Re: [PATCH] Re: [wwwdocs] Add common C++ issues to /gcc-6/porting_to.html

[Jonathan Wakely <jwakely at redhat dot com>]

On 03/02/16 12:13 -0500, David Malcolm wrote:
> On Tue, 2016-02-02 at 20:36 +0000, Jonathan Wakely wrote:
>
> I had some difficulty reading the new section; mostly due to the
> leapfrogging of C++11 by the default (my immediate reaction was "why is
> it talking about C++11 when the option says GNU++14?")
>
> I'm attaching a patch which I hope clarifies it, for people like me who
> aren't experts in C++ standards (with that as a caveat... I'm not an
> expert in C++ standards).
>
> Is the attached OK to commit? (it validates)
>
> Some other notes below.
>
> > Index: htdocs/gcc-6/porting_to.html
> > ===================================================================
> > RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
> > retrieving revision 1.2
> > diff -u -r1.2 porting_to.html
> > --- htdocs/gcc-6/porting_to.html	27 Jan 2016 14:40:26 -0000	
> > 1.2
> > +++ htdocs/gcc-6/porting_to.html	2 Feb 2016 20:32:29 -0000
>
> [...snip...]
>
> > +<h4>Cannot convert 'bool' to 'T*'</h4>
> > +
> > +<p>
> > +The current C++ standard only allows integer literals to be used as
> > null
>
> Which standard?

The current one :-)

That was deliberate, because formally the C++11 standard _did_ allow
false and (1 - 1) as null pointer constants. That was changed for
C++14, but was the subject of a defect report against C++11 and so G++
implements the C++14 rule even in C++11 mode (as does Clang).

So it would be wrong to say C++11, and misleading to say C++14.

Maybe we should just say "The C++ standard no longer allows ..."


> > +pointer constants, so other constants such as <code>false</code> and
> > +<code>(1 - 1)</code> cannot be used where a null pointer is desired.
> > Code that
> > +fails to compile with this error should be changed to use
> > <code>nullptr</code>,
> > +or <code>0</code>, or <code>NULL</code>.
> > +</p>
> > +
> > +<h4>Cannot convert 'std::ostream' to 'bool'</h4>
> > +
> > +<p>
> > +Since C++11 iostream classes are no longer implicitly convertible to
> > +<code>void*</code> so it is no longer valid to do something like:
>
> Should there be a comma between "C++" and "iostream" here?  (or maybe
> rewrite as: "As of C++, iostream classes"... ?)

As long as we keep the "11" that would be an improvement, yes.

> [...snip...]
>
>
> Hope this is constructive
> Dave

> Index: htdocs/gcc-6/porting_to.html
> ===================================================================
> RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
> retrieving revision 1.3
> diff -u -p -r1.3 porting_to.html
> --- htdocs/gcc-6/porting_to.html	2 Feb 2016 20:34:14 -0000	1.3
> +++ htdocs/gcc-6/porting_to.html	3 Feb 2016 17:04:50 -0000
> @@ -36,8 +36,10 @@ manner. Additions and suggestions for im
> <h3>Default standard is now GNU++14</h3>
>
> <p>
> -GCC defaults to <code>-std=gnu++14</code> instead of <code>-std=gnu++98</code>.
> -This brings several changes that users should be aware of.  The following
> +GCC 6 defaults to <code>-std=gnu++14</code> instead of <code>-std=gnu++98</code>:
> +the C++14 standard, plus GNU extensions.
> +This brings several changes that users should be aware of, some new with the C++14
> +standard, others that appeared with the C++11 standard.  The following
> paragraphs describe some of these changes and suggest how to deal with them.
> </p>

Looks good to me.

> @@ -45,6 +47,10 @@ paragraphs describe some of these change
> use the <code>-std=gnu++98</code> command-line option, perhaps by putting it
> in <code>CXXFLAGS</code> or similar variables in Makefiles.</p>
>
> +<p>Alternatively, you might prefer to update to gnu++11, bringing in the C++11
> +changes but not the C++14 ones.  If so, use the <code>-std=gnu++11</code>
> +command-line option.</p>

I see no harm in adding this, although the changes from C++98 to C++11
are huge, and the changes from C++11 to C++14 are tiny, so for the
purposes of porting code to work with GCC 6 it's unlikely to help.