Re: [RFA 1/6] change gmp_string_asprintf to return an std::string

[Simon Marchi <simark@simark.ca>]

On 2020-11-15 3:49 a.m., Joel Brobecker wrote:
> This was suggested by Simon during a code review of this package upstream.
> The upside is that this makes the function's API more natural and C++.
> The downside is an extra malloc, which might be the reason why we went
> for using a unique_xmalloc_ptr in the first place. Since this function
> is not expected to be called frequently, the API improvement might be
> worth the performance impact.

In fact, I was thinking of doing the exact same thing as the
string_print function.  By calling vsnprintf with a NULL buffer, you can
get how many bytes would be required to hold the result.  You can then
allocate the required buffer and call vsprintf with it.

So, there's no extra allocation, although it requires formatting twice.
But if it's good enough for string_printf (which is heavily used), it's
good enough for gmp_string_printf.

I was just wondering whether gmp_vsnprintf supported passing a NULL
buffer like vsnprintf does, and it seems like yes.  I tried the
implementation shown below and it works (with gmp 6.2.0 at least).

I would suggest renaming the function to gmp_string_printf for symmetry
with plain string_printf.

std::string
gmp_string_asprintf (const char *fmt, ...)
{
  va_list vp;

  va_start (vp, fmt);
  int size = gmp_vsnprintf (NULL, 0, fmt, vp);
  va_end (vp);

  std::string str (size, '\0');

  /* C++11 and later guarantee std::string uses contiguous memory and
     always includes the terminating '\0'.  */
  va_start (vp, fmt);
  gmp_vsprintf (&str[0], fmt, vp);
  va_end (vp);

  return str;
}

Simon