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Subject: Re: [boost] [units] gcc warnings
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-06-17 07:44:32

Emil Dotchevski wrote:
> On Wed, Jun 16, 2010 at 10:16 AM, Stewart, Robert
> <Robert.Stewart_at_[hidden]> wrote:
> > I don't see how static_cast would be able to hide an error.
> The general point is that casts do more than simply silence
> warnings. Example:
> void foo( short x );
> int value();
> ....
> foo(static_cast<short>(value())); //static_cast used to
> //suppress a warning
> Later under maintenance we introduce an additional foo overload:
> void foo( long x );
> Now the call to foo with the static_cast<short> will continue to
> compile while without the static_cast you'd get a compile error.

That makes sense. However, in order for that code to be useful after the maintenance change, the result of calling value() would need to be cast to either short or long to avoid the ambiguity and casting to short could be justifiable. Doing so, before or after the maintenance, falls within the discussion below.

> > I suggested an assertion in addition to the static_cast to
> > verify the behavior as a defense against maintenance effects.
> > Assuming the type is always short, then asserting the current
> > value less-than std::numeric_limits<short>::max() would
> > suffice. To account for your change of type scenario, a
> > compile-time assertion that the type is short would also be
> > needed.
> void foo( short a, short b )
> {
> short result=a+b;
> ....
> }
> In general, I find it difficult to accept this instead:
> void foo( short a, short b )
> {
> int tmp=a+b;
> short result=static_cast<short>(tmp);
> assert(result==tmp);
> ....
> }

This would be more palatable:

   short const result(a + b);
   assert(result == a + b);

(That might warrant a comment, though, as it looks odd.)

> In some specific case -- maybe, but I can't justify asserts every time
> I deal with shorts (which admittedly isn't very often.)

If you don't do something like that, when using shorts, then the code is unsafe, right? Why wouldn't you want to help to ensure that your code does the right thing?

The result of this is to recognize that doing math like this with shorts is dangerous because so many things like to be int. (There are problems with all signed types, but those smaller than int are clearly worse.)

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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