Public default constructor and comparison operator for QgsExpression

python/core/qgsexpression.sip

@@ -14,6 +14,14 @@ class QgsExpression
     QgsExpression( const QString& expr );
     ~QgsExpression();
 
+    /**
+     * Checks if this expression is valid.
+     * A valid expression could be parsed but does not necessarily evaluate properly.
+     *
+     * @note Added in QGIS 3.0
+     */
+    bool isValid() const;
+
     //! Returns true if an error occurred when parsing the input expression
     bool hasParserError() const;
     //! Returns parser error

src/core/qgsexpression.cpp

@@ -3696,6 +3696,18 @@ QgsExpression::~QgsExpression()
     delete d;
 }
 
+bool QgsExpression::operator==( const QgsExpression& other ) const
+{
+  if ( d == other.d || d->mExp == other.d->mExp )
+    return true;
+  return false;
+}
+
+bool QgsExpression::isValid() const
+{
+  return d->mRootNode;
+}
+
 bool QgsExpression::hasParserError() const { return !d->mParserErrorString.isNull(); }
 
 QString QgsExpression::parserErrorString() const { return d->mParserErrorString; }
@@ -3874,7 +3886,7 @@ int QgsExpression::currentRowNumber() { return d->mRowNumber; }
 QString QgsExpression::dump() const
 {
   if ( !d->mRootNode )
-    return tr( "(no root)" );
+    return QString();
 
   return d->mRootNode->dump();
 }

src/core/qgsexpression.h

@@ -113,7 +113,7 @@ class CORE_EXPORT QgsExpression
     /**
      * Creates a new expression based on the provided string.
      * The string will immediately be parsed. For optimization
-     * {@link prepare()} should alwys be called before every
+     * {@link prepare()} should always be called before every
      * loop in which this expression is used.
      */
     QgsExpression( const QString& expr );
@@ -130,8 +130,30 @@ class CORE_EXPORT QgsExpression
      * it does not need to be re-parsed.
      */
     QgsExpression& operator=( const QgsExpression& other );
+
+    /**
+     * Create an empty expression
+     */
+    QgsExpression();
+
     ~QgsExpression();
 
+    /**
+     * Compares two expressions. The operator returns true
+     * if the expression string is equal.
+     *
+     * @note Added in QGIS 2.18
+     */
+    bool operator==( const QgsExpression& other ) const;
+
+    /**
+     * Checks if this expression is valid.
+     * A valid expression could be parsed but does not necessarily evaluate properly.
+     *
+     * @note Added in QGIS 2.18
+     */
+    bool isValid() const;
+
     //! Returns true if an error occurred when parsing the input expression
     bool hasParserError() const;
     //! Returns parser error
@@ -251,6 +273,13 @@ class CORE_EXPORT QgsExpression
      */
     static bool isValid( const QString& text, const QgsExpressionContext* context, QString &errorMessage );
 
+    /**
+     * Set the expression string, will reset the whole internal structure.
+     *
+     * @note Added in QGIS 2.18
+     */
+    void setExpression( const QString& expression );
+
     void setScale( double scale );
 
     double scale();
@@ -1371,11 +1400,6 @@ class CORE_EXPORT QgsExpression
     static QString formatPreviewString( const QVariant& value );
 
   protected:
-    /**
-     * Used by QgsOgcUtils to create an empty
-     */
-    QgsExpression();
-
     void initGeomCalculator();
 
     static QMap<QString, QVariant> gmSpecialColumns;

tests/src/python/test_qgsexpression.py

@@ -187,5 +187,13 @@ def testComment(self):
             result = exp.evaluate()
             self.assertEqual(exp_res, result)
 
+    def testValid(self):
+        e = QgsExpression()
+        self.assertFalse(e.isValid())
+        e.setExpression('asdf||#@¼')
+        self.assertFalse(e.isValid())
+        e.setExpression('1')
+        self.assertTrue(e.isValid())
+
 if __name__ == "__main__":
     unittest.main()