If one of the C PL_*() functions in SWI-Prolog.h returns
failure, this can be either a Prolog-style failure (e.g. from
PL_unify() or PL_next_solution()) or an error. If the
failure is due to an error, it's usually best to immediately return to
Prolog - and this can be done with the PlEx()
function, which turns a Prolog error into a C++ PlException. PlCheckFail()
calls PlEx() and additionally throws PlFail()
if the failure is for Prolog failure.
PlEx() calls PL_exception()
to see if there is a Prolog exception; if so, the Prolog exception is
converted to a
PlException object, which is then thrown. For more details
on the C++ exceptions, see section 1.15.
PlWrap<int>() is similar to PlEx()
except that it returns the result of the call. This is useful for PlQuery::next_solution(),
which doesn't do a check for whether the‘false` return value is
due to a failed query or an exception (PlQuery::next_solution()
can return values other than true or false if
the PlQuery object was created
with the PL_Q_EXT_STATUS flag.
- void PlCheckFail(bool
rc)
- If rc is
false, throw PlFail to
return control to Prolog with failure.
- C_t PlWrap(C_t
rc, qid_t qid = 0)
- If rc indicates failure or an error, check for an error and
throw a
PlException if there was one; otherwise, return the rc.
- void PlEx(C_t
rc, qid_t qid = 0)
- If rc is “false” (non-zero), throw
PlFail
to return control to Prolog with failure. This is the same as PlCheckFail()
except it can also specify a
qid_t query ID.
