added toString method for ShieldExpression

src/storm/logic/ShieldExpression.cpp

@@ -45,8 +45,12 @@ namespace storm {
             }
         }
 
+        std::string ShieldExpression::toString() const {
+            return "<" + typeToString() + ", " + comparisonToString() + "=" + std::to_string(value) + ">";
+        }
+
         std::ostream& operator<<(std::ostream& out, ShieldExpression const& shieldExpression) {
-            out << "<" << shieldExpression.typeToString() << ", " << shieldExpression.comparisonToString() << "=" << std::to_string(shieldExpression.value) << ">";
+            out << shieldExpression.toString();
             return out;
         }
     }

src/storm/logic/ShieldExpression.h

@@ -28,7 +28,7 @@ namespace storm {
 
             std::string typeToString() const;
             std::string comparisonToString() const;
-
+            std::string toString() const;
             friend std::ostream& operator<<(std::ostream& stream, ShieldExpression const& shieldExpression);
 
         private: