diff --git a/gtsam/nonlinear/Expression.h b/gtsam/nonlinear/Expression.h
index afbc5e93a..d24a568f5 100644
--- a/gtsam/nonlinear/Expression.h
+++ b/gtsam/nonlinear/Expression.h
@@ -60,7 +60,9 @@ private:
 public:
 
   // Expressions wrap trees of functions that can evaluate their own derivatives.
-  // The meta-functions below provide a handy to specify the type of those functions
+  // The meta-functions below are useful to specify the type of those functions.
+  // Example, a function taking a camera and a 3D point and yielding a 2D point:
+  //   Expression<Point2>::BinaryFunction<SimpleCamera,Point3>::type
   template<class A1>
   struct UnaryFunction {
     typedef boost::function<
@@ -135,6 +137,10 @@ public:
           typename MakeOptionalJacobian<T, A3>::type) const,
       const Expression<A2>& expression2, const Expression<A3>& expression3);
 
+  /// Destructor
+  virtual ~Expression() {
+  }
+
   /// Return keys that play in this expression
   std::set<Key> keys() const;
 
diff --git a/gtsam/nonlinear/internal/ExpressionNode.h b/gtsam/nonlinear/internal/ExpressionNode.h
index 6e47adef0..e7aa34db0 100644
--- a/gtsam/nonlinear/internal/ExpressionNode.h
+++ b/gtsam/nonlinear/internal/ExpressionNode.h
@@ -129,6 +129,10 @@ class ConstantExpression: public ExpressionNode<T> {
 
 public:
 
+  /// Destructor
+  virtual ~ConstantExpression() {
+  }
+
   /// Return value
   virtual T value(const Values& values) const {
     return constant_;
@@ -154,12 +158,15 @@ class LeafExpression: public ExpressionNode<T> {
   LeafExpression(Key key) :
       key_(key) {
   }
-  // todo: do we need a virtual destructor here too?
 
   friend class Expression<T> ;
 
 public:
 
+  /// Destructor
+  virtual ~LeafExpression() {
+  }
+
   /// Return keys that play in this expression
   virtual std::set<Key> keys() const {
     std::set<Key> keys;
@@ -205,18 +212,23 @@ class UnaryExpression: public ExpressionNode<T> {
   boost::shared_ptr<ExpressionNode<A1> > expression1_;
   Function function_;
 
-public:
-
   /// Constructor with a unary function f, and input argument e1
   UnaryExpression(Function f, const Expression<A1>& e1) :
       expression1_(e1.root()), function_(f) {
     ExpressionNode<T>::traceSize_ = upAligned(sizeof(Record)) + e1.traceSize();
   }
 
+  friend class Expression<T> ;
+
+public:
+
+  /// Destructor
+  virtual ~UnaryExpression() {
+  }
+
   /// Return value
   virtual T value(const Values& values) const {
-    using boost::none;
-    return function_(expression1_->value(values), none);
+    return function_(expression1_->value(values), boost::none);
   }
 
   /// Return keys that play in this expression
@@ -300,8 +312,6 @@ class BinaryExpression: public ExpressionNode<T> {
   boost::shared_ptr<ExpressionNode<A2> > expression2_;
   Function function_;
 
-public:
-
   /// Constructor with a binary function f, and two input arguments
   BinaryExpression(Function f, const Expression<A1>& e1,
       const Expression<A2>& e2) :
@@ -310,8 +320,15 @@ public:
         upAligned(sizeof(Record)) + e1.traceSize() + e2.traceSize();
   }
 
+  friend class Expression<T> ;
   friend class ::ExpressionFactorBinaryTest;
 
+public:
+
+  /// Destructor
+  virtual ~BinaryExpression() {
+  }
+
   /// Return value
   virtual T value(const Values& values) const {
     using boost::none;
@@ -394,8 +411,6 @@ class TernaryExpression: public ExpressionNode<T> {
   boost::shared_ptr<ExpressionNode<A3> > expression3_;
   Function function_;
 
-public:
-
   /// Constructor with a ternary function f, and two input arguments
   TernaryExpression(Function f, const Expression<A1>& e1,
       const Expression<A2>& e2, const Expression<A3>& e3) :
@@ -405,6 +420,14 @@ public:
         e1.traceSize() + e2.traceSize() + e3.traceSize();
   }
 
+  friend class Expression<T> ;
+
+public:
+
+  /// Destructor
+  virtual ~TernaryExpression() {
+  }
+
   /// Return value
   virtual T value(const Values& values) const {
     using boost::none;