diff --git a/include/rotgen/impl/matrix_impl64.hpp b/include/rotgen/impl/matrix_impl64.hpp
index e814c7c..07a3fce 100644
--- a/include/rotgen/impl/matrix_impl64.hpp
+++ b/include/rotgen/impl/matrix_impl64.hpp
@@ -44,7 +44,7 @@ namespace rotgen
 
     matrix_impl64& operator+=(matrix_impl64 const& rhs);
     matrix_impl64& operator-=(matrix_impl64 const& rhs);
-    matrix_impl64& operator-();
+    matrix_impl64 operator-() const;
     matrix_impl64& operator*=(matrix_impl64 const& rhs);
     matrix_impl64& operator*=(double  d);
     matrix_impl64& operator/=(double  d);
diff --git a/include/rotgen/matrix.hpp b/include/rotgen/matrix.hpp
index 6018b13..9c78d00 100644
--- a/include/rotgen/matrix.hpp
+++ b/include/rotgen/matrix.hpp
@@ -29,6 +29,8 @@ namespace rotgen
        if constexpr(Cols != -1) assert(c == Cols &&  "Mismatched between dynamic and static column size");
     }
 
+    matrix(parent const& base) : parent(base) {}
+
     matrix(std::initializer_list<std::initializer_list<Scalar>> init) : parent(init)
     {
       if constexpr(Rows != -1) assert(init.size() == Rows &&  "Mismatched between dynamic and static row size");
@@ -75,9 +77,9 @@ namespace rotgen
       return *this;
     }
 
-    matrix& operator-()
+    matrix operator-() const
     {
-      return static_cast<parent const&>(*this).operator-();
+      return matrix(static_cast<parent const&>(*this).operator-());
     }
 
     matrix& operator*=(matrix const& rhs)
diff --git a/src/matrix_impl64.cpp b/src/matrix_impl64.cpp
index a5a69ad..658b22c 100644
--- a/src/matrix_impl64.cpp
+++ b/src/matrix_impl64.cpp
@@ -101,8 +101,7 @@ namespace rotgen
     return *this;
   }
 
-  matrix_impl64& matrix_impl64::operator-()
-  {
+  matrix_impl64 matrix_impl64::operator-() const {
     matrix_impl64 result(*this);
     result.storage_->data = -result.storage_->data;
     return result;