1.10.1 The class PlException

The C++ model of exceptions and the Prolog model of exceptions are different. Wherever the underlying function returns a "fail" return code, the C++ API does a further check for whether there's an exception and, if so, does a C++ throw of a PlException object. You can use C++ try-catch to intercept this and examine the

This subclass of PlTerm is used to represent exceptions. Currently defined methods are:

PlException :: PlException()

Create an exception term using PL_exception(0). The method is_null() succeeds if there was simple failure (e.g., from unification failing) and not_null() succeeds if there was an exception.

PlException :: PlException(const PlTerm &)

Create an exception from a general Prolog term. This provides the interface for throwing any Prolog terms as an exception.

PlException ::operator wchar_t *(void)

PlException ::operator char *(void)

The exception is translated into a message as produced by print_message/2. The character data is stored in a ring. Example:

  ...;
  try
  { PlCall("consult(load)");
  } catch ( PlException &ex )
  { cerr << (char *) ex << endl;
  }

int plThrow()

Used in the PREDICATE() wrapper to pass the exception to Prolog. See PL_raise_exeption().

int cppThrow()

Used by PlQuery::next_solution() to refine a generic PlException representing a specific class of Prolog exceptions to the corresponding C++ exception class and finally then executes throw(). Thus, if a PlException represents the term

error(type_error(Expected, Actual), Context)

PlException::cppThrow() throws a PlTypeEror exception. This ensures consistency in the exception-class whether the exception is generated by the C++-interface or returned by Prolog.

The following example illustrates this behaviour:

PREDICATE(call_atom, 1)
{ try
  { return PlCall((char *)A1);
  } catch ( PlTypeError &ex )
  { cerr << "Type Error caugth in C++" << endl;
    cerr << "Message: \"" << (char *)ex << "\"" << endl;
    return FALSE;
  }
}