When a blob is used in the context of a PREDICATE() macro, it can
raise a C++ exception (PlFail or PlException)
and the PREDICATE() code will convert it to the appropriate Prolog
failure or error; memory allocation exceptions are also handled.
Blobs have callbacks, which can run outside the context of a
PREDICATE(). Their exception handling is as follows:
- acquire(), which is called from PlBlobV<MyBlob>::acquire(),
can throw a C++ exception.
- compare_fields(), which is called from PlBlobV<MyBlob>::compare(),
should not throw an exception. A Prolog error won't work as it uses "raw
pointers" and thus a GC or stack shift triggered by creating the
exception will upset the system.
- write_fields(), which is called from PlBlobV<MyBlob>::write(),
can throw an exception, just like code inside a PREDICATE(). In
particular, you can wrap calls to Sfprintf() in PlCheckFail(),
although the calling context will check for errors on the stream, so
checking the Sfprintf() result isn't necessary.
- save() can throw a C++ exception, including PlFail().
- load() can throw a C++ exception, which is converted to a return
value of
PlAtom::null, which is interpreted by Prolog as
failure.