fix compile error on windows

Author: Huangzizhou

src/ipc/smooth_contact/collisions/smooth_collision.cpp

@@ -259,13 +259,14 @@ auto SmoothCollisionTemplate<max_vert, PrimitiveA, PrimitiveB>::gradient(
 }
 
 template <int max_vert, typename PrimitiveA, typename PrimitiveB>
-auto SmoothCollisionTemplate<max_vert, PrimitiveA, PrimitiveB>::hessian(
+Eigen::MatrixXd
+SmoothCollisionTemplate<max_vert, PrimitiveA, PrimitiveB>::hessian(
     const Vector<
         double,
         -1,
         SmoothCollisionTemplate<max_vert, PrimitiveA, PrimitiveB>::max_size>&
         positions,
-    const ParameterType& params) const -> MatrixMax<double, max_size, max_size>
+    const ParameterType& params) const
 {
     const auto core_indices = get_core_indices();
 

src/ipc/smooth_contact/collisions/smooth_collision.hpp

@@ -48,25 +48,15 @@ template <int max_vert> class SmoothCollision : public Collision<max_vert> {
 
     virtual double operator()(
         const Vector<double, -1, max_size>& positions,
-        const ParameterType& params) const
-    {
-        return 0.;
-    }
+        const ParameterType& params) const = 0;
 
     virtual Vector<double, -1, max_size> gradient(
         const Vector<double, -1, max_size>& positions,
-        const ParameterType& params) const
-    {
-        return Vector<double, -1, max_size>::Zero(positions.size());
-    }
+        const ParameterType& params) const = 0;
 
-    virtual MatrixMax<double, max_size, max_size> hessian(
+    virtual Eigen::MatrixXd hessian(
         const Vector<double, -1, max_size>& positions,
-        const ParameterType& params) const
-    {
-        return MatrixMax<double, max_size, max_size>::Zero(
-            positions.size(), positions.size());
-    }
+        const ParameterType& params) const = 0;
 
     // ---- distance ----
 
@@ -170,7 +160,7 @@ class SmoothCollisionTemplate : public SmoothCollision<max_vert> {
         const Vector<double, -1, max_size>& positions,
         const ParameterType& params) const override;
 
-    MatrixMax<double, max_size, max_size> hessian(
+    Eigen::MatrixXd hessian(
         const Vector<double, -1, max_size>& positions,
         const ParameterType& params) const override;