[antlr-interest] rethrowing of exceptions in ANTLR default exception handlers

[Dennis Marsa]

Hello,

An ANTLR C++ default exception handler generally looks something
like this:

  catch (RecognitionException& ex) {
    if( inputState->guessing == 0 ) {
      reportError(ex);
      consume();
      consumeUntil(_tokenSet_6);
    } else {
      throw ex;
    }
  }

When guessing, the handler re-throws the exception with
the "throw ex;" statement in the else clause.

"throw ex;" actually throws a copy of ex (even though ex
is a reference).  If the caught exception is actually a
subclass of RecognitionException (e.g. MismatchedTokenException),
the copy made by the "throw ex" will only be a copy of the
RecognitionException part of the original exception.

Thus, the original exception can be sliced to a base class,
possibly losing information from the original exception, when
it is rethrown.

I believe the appropriate fix is to use throw with no operand:

   throw;   // no operand

which by definition rethrows the caught exception (ex)
without copying it.

  catch (RecognitionException& ex) {
    if( inputState->guessing == 0 ) {
      reportError(ex);
      consume();
      consumeUntil(_tokenSet_6);
    } else {
      throw;  // rethrows ex without copying
    }
  }

Attached is a patch against the 2.7.2a2 C++ code generator
that addresses this issue.

Regards,

Dennis
 

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-------------- next part --------------
*** CppCodeGenerator.java.orig	Thu Mar 21 14:37:16 2002
--- CppCodeGenerator.java	Thu Mar 21 14:37:31 2002
***************
*** 2267,2273 ****
  				println("} else {");
  				tabs++;
  				// When guessing, rethrow exception
! 				println("throw "+extractIdOfAction(handler.exceptionTypeAndName)+";");
  				tabs--;
  				println("}");
  			}
--- 2267,2273 ----
  				println("} else {");
  				tabs++;
  				// When guessing, rethrow exception
! 				println("throw;");
  				tabs--;
  				println("}");
  			}
***************
*** 3326,3332 ****
  				// When guessing, rethrow exception
  				println("} else {");
  				tabs++;
! 				println("throw ex;");
  				tabs--;
  				println("}");
  			}
--- 3326,3332 ----
  				// When guessing, rethrow exception
  				println("} else {");
  				tabs++;
! 				println("throw;");
  				tabs--;
  				println("}");
  			}