diff --git a/src/formula/AP.h b/src/formula/AP.h
index 8bda0df06..8c9f7ad16 100644
--- a/src/formula/AP.h
+++ b/src/formula/AP.h
@@ -59,7 +59,7 @@ public:
 	/*!
 	 * @returns the name of the atomic proposition
 	 */
-	std::string getAP() {
+	const std::string& getAP() {
 		return ap;
 	}
 
diff --git a/src/formula/And.h b/src/formula/And.h
index 16f0b6d8e..12d8d38da 100644
--- a/src/formula/And.h
+++ b/src/formula/And.h
@@ -91,15 +91,15 @@ public:
 	/*!
 	 * @returns a pointer to the left child node
 	 */
-	PCTLStateFormula<T>* getLeft() {
-		return left;
+	PCTLStateFormula<T>& getLeft() {
+		return *left;
 	}
 
 	/*!
 	 * @returns a pointer to the right child node
 	 */
-	PCTLStateFormula<T>* getRight() {
-		return right;
+	PCTLStateFormula<T>& getRight() {
+		return *right;
 	}
 
 	/*!
diff --git a/src/formula/BoundedUntil.h b/src/formula/BoundedUntil.h
index c0061691c..9d7cf65f3 100644
--- a/src/formula/BoundedUntil.h
+++ b/src/formula/BoundedUntil.h
@@ -97,15 +97,15 @@ public:
 	/*!
 	 * @returns a pointer to the left child node
 	 */
-	PCTLStateFormula<T>* getLeft() {
-		return left;
+	PCTLStateFormula<T>& getLeft() {
+		return *left;
 	}
 
 	/*!
 	 * @returns a pointer to the right child node
 	 */
-	PCTLStateFormula<T>* getRight() {
-		return right;
+	PCTLStateFormula<T>& getRight() {
+		return *right;
 	}
 
 	/*!
diff --git a/src/formula/Next.h b/src/formula/Next.h
index 4ec120b80..98f5d9533 100644
--- a/src/formula/Next.h
+++ b/src/formula/Next.h
@@ -65,8 +65,8 @@ public:
 	/*!
 	 * @returns the child node
 	 */
-	PCTLStateFormula<T>* getChild() {
-		return child;
+	PCTLStateFormula<T>& getChild() {
+		return *child;
 	}
 
 	/*!
diff --git a/src/formula/Not.h b/src/formula/Not.h
index dfcb7c100..09782530a 100644
--- a/src/formula/Not.h
+++ b/src/formula/Not.h
@@ -60,8 +60,8 @@ public:
 	/*!
 	 * @returns The child node
 	 */
-	PCTLStateFormula<T>* getChild() {
-		return child;
+	PCTLStateFormula<T>& getChild() {
+		return *child;
 	}
 
 	/*!
diff --git a/src/formula/Or.h b/src/formula/Or.h
index 51a2f88fd..a5072b02a 100644
--- a/src/formula/Or.h
+++ b/src/formula/Or.h
@@ -90,15 +90,15 @@ public:
 	/*!
 	 * @returns a pointer to the left child node
 	 */
-	PCTLStateFormula<T>* getLeft() {
-		return left;
+	PCTLStateFormula<T>& getLeft() {
+		return *left;
 	}
 
 	/*!
 	 * @returns a pointer to the right child node
 	 */
-	PCTLStateFormula<T>* getRight() {
-		return right;
+	PCTLStateFormula<T>& getRight() {
+		return *right;
 	}
 
 	/*!
diff --git a/src/formula/ProbabilisticOperator.h b/src/formula/ProbabilisticOperator.h
index b0585fd58..622c2effc 100644
--- a/src/formula/ProbabilisticOperator.h
+++ b/src/formula/ProbabilisticOperator.h
@@ -73,21 +73,21 @@ public:
 	/*!
 	 * @returns the child node (representation of a PCTL path formula)
 	 */
-	PCTLPathFormula<T>* getPathFormula () {
-		return pathFormula;
+	PCTLPathFormula<T>& getPathFormula () {
+		return *pathFormula;
 	}
 
 	/*!
 	 * @returns the lower bound for the probability
 	 */
-	T getLowerBound() {
+	const T& getLowerBound() {
 		return lower;
 	}
 
 	/*!
 	 * @returns the upper bound for the probability
 	 */
-	T getUpperBound() {
+	const T& getUpperBound() {
 		return upper;
 	}
 
diff --git a/src/formula/Until.h b/src/formula/Until.h
index df80ee0e0..be02eb5d9 100644
--- a/src/formula/Until.h
+++ b/src/formula/Until.h
@@ -90,15 +90,15 @@ public:
 	/*!
 	 * @returns a pointer to the left child node
 	 */
-	PCTLStateFormula<T>* getLeft() {
-		return left;
+	PCTLStateFormula<T>& getLeft() {
+		return *left;
 	}
 
 	/*!
 	 * @returns a pointer to the right child node
 	 */
-	PCTLStateFormula<T>* getRight() {
-		return right;
+	PCTLStateFormula<T>& getRight() {
+		return *right;
 	}
 
 	/*!