From: Warren Young
Date: April 8 2008 10:28pm
Subject: RFC: Remove, change, or leave Query safe bool stuff alone?
List-Archive: http://lists.mysql.com/plusplus/7570
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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: 
http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.1

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.