Re: C++ string::find functions and size_t

[Daniel J Sebald]

On 02/27/2015 05:56 PM, John W. Eaton wrote:
> On 02/27/2015 06:37 PM, Andreas Weber wrote:
> > Am 27.02.2015 um 18:47 schrieb rik:
> > > 2/27/15
> > >
> > > Dan Sebald found a subtle bug in the use of the C++ string find
> > > functions
> > > that was leading to segfaults.
> > >
> > > -- Code --
> > > size_t pos = file.find_first_not_of ("|");
> > > if (pos > 0)
> > > file = file.substr (pos);
> > > else
> > > -- End Code --
> >
> > I think the idea here was to check if the first char in file is "|" and
> > omit the check (because we are building a pipe) for an existent
> > directory in the else path.
> >
> > > The issue is that the size_t is an unsigned quantity and the find
> > > functions
> > > do not return -1 on failure as do regular C functions. Instead they
> > > return
> > > std::string::npos (a very large number) when they fail to find the
> > > search term.
> > >
> > > This was easily corrected to
> > >
> > > -- Code --
> > > size_t pos = file.find_first_not_of ("|");
> > > if (pos != std::string::npos)
> > > file = file.substr (pos);
> > > else
> > > -- End Code --
> >
> > Please correct me if I'm wrong but shouldn't this then be
> >
> > -- Code --
> > size_t pos = file.find_first_not_of ("|");
> > if (pos != std::string::npos && pos > 0)
> > -- End Code --
>
> Yes, I think that's correct. I also thought maybe it would be clearer to
> write if (file.length () > 0 && file[0] == '|'), then just extract get
> file.substr (1) as the command. Shouldn't that work? It looks simpler to
> me.

After thinking about it, a change there would be good.  The logic 
associated with finding "anything but" makes the conditional a little 
unusual.  I'm not sure even that is foolproof though.  (If the user's 
not going to enter this string, then it doesn't matter I suppose.)  What 
about cases like " | foo.out", if that is a possible user entry somehow. 
 Maybe there should be:

  size_t pos = file.find_first_not_of (" ");
  if (pos != std::string::npos)
    {
      if (file[pos] == '|' && file.length () > (pos + 1) &&
          file.find_first_not_of (" ", ) != std::string::npos)
        {
          file = file.substr (pos);
        }
      else if (file[pos] != '|')
        {
          pos = file.find_last_of (file_ops::dir_sep_chars ());
          [etc]
        }
      else
        error[empty pipe spec, obviating "broken pipe" error]
    }
  else
    error[empty output, obviating "broken pipe" error]


And I'm not sure that fully does it still because there is this:

  pos = file.find_last_of (file_ops::dir_sep_chars ());

which I think is meant to find a device file (??).  What if a file name 
is given but there is no directory character?  Or does there have to be 
a directory character by nature of the OS file system?

Dan