Re: Where did the warning go?

[Tom St Denis <tstdenis at ellipticsemi dot com>]

Eivind LM wrote:
> On Tue, 24 Feb 2009 15:13:55 +0100, Tom St Denis 
> <tstdenis@ellipticsemi.com> wrote:
>
> > Eivind LM wrote:
> > > It does of course depend on each case whether a warning is easy to 
> > > avoid or not, and also if it warns about a real problem or not. But 
> > > in any case, a warning can help me write code that more surely will 
> > > do what I intend. For example the following code is not doing what I 
> > > intended:
> > >
> > >   #include <stdio.h>
> > >   void print(int a) { printf("%d\n", a); };
> > >   int main() { print(2.5); return 0; };
> > >
> > > I think the problem in the code is both easy to avoid and serious, 
> > > but I get no warning with -Wall and -Wextra (g++ 4.3).
> > That's because it's not undefined behaviour.  "default" warnings 
> > should be for things that are not guaranteed to have a known meaning 
> > or behaviour.
>
> I think the warning should be part of -Wall, but I understand if you 
> don't agree. Anyway this is not my point. I trust someone has good 
> reasons for the selection of warnings included in -Wall.
Because warnings for things that have defined behaviour [and aren't 
specifically likely to be wrong] are annoying, see "splint" for examples 
of useless warnings.


> > int a;
> > if (a == 4) { ... }
> >
> > Is undefined behaviour since a is unitialized.
>
> Compiling this example with -Wall and -Wextra gives me no warning with 
> g++ (Debian 4.3.2-1.1) 4.3.2. Installed by apt-get today from Debian 
> repositories.

You have to turn on the optimizer for the warning to come out (otherwise 
the compiler can't detect it's uninitialized).

> > float a;
> > a = 0;
> > if (a == 4) { ... }
> >
> > Is fine, but I should also point out "4" is an integral type that is 
> > converted to float for the purpose of the expression "a == 4" .  So 
> > by your logic, it should also generate a warning.
>
> Yes, I would like a warning if I compare a float to an int.
Why?  It's perfectly valid (well yeah not the == bit, but try a > 4).


> I would probably not compare two floats on equality anyway, but if I 
> really had to, then I would write "4" as 4.0f.

You should learn about type promotion. 

That's like saying

long a;
a = 3;

Should be a warning since I didn't write 3L or (long)3.

> > -Wall should really just enable every warning that has something to 
> > do with behaviour that is not predictable.
>
> Fine, but the name should be changed.
I dunno, there are a lot of people who use -Wall -W and rely on the fact 
that the warnings it produces are USEFUL not just chatty.

Tom