Re: [boost] Current Guidance on Compiler Warnings?

Subject: Re: [boost] Current Guidance on Compiler Warnings?
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2018-11-23 19:58:44

On Thu, Nov 22, 2018 at 12:56 AM Jayesh Badwaik via Boost <
boost_at_[hidden]> wrote:
>
> On Monday, 19 November 2018 21:02:55 CET Emil Dotchevski via Boost wrote:
> > In that context there are probably legal reason for zero-warning policy,
> > but it is not true that lack of warnings means fewer errors, in fact it
> > could easily lead to more errors. For example warnings about implicit
> > conversions are frequently "fixed" by replacing the implicit conversion
> > with explicit conversion (cast). But the two are semantically very
> > different, and therefore one of them is incorrect, and very likely that
is
> > the explicit one.
>
> Didn't know that. Can I see an example where the explicit warning is
> incorrect as opposed to implicit warning?

I'm guessing you want an example where the explicit conversion is
incorrect, and you want to use an implicit conversion even though you get a
warning. Here:

unsigned f();

void g( int x )
{
  if( x < f() ) //warning C4018: '<': signed/unsigned mismatch
  {
    ....
  }
}

So you change that to:

if( static_cast<unsigned>(x) < f() )

Then under refactoring both f and g get changed:

unsigned long f();

void g( long x )
{
  if( static_cast<unsigned>(x) < f() )
  {
    ....
  }
}

And now you probably have a bug, and no warning. I say probably, because if
this was my code, I'd know the cast is correct because my choice of
implicit vs. explicit conversion does not depend on what warnings I get
from the compiler, I do what is right. But if you've instructed your
programmers to use casts to silence warnings, you never know why that cast
sits there.

Assuming the implicit conversion is correct (it probably is, but you should
always know), this should be written as:

assert(x>=0);
if( x < f() )