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

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)

	~QgsExpression ()

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

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

QString	dump () const
	Return the expression string that represents this QgsExpression. 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...

const QString	expression () const
	Alias for dump() More...

QgsDistanceArea *	geomCalculator ()
	Return calculator used for distance and area calculations (used by internal functions) 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...

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 ()

Q_DECL_DEPRECATED void	setCurrentRowNumber (int rowNumber)
	Set the number for $rownum special column. 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 in evaluation of 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 int	functionCount ()
	Returns the number of functions defined in the parser. More...

static int	functionIndex (const QString &name)

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)

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)
	return quoted column reference (in double quotes) More...

static QString	quotedString (QString text)
	return quoted string (in single quotes) 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=0, const QgsDistanceArea distanceArea=0)
	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=0, const QgsDistanceArea *distanceArea=0)
	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	initGeomCalculator ()

Static Protected Member Functions
static void	initFunctionHelp ()

static void	initVariableHelp ()

Protected Attributes
QgsDistanceArea *	mCalc

QString	mEvalErrorString

QString	mExp

QString	mParserErrorString

Node *	mRootNode

int	mRowNumber

double	mScale

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
}

Possible QVariant value types:

invalid (null)
int
double
string
geometry

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.

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 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 toint, toreal, tostring 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.

Arithmetic operators do integer arithmetics if both operands are integer. That is 2+2 yields integer 4, but 2.0+2 returns real number 4.0. There are also two versions of division and modulo operators: 1.0/2 returns 0.5 while 1/2 returns 0.

Definition at line 92 of file qgsexpression.h.

Member Typedef Documentation

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

Definition at line 343 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 347 of file qgsexpression.h.

typedef QList<WhenThen*> QgsExpression::WhenThenList

Definition at line 859 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 294 of file qgsexpression.h.

enum QgsExpression::NodeType

Enumerator
ntUnaryOperator
ntBinaryOperator
ntInOperator
ntFunction
ntLiteral
ntColumnRef
ntCondition

Definition at line 555 of file qgsexpression.h.

enum QgsExpression::SpatialOperator

Enumerator
soBbox
soIntersects
soContains
soCrosses
soEquals
soDisjoint
soOverlaps
soTouches
soWithin

Definition at line 327 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 284 of file qgsexpression.h.

Constructor & Destructor Documentation

QgsExpression::QgsExpression ( const QString & expr )

Definition at line 2468 of file qgsexpression.cpp.

QgsExpression::~QgsExpression ( )

Definition at line 2480 of file qgsexpression.cpp.

QgsExpression::QgsExpression ( )

inlineprotected

Used by QgsOgcUtils to create an empty.

Definition at line 925 of file qgsexpression.h.

Member Function Documentation

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

Entry function for the visitor pattern.

Definition at line 2630 of file qgsexpression.cpp.

const QStringList & QgsExpression::BuiltinFunctions ( )

static

Definition at line 2143 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 2135 of file qgsexpression.cpp.

Q_DECL_DEPRECATED int QgsExpression::currentRowNumber ( )

inline

Return the number used for $rownum special column.

Definition at line 172 of file qgsexpression.h.

QString QgsExpression::dump ( ) const

Return the expression string that represents this QgsExpression.

Definition at line 2622 of file qgsexpression.cpp.

QString QgsExpression::evalErrorString ( ) const

inline

Returns evaluation error.

Definition at line 165 of file qgsexpression.h.

QVariant QgsExpression::evaluate ( const QgsFeature * f )