType

Enumerator
ntUnaryOperator
ntBinaryOperator
ntInOperator
ntFunction
ntLiteral
ntColumnRef
ntCondition

Definition at line 675 of file qgsexpression.h.

enum QgsExpression::SpatialOperator

Enumerator
soBbox
soIntersects
soContains
soCrosses
soEquals
soDisjoint
soOverlaps
soTouches
soWithin

Definition at line 408 of file qgsexpression.h.

enum QgsExpression::UnaryOperator

list of unary operators

Note: if any change is made here, the definition of QgsExpression::UnaryOperatorText[] must be adapted.

Enumerator
uoNot
uoMinus

Definition at line 364 of file qgsexpression.h.

Constructor & Destructor Documentation

QgsExpression::QgsExpression ( const QString & expr )

Creates a new expression based on the provided string.

The string will immediately be parsed. For optimization prepare() should alwys be called before every loop in which this expression is used.

Definition at line 3198 of file qgsexpression.cpp.

QgsExpression::QgsExpression ( const QgsExpression & other )

Create a copy of this expression.

This is preferred over recreating an expression from a string since it does not need to be re-parsed.

Definition at line 3206 of file qgsexpression.cpp.

QgsExpression::~QgsExpression ( )

Definition at line 3224 of file qgsexpression.cpp.

QgsExpression::QgsExpression ( )

protected

Used by QgsOgcUtils to create an empty.

Definition at line 3219 of file qgsexpression.cpp.

Member Function Documentation

void QgsExpression::acceptVisitor ( QgsExpression::Visitor & v ) const

Entry function for the visitor pattern.

Definition at line 3439 of file qgsexpression.cpp.

QgsUnitTypes::AreaUnit QgsExpression::areaUnits ( ) const

Returns the desired areal units for calculations involving geomCalculator(), eg "$area".

Note: areas are only converted when a geomCalculator() has been set; added in QGIS 2.14

See also: setAreaUnits(); distanceUnits()

Definition at line 3429 of file qgsexpression.cpp.

const QStringList & QgsExpression::BuiltinFunctions ( )

static

Definition at line 2800 of file qgsexpression.cpp.

void QgsExpression::cleanRegisteredFunctions ( )

static

Deletes all registered functions whose ownership have been transferred to the expression engine.

Note: added in QGIS 2.12

Definition at line 2792 of file qgsexpression.cpp.

int QgsExpression::currentRowNumber ( )

Return the number used for $rownum special column.

Deprecated:: use QgsExpressionContext to retrieve row number instead

Definition at line 3404 of file qgsexpression.cpp.

void QgsExpression::detach ( )

protected

Helper for implicit sharing.

When called will create a new deep copy of this expression.

Note: not available in Python bindings

Definition at line 3276 of file qgsexpression.cpp.

QGis::UnitType QgsExpression::distanceUnits ( ) const

Returns the desired distance units for calculations involving geomCalculator(), eg "$length" and "$perimeter".

Note: distances are only converted when a geomCalculator() has been set; added in QGIS 2.14

See also: setDistanceUnits(); areaUnits()

Definition at line 3419 of file qgsexpression.cpp.

QString QgsExpression::dump ( ) const

Return an expression string, constructed from the internal abstract syntax tree.

This does not contain any nice whitespace formatting or comments. In general it is preferrable to use expression() instead.

Definition at line 3406 of file qgsexpression.cpp.

QString QgsExpression::evalErrorString ( ) const

Returns evaluation error.

Definition at line 3389 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( const QgsFeature * f )