add class for exceptions

src/DataTree.hh

@@ -142,13 +142,17 @@ public:
   expr_t MinusOne, MinusInfinity;
 
   //! Raised when a local parameter is declared twice
-  struct LocalVariableException
+  class LocalVariableException
   {
+  public:
     string name;
+    LocalVariableException(str name_arg) : name(name_arg) {}
   };
 
   class DivisionByZeroException
   {
+  public:
+    DivisionByZeroException() {}
   };
 
   inline expr_t AddPossiblyNegativeConstant(double val);
@@ -284,6 +288,8 @@ public:
   //! Thrown when trying to access an unknown variable by deriv_id
   class UnknownDerivIDException
   {
+  public:
+    UnknownDerivIDException() {}
   };
 
   //! Raised when a trend is declared twice