Re: [patch] Make vector::at() assertion message more useful

[Chris Jefferson <chris at bubblescope dot net>]

On 18/08/13 11:42, Oleg Endo wrote:
> On Sun, 2013-08-18 at 11:38 +0100, Chris Jefferson wrote:
> > On 18/08/13 08:09, VÃclav Zeman wrote:
> > > On 08/17/2013 05:10 PM, Gabriel Dos Reis wrote:
> > > > On Sat, Aug 17, 2013 at 5:41 AM, Paolo Carlini <paolo.carlini@oracle.com> wrote:
> > > > > Hi,
> > > > >
> > > > > Paul Pluzhnikov <ppluzhnikov@google.com> ha scritto: __throw_out_of_range(__N("vector::_M_range_check"));
> > > > > > +        {
> > > > > > +          char __s[256];
> > > > > > +          __builtin_snprintf(__s, sizeof(__s),
> > > > > > +                             __N("vector::_M_range_check: %zu >= %zu"),
> > > > > > +                             __n, this->size());
> > > > > > +          __throw_out_of_range(__s);
> > > > > > +        }
> > > > > The idea makes sense, but while we are at it I think the message could be more clear, say what the two numbers are. Also, I don't think we can unconditionally call snprintf, it's C99 and supported targets lack it. Maybe with some libiberty magic? I don't think such magic automagically triggers when __builtin_snprintf is expanded, or does it? Please investigate that.
> > > > I agree that we can't embark a significant IO library as part of a purely
> > > > data structure components.
> > > >
> > > > Furthermore, __builtin_snprintf doesn't always expand to a
> > > > compiler magic -- it can be a normal function call.
> > > >
> > > > This would be a regression, not an improvement.
> > > What about adding a simple template function that converts integer into
> > > std::string using division by 10? You could then concatenate the message
> > > from std::string fragments, avoiding IO library but having to pull in
> > > std::string instead. Would that be acceptable?
> > Rather than making up new functions, why not use a std::ostringstream?
> Because that would pull in the IO library parts, which is undesired in
> this case.

There seems to be two seperate issues here (unless I misunderstand).

1) The problem with bringing in snprintf is that it is not supported on 
all targets. Therefore we have to worry about conditional support.

2) ostringstream is supported on all platforms, but bringing it in 
includes a largish header and a large chunk of code, although only in 
debug mode.


Obviously if there are platforms that do not support ostringstream, we 
should not use it as we get back to the same conditional support issues 
of snprintf. However, I don't see any significant disadvantages to using 
it in debug mode (which already, by necessity, produces larger and much 
slower code).

Chris