Add QString operator to QgsExpression

Author: m-kuhn

python/core/expression/qgsexpression.sip

@@ -98,6 +98,7 @@ Implicit sharing was added in 2.14
 %End
 
 
+
     QgsExpression();
 %Docstring
  Create an empty expression.

src/core/expression/qgsexpression.cpp

@@ -218,6 +218,11 @@ QgsExpression &QgsExpression::operator=( const QgsExpression &other )
   return *this;
 }
 
+QgsExpression::operator QString() const
+{
+  return d->mExp;
+}
+
 QgsExpression::QgsExpression()
   : d( new QgsExpressionPrivate )
 {

src/core/expression/qgsexpression.h

@@ -137,6 +137,13 @@ class CORE_EXPORT QgsExpression
      */
     QgsExpression &operator=( const QgsExpression &other );
 
+    /**
+     * Automatically convert this expression to a string where requested.
+     *
+     * \since QGIS 3.0
+     */
+    operator QString() const SIP_SKIP;
+
     /**
      * Create an empty expression.
      *