Re: Verify and expression

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++.moderated
Date:
16 Oct 2006 08:36:24 -0400
Message-ID:
<4pg2juFisub6U1@individual.net>
* Mark Van Peteghem:

Alf P. Steinbach schreef:

* Mark Van Peteghem:
  

Another pitfall is that the ability to use it as an expression makes
it not so extensible. When you want to add expressions, a level, a
group and or an optional action, you need to wrap it in a if-else
construct, so it can no longer be used as expression.
    

Could you elaborate on this?

As far as I know this is a valid C++ statement:

    42;


I didn't talk about statements, only expressions. Maybe I didn't
explain too well what I mean with adding expressions: I meant
expressions that are logged and displayed along with the other
information about a failed assertion; in ModAssert you can do that
with

     MOD_ASSERT_P(a << b, a+b==10);

where a and b would be logged and displayed if the condition failed.
To be able to do that, MOD_ASSERT_P has to be defined as a if-else
construct, so it can no longer be used as an expression.


I don't understand (but then, what else is new?).

Let's say I define MOD_ASSERT_P like this -- off the cuff:

   #define MOD_ASSERT_P( sideEffect, assumption ) \
      ((assumption)? true : (sideEffect, std::terminate(), true))

perhaps conditionally defined with a do-nothing variant for NDEBUG.

I don't know the spec for MOD_ASSERT_P, but just assume the above
definition, unless it's way too different from what the real
MOD_ASSERT_P does.

The only potential problem I can think of at the moment is that the
value of 'assumption' is used more than once but shouldn't be evaluated
more than once, for fear of side-effects. And that one of those
potential evaluations is as an argument to 'assert'. But for that there
is both the solution of global variable + stringizing, and the solution
of requiring 'assumption' to have no side effects, which I think best.

A stringizing solution to that potential problem (not using a global
variable because here there's just one evaluation in addition to assert)
  -- not that I would necessarily choose this solution -- might look
like this:

     inline bool trueSansWarning() { return true; }

     #define BECAUSE_NOT_TRUE( e ) (void(#e), false)

     #define MOD_ASSERT_P( sideEffect, assumption ) \
         ((assumption)? trueSansWarning() : ( \
             sideEffect, \
             assert( BECAUSE_NOT_TRUE(assumption) ), std::terminate(), \
             trueSansWarning() \
             ) \
         )

Admittedly, I did have to put this last example through the compiler(s),
and admittedly, for MOD_ASSERT_P the ability to serve as an expression
seems to me to be of dubious value (as opposed to e.g. a TERMINATE macro
without any assumption to be tested), but what's the problem?

Well, except for Visual C++ warning about unreachable code (one of the
countless silly-warnings that should simply be turned off), I mean?

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