In another thread (http://lists.mysql.com/plusplus/7562) Dustin Moore
discovered that the new safe bool stuff in Query isn't doing what it was
designed to do because it conflicts with a similar mechanism in one of
Query's base classes, basic_ios. (The Standard C++ Library derivation
demon rises again...)
I thought about changing the method to be operator void*() instead,
overriding the basic_ios version. The code would then compile and do
what was originally intended, but this has problems:
1. basic_ios::operator void*() isn't virtual, so it would be hidden.
I'm not convinced that this is a serious problem due to the nature of
this method, but the C++ FAQ says this is evil:
2. I'm not sure testing the Query object in bool context makes a great
deal of sense anyway. This operation doesn't tell you anything you
can't figure out by some combination of testing the query *result* in
bool context, catching exceptions, and calling Query::affected_rows().
It's just syntactic sugar.
On the other hand, if I remove the safe bool stuff, it breaks the ABI
yet again. Also, it allows calling the base class version which can
result in a run-time failure by masking Query execution errors if you
expected the test to check for them. I'm tempted to override operator
void* but make it private so the build breaks in an "official" way. The
decision rests on whether people are relying on "Query is-a ostream" in
this regard: is anyone testing it in bool context, expecting to test
only the status of the underlying ostream? Seems unlikely to me.
So, here are the options:
1. Leave it alone, relying on the compile error to catch a likely bug.
Downside: ugly, confusing error message.
2. Break the ABI, providing the current test through operator void*.
Downside: fails if you test a Query object through a base class pointer.
3. Break the ABI, hiding operator void* as private in Query, so the test
isn't available; the only way to find success is through Query's
existing interfaces, not through the base class interface. Downside:
breaks the is-a relationship between Query and ostream still further.
|• RFC: Remove, change, or leave Query safe bool stuff alone?||Warren Young||9 Apr|