diff --git a/gtsam/discrete/DiscreteFactor.cpp b/gtsam/discrete/DiscreteFactor.cpp
index c60594f57..2b1bc36a3 100644
--- a/gtsam/discrete/DiscreteFactor.cpp
+++ b/gtsam/discrete/DiscreteFactor.cpp
@@ -28,11 +28,6 @@ using namespace std;
 
 namespace gtsam {
 
-/* ************************************************************************* */
-const DiscreteValues& GetDiscreteValues(const HybridValues& c) {
-  return c.discrete();
-}
-
 /* ************************************************************************* */
 double DiscreteFactor::error(const DiscreteValues& values) const {
   return -std::log((*this)(values));
@@ -40,7 +35,7 @@ double DiscreteFactor::error(const DiscreteValues& values) const {
 
 /* ************************************************************************* */
 double DiscreteFactor::error(const HybridValues& c) const {
-  return DiscreteFactor::error(GetDiscreteValues(c));
+  return this->error(c.discrete());
 }
 
 /* ************************************************************************* */
diff --git a/gtsam/discrete/DiscreteFactor.h b/gtsam/discrete/DiscreteFactor.h
index dc0adbab4..712e1ff75 100644
--- a/gtsam/discrete/DiscreteFactor.h
+++ b/gtsam/discrete/DiscreteFactor.h
@@ -29,9 +29,6 @@ class DecisionTreeFactor;
 class DiscreteConditional;
 class HybridValues;
 
-// Forward declaration of function to extract Values from HybridValues.
-const DiscreteValues& GetDiscreteValues(const HybridValues& c);
-
 /**
  * Base class for discrete probabilistic factors
  * The most general one is the derived DecisionTreeFactor
diff --git a/gtsam/linear/GaussianFactor.cpp b/gtsam/linear/GaussianFactor.cpp
index 74582ecdc..e60e626a1 100644
--- a/gtsam/linear/GaussianFactor.cpp
+++ b/gtsam/linear/GaussianFactor.cpp
@@ -24,12 +24,14 @@
 
 namespace gtsam {
 
-/* ************************************************************************* */
-const VectorValues& GetVectorValues(const HybridValues& c) {
-  return c.continuous();
+double GaussianFactor::error(const VectorValues& c) const {
+  throw std::runtime_error("GaussianFactor::error is not implemented");
+}
+
+double GaussianFactor::error(const HybridValues& c) const {
+  return this->error(c.continuous());
 }
 
-/* ************************************************************************* */
 VectorValues GaussianFactor::hessianDiagonal() const {
   VectorValues d;
   hessianDiagonalAdd(d);
diff --git a/gtsam/linear/GaussianFactor.h b/gtsam/linear/GaussianFactor.h
index 8a50541f5..eb4ea7e06 100644
--- a/gtsam/linear/GaussianFactor.h
+++ b/gtsam/linear/GaussianFactor.h
@@ -31,9 +31,6 @@ namespace gtsam {
   class Scatter;
   class SymmetricBlockMatrix;
 
-  // Forward declaration of function to extract VectorValues from HybridValues.
-  const VectorValues& GetVectorValues(const HybridValues& c);
-
   /**
    * An abstract virtual base class for JacobianFactor and HessianFactor. A GaussianFactor has a
    * quadratic error function. GaussianFactor is non-mutable (all methods const!). The factor value
@@ -73,17 +70,13 @@ namespace gtsam {
      *   0.5*(A*x-b)'*D*(A*x-b) - log(k)
      * for a \class GaussianConditional, where k is the normalization constant.
      */
-    virtual double error(const VectorValues& c) const {
-      throw std::runtime_error("GaussianFactor::error::error is not implemented");
-    }
+    virtual double error(const VectorValues& c) const;
 
     /**
      * The Factor::error simply extracts the \class VectorValues from the
      * \class HybridValues and calculates the error.
      */
-    double error(const HybridValues& c) const override {
-      return GaussianFactor::error(GetVectorValues(c));
-    }
+    double error(const HybridValues& c) const override;
 
     /** Return the dimension of the variable pointed to by the given key iterator */
     virtual DenseIndex getDim(const_iterator variable) const = 0;
diff --git a/gtsam/nonlinear/NonlinearFactor.cpp b/gtsam/nonlinear/NonlinearFactor.cpp
index ec943e9b1..b669be472 100644
--- a/gtsam/nonlinear/NonlinearFactor.cpp
+++ b/gtsam/nonlinear/NonlinearFactor.cpp
@@ -24,8 +24,13 @@
 namespace gtsam {
 
 /* ************************************************************************* */
-const Values& GetValues(const HybridValues& c) {
-  return c.nonlinear();
+double NonlinearFactor::error(const Values& c) const {
+  throw std::runtime_error("NonlinearFactor::error is not implemented");
+}
+
+/* ************************************************************************* */
+double NonlinearFactor::error(const HybridValues& c) const {
+  return this->error(c.nonlinear());
 }
 
 /* ************************************************************************* */
diff --git a/gtsam/nonlinear/NonlinearFactor.h b/gtsam/nonlinear/NonlinearFactor.h
index d046b91ce..9ccd0246f 100644
--- a/gtsam/nonlinear/NonlinearFactor.h
+++ b/gtsam/nonlinear/NonlinearFactor.h
@@ -34,9 +34,6 @@ namespace gtsam {
 
 /* ************************************************************************* */
 
-// Forward declaration of function to extract Values from HybridValues.
-const Values& GetValues(const HybridValues& c);
-
 /**
  * Nonlinear factor base class
  *
@@ -97,17 +94,13 @@ public:
    * and calculates the error by asking the user to implement the method
    * \code double evaluateError(const Values& c) const \endcode.
    */
-  virtual double error(const Values& c) const {
-    throw std::runtime_error("NonlinearFactor::error is not implemented");
-  }
+  virtual double error(const Values& c) const;
 
   /**
    * The Factor::error simply extracts the \class Values from the
    * \class HybridValues and calculates the error.
    */
-  double error(const HybridValues& c) const override {
-    return NonlinearFactor::error(GetValues(c));
-  }
+  double error(const HybridValues& c) const override;
 
   /** get the dimension of the factor (number of rows on linearization) */
   virtual size_t dim() const = 0;