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QgsExpression Class Reference

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

#include <qgsexpression.h>

Inheritance diagram for QgsExpression:
Inheritance graph
[legend]

Classes

class  Function
 A abstract base class for defining QgsExpression functions. More...
 
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...
 
QgsDistanceAreageomCalculator ()
 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 NoderootNode () 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 QStringListBuiltinFunctions ()
 
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 (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 void setSpecialColumn (const QString &name, QVariant value)
 Assign a special column. More...
 
static 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 (QString name)
 Unregisters a function from the expression engine. More...
 
static 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

QgsDistanceAreamCalc
 
QString mEvalErrorString
 
QString mExp
 
QString mParserErrorString
 
NodemRootNode
 
int mRowNumber
 
double mScale
 

Static Protected Attributes

static QHash< QString, QStringgFunctionHelpTexts
 
static QHash< QString, QStringgGroups
 
static QMap< QString, QStringgmSpecialColumnGroups
 
static QMap< QString, QVariantgmSpecialColumns
 
static QHash< QString, QStringgVariableHelpTexts
 

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:

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 88 of file qgsexpression.h.

Member Typedef Documentation

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

Definition at line 328 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 332 of file qgsexpression.h.

Definition at line 841 of file qgsexpression.h.

Member Enumeration Documentation

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 279 of file qgsexpression.h.

Enumerator
ntUnaryOperator 
ntBinaryOperator 
ntInOperator 
ntFunction 
ntLiteral 
ntColumnRef 
ntCondition 

Definition at line 537 of file qgsexpression.h.

Enumerator
soBbox 
soIntersects 
soContains 
soCrosses 
soEquals 
soDisjoint 
soOverlaps 
soTouches 
soWithin 

Definition at line 312 of file qgsexpression.h.

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 269 of file qgsexpression.h.

Constructor & Destructor Documentation

QgsExpression::QgsExpression ( const QString expr)

Definition at line 2139 of file qgsexpression.cpp.

QgsExpression::~QgsExpression ( )

Definition at line 2151 of file qgsexpression.cpp.

QgsExpression::QgsExpression ( )
inlineprotected

Used by QgsOgcUtils to create an empty.

Definition at line 907 of file qgsexpression.h.

Member Function Documentation

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

Entry function for the visitor pattern.

Definition at line 2301 of file qgsexpression.cpp.

const QStringList & QgsExpression::BuiltinFunctions ( )
static

Definition at line 1807 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 1799 of file qgsexpression.cpp.

Q_DECL_DEPRECATED int QgsExpression::currentRowNumber ( )
inline

Return the number used for $rownum special column.

Definition at line 167 of file qgsexpression.h.

QString QgsExpression::dump ( ) const

Return the expression string that represents this QgsExpression.

Definition at line 2293 of file qgsexpression.cpp.

QString QgsExpression::evalErrorString ( ) const
inline

Returns evaluation error.

Definition at line 160 of file qgsexpression.h.

QVariant QgsExpression::evaluate ( const QgsFeature f)

Evaluate the feature and return the result.

Note
prepare() should be called before calling this method

Definition at line 2230 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( const QgsFeature f)

Evaluate the feature and return the result.

Note
prepare() should be called before calling this method
available in python bindings as evaluatePrepared

Definition at line 2243 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( const QgsFeature f,
const QgsFields fields 
)

Evaluate the feature and return the result.

Note
this method does not expect that prepare() has been called on this instance

Definition at line 2250 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( const QgsFeature f,
const QgsFields fields 
)
inline

Evaluate the feature and return the result.

Note
this method does not expect that prepare() has been called on this instance
not available in python bindings

Definition at line 2262 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( )

Evaluate the feature and return the result.

Note
this method does not expect that prepare() has been called on this instance
added in QGIS 2.12

Definition at line 2269 of file qgsexpression.cpp.

QVariant QgsExpression::evaluate ( const QgsExpressionContext context)

Evaluate the expression against the specified context and return the result.

Parameters
contextcontext for evaluating expression
Note
prepare() should be called before calling this method.
added in QGIS 2.12

Definition at line 2281 of file qgsexpression.cpp.

double QgsExpression::evaluateToDouble ( const QString text,
const double  fallbackValue 
)
static

Attempts to evaluate a text string as an expression to a resultant double value.

Parameters
texttext to evaluate as expression
fallbackValuevalue to return if text can not be evaluated as a double
Returns
evaluated double value, or fallback value
Note
added in QGIS 2.7
this method is inefficient for bulk evaluation of expressions, it is intended for one-off evaluations only.

Definition at line 2386 of file qgsexpression.cpp.

const QString QgsExpression::expression ( ) const
inline

Alias for dump()

Definition at line 200 of file qgsexpression.h.

int QgsExpression::functionCount ( )
static

Returns the number of functions defined in the parser.

Returns
The number of function defined in the parser.

Definition at line 2133 of file qgsexpression.cpp.

int QgsExpression::functionIndex ( const QString name)
static

Definition at line 2117 of file qgsexpression.cpp.

const QList< QgsExpression::Function * > & QgsExpression::Functions ( )
static

Definition at line 1853 of file qgsexpression.cpp.

QgsDistanceArea* QgsExpression::geomCalculator ( )
inline

Return calculator used for distance and area calculations (used by internal functions)

Definition at line 213 of file qgsexpression.h.

QString QgsExpression::group ( QString  group)
static

Returns the translated name for a function group.

Parameters
groupuntranslated group name

Definition at line 3202 of file qgsexpression.cpp.

bool QgsExpression::hasEvalError ( ) const
inline

Returns true if an error occurred when evaluating last input.

Definition at line 158 of file qgsexpression.h.

bool QgsExpression::hasParserError ( ) const
inline

Returns true if an error occurred when parsing the input expression.

Definition at line 95 of file qgsexpression.h.

bool QgsExpression::hasSpecialColumn ( const QString name)
static

Check whether a special column exists.

Note
added in 2.2

Definition at line 2037 of file qgsexpression.cpp.

QString QgsExpression::helptext ( QString  name)
static

Returns the help text for a specified function.

Parameters
namefunction name
See Also
variableHelpText()

Definition at line 3125 of file qgsexpression.cpp.

void QgsExpression::initFunctionHelp ( )
staticprotected

Definition at line 6 of file qgsexpression_texts.cpp.

void QgsExpression::initGeomCalculator ( )
protected

Definition at line 2188 of file qgsexpression.cpp.

void QgsExpression::initVariableHelp ( )
staticprotected

Definition at line 3133 of file qgsexpression.cpp.

bool QgsExpression::isField ( ) const
inline

Checks whether an expression consists only of a single field reference.

Note
added in 2.9

Definition at line 182 of file qgsexpression.h.

bool QgsExpression::isFunctionName ( const QString name)
static

Definition at line 2112 of file qgsexpression.cpp.

bool QgsExpression::isValid ( const QString text,
const QgsFields fields,
QString errorMessage 
)
static

Definition at line 2072 of file qgsexpression.cpp.

bool QgsExpression::isValid ( const QString text,
const QgsExpressionContext context,
QString errorMessage 
)
static

Tests whether a string is a valid expression.

Parameters
textstring to test
contextoptional expression context
errorMessagewill be filled with any error message from the validation
Returns
true if string is a valid expression
Note
added in QGIS 2.12

Definition at line 2078 of file qgsexpression.cpp.

bool QgsExpression::needsGeometry ( ) const

Returns true if the expression uses feature geometry for some computation.

Definition at line 2181 of file qgsexpression.cpp.

QString QgsExpression::parserErrorString ( ) const
inline

Returns parser error.

Definition at line 97 of file qgsexpression.h.

bool QgsExpression::prepare ( const QgsFields fields)

Get the expression ready for evaluation - find out column indexes.

Definition at line 2204 of file qgsexpression.cpp.

bool QgsExpression::prepare ( const QgsExpressionContext context)

Get the expression ready for evaluation - find out column indexes.

Parameters
contextcontext for preparing expression
Note
added in QGIS 2.12

Definition at line 2210 of file qgsexpression.cpp.

QString QgsExpression::quotedColumnRef ( QString  name)
static

return quoted column reference (in double quotes)

Definition at line 2098 of file qgsexpression.cpp.

QString QgsExpression::quotedString ( QString  text)
static

return quoted string (in single quotes)

Definition at line 2103 of file qgsexpression.cpp.

QStringList QgsExpression::referencedColumns ( ) const

Get list of columns referenced by the expression.

Note
if the returned list contains the QgsFeatureRequest::AllAttributes constant then all attributes from the layer are required for evaluation of the expression. QgsFeatureRequest::setSubsetOfAttributes automatically handles this case.

Definition at line 2157 of file qgsexpression.cpp.

bool QgsExpression::registerFunction ( QgsExpression::Function function,
bool  transferOwnership = false 
)
static

Registers a function to the expression engine.

This is required to allow expressions to utilise the function.

Parameters
functionfunction to register
transferOwnershipset to true to transfer ownership of function to expression engine
Returns
true on successful registration
See Also
unregisterFunction

Definition at line 1770 of file qgsexpression.cpp.

QString QgsExpression::replaceExpressionText ( const QString action,
const QgsFeature feat,
QgsVectorLayer layer,
const QMap< QString, QVariant > *  substitutionMap = 0,
const QgsDistanceArea distanceArea = 0 
)
static

This function currently replaces each expression between [% and %] in the string with the result of its evaluation on the feature passed as argument.

Additional substitutions can be passed through the substitutionMap parameter

Parameters
action
feat
layer
substitutionMap
distanceAreaoptional QgsDistanceArea. If specified, the QgsDistanceArea is used for distance and area conversion

Definition at line 2307 of file qgsexpression.cpp.

QString QgsExpression::replaceExpressionText ( const QString action,
const QgsExpressionContext context,
const QMap< QString, QVariant > *  substitutionMap = 0,
const QgsDistanceArea distanceArea = 0 
)
static

This function replaces each expression between [% and %] in the string with the result of its evaluation with the specified context.

Additional substitutions can be passed through the substitutionMap parameter

Parameters
action
contextexpression context
substitutionMap
distanceAreaoptional QgsDistanceArea. If specified, the QgsDistanceArea is used for distance and area conversion
Note
added in QGIS 2.12

Definition at line 2315 of file qgsexpression.cpp.

const Node* QgsExpression::rootNode ( ) const
inline

Returns root node of the expression. Root node is null is parsing has failed.

Definition at line 102 of file qgsexpression.h.

double QgsExpression::scale ( )
inline

Definition at line 197 of file qgsexpression.h.

Q_DECL_DEPRECATED void QgsExpression::setCurrentRowNumber ( int  rowNumber)
inline

Set the number for $rownum special column.

Definition at line 165 of file qgsexpression.h.

void QgsExpression::setEvalErrorString ( const QString str)
inline

Set evaluation error (used internally by evaluation functions)

Definition at line 162 of file qgsexpression.h.

void QgsExpression::setGeomCalculator ( const QgsDistanceArea calc)

Sets the geometry calculator used in evaluation of expressions,.

Definition at line 2198 of file qgsexpression.cpp.

void QgsExpression::setScale ( double  scale)
inline

Definition at line 195 of file qgsexpression.h.

void QgsExpression::setSpecialColumn ( const QString name,
QVariant  value 
)
static

Assign a special column.

Definition at line 2001 of file qgsexpression.cpp.

QVariant QgsExpression::specialColumn ( const QString name)
static

Return the value of the given special column or a null QVariant if undefined.

Definition at line 2021 of file qgsexpression.cpp.

QList< QgsExpression::Function * > QgsExpression::specialColumns ( )
static

Returns a list of special Column definitions.

Definition at line 2086 of file qgsexpression.cpp.

bool QgsExpression::unregisterFunction ( QString  name)
static

Unregisters a function from the expression engine.

The function will no longer be usable in expressions.

Parameters
namefunction name
See Also
registerFunction

Definition at line 1783 of file qgsexpression.cpp.

void QgsExpression::unsetSpecialColumn ( const QString name)
static

Unset a special column.

Definition at line 2012 of file qgsexpression.cpp.

QString QgsExpression::variableHelpText ( const QString variableName,
bool  showValue = true,
const QVariant value = QVariant() 
)
static

Returns the help text for a specified variable.

Parameters
variableNamename of variable
showValueset to true to include current value of variable in help text
valuecurrent value of variable to show in help text
See Also
helptext()
Note
added in QGIS 2.12

Definition at line 3176 of file qgsexpression.cpp.

Friends And Related Function Documentation

friend class QgsOgcUtils
friend

Definition at line 932 of file qgsexpression.h.

Member Data Documentation

const char * QgsExpression::BinaryOperatorText
static
Initial value:
=
{
"OR", "AND",
"=", "<>", "<=", ">=", "<", ">", "~", "LIKE", "NOT LIKE", "ILIKE", "NOT ILIKE", "IS", "IS NOT",
"+", "-", "*", "/", "//", "%", "^",
"||"
}

Definition at line 325 of file qgsexpression.h.

QHash< QString, QString > QgsExpression::gFunctionHelpTexts
staticprotected

Definition at line 925 of file qgsexpression.h.

QHash< QString, QString > QgsExpression::gGroups
staticprotected

Definition at line 927 of file qgsexpression.h.

QStringList QgsExpression::gmBuiltinFunctions
static

Definition at line 487 of file qgsexpression.h.

QList< QgsExpression::Function * > QgsExpression::gmFunctions
static

Definition at line 484 of file qgsexpression.h.

QList< QgsExpression::Function * > QgsExpression::gmOwnedFunctions
static

List of functions owned by the expression engine.

Definition at line 505 of file qgsexpression.h.

QMap< QString, QString > QgsExpression::gmSpecialColumnGroups
staticprotected

Definition at line 923 of file qgsexpression.h.

QMap< QString, QVariant > QgsExpression::gmSpecialColumns
staticprotected

Definition at line 922 of file qgsexpression.h.

QHash< QString, QString > QgsExpression::gVariableHelpTexts
staticprotected

Definition at line 926 of file qgsexpression.h.

QgsDistanceArea* QgsExpression::mCalc
protected

Definition at line 920 of file qgsexpression.h.

QString QgsExpression::mEvalErrorString
protected

Definition at line 914 of file qgsexpression.h.

QString QgsExpression::mExp
protected

Definition at line 918 of file qgsexpression.h.

QString QgsExpression::mParserErrorString
protected

Definition at line 913 of file qgsexpression.h.

Node* QgsExpression::mRootNode
protected

Definition at line 911 of file qgsexpression.h.

int QgsExpression::mRowNumber
protected

Definition at line 916 of file qgsexpression.h.

double QgsExpression::mScale
protected

Definition at line 917 of file qgsexpression.h.

const char * QgsExpression::UnaryOperatorText
static
Initial value:
=
{
"NOT", "-"
}

Definition at line 326 of file qgsexpression.h.


The documentation for this class was generated from the following files: