A better error message for an invalid use of dynamic_cast

Ian Lance Taylor ian@zembu.com
Thu Oct 28 13:28:00 GMT 1999

Hi.  I never got a reply for this patch submission.  I just want to
check that it hasn't been forgotten or silently discarded.  I don't
insist on this particular error message, but I think a nicer error
message would be more helpful.


------- Start of forwarded message -------
Date: 24 Jun 1999 16:14:10 -0400
From: Ian Lance Taylor <ian@airs.com>
To: egcs-patches@egcs.cygnus.com
CC: ian@airs.com
Subject: A better error message for an invalid use of dynamic_cast

In the current CVS sources, when I compile this C++ code:

class c1

class c2 : public c1

c2 *
fn (c1 *p1)
  return dynamic_cast<c2 *> (p1);

I get this error message:

/home/ian/foo1.cc: In function `class c2 * fn(class c1 *)':
/home/ian/foo1.cc:12: cannot dynamic_cast `p1' (of type `class c1 *') to type `class c2 *'

This is not especially informative, and it took me a while to realize
that the problem could be solved by adding a virtual function to class

I propose the appended patch.  This changes the error message to the

/home/ian/foo1.cc: In function `class c2 * fn(c1 *)':
/home/ian/foo1.cc:12: dynamic_cast of non-polymorphic type `class c1 *'


1999-06-24  Ian Lance Taylor  <ian@zembu.com>

	* rtti.c (build_dynamic_cast_1): Give a better error message for
	an attempt to dynamic_cast a polymorphic type.

Index: rtti.c
RCS file: /cvs/egcs/egcs/gcc/cp/rtti.c,v
retrieving revision 1.32
diff -u -p -r1.32 rtti.c
- --- rtti.c	1999/04/27 09:41:31	1.32
+++ rtti.c	1999/06/24 20:13:34
@@ -693,6 +693,9 @@ build_dynamic_cast_1 (type, expr)
+  cp_error ("dynamic_cast of non-polymorphic type `%#T'", exprtype);
+  return error_mark_node;
   cp_error ("cannot dynamic_cast `%E' (of type `%#T') to type `%#T'",
 	    expr, exprtype, type);
------- End of forwarded message -------

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