Re: Introductory Presentations, especially aimed at C++ programmers!

[Robert A Duff <bobduff@TheWorld.com>]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> writes:

> On 12/12/2016 05:53 PM, Randy Brukardt wrote:
>>
>> A Get_Line like the above
>> generally requires some sort of incremental allocation (a buffer is
>> allocated, if its not big enough a bigger one is allocated, the first one is
>> copied into it, and then we repeat until a <LF> appears),
>
> As the person who created the Get_Line function, I can assure that's not
> how most versions are implemented.

That's how it's done in GNAT.  How is it done in "most versions"?

From a-textio.adb:

   function Get_Line (File : File_Type) return String is
      function Get_Rest (S : String) return String;
      --  This is a recursive function that reads the rest of the line and
      --  returns it. S is the part read so far.

      --------------
      -- Get_Rest --
      --------------

      function Get_Rest (S : String) return String is

         --  The first time we allocate a buffer of size 500. Each following
         --  time we allocate a buffer the same size as what we have read so
         --  far. This limits us to a logarithmic number of calls to Get_Rest
         --  and also ensures only a linear use of stack space.

         Buffer : String (1 .. Integer'Max (500, S'Length));
         Last   : Natural;

      begin
         Get_Line (File, Buffer, Last);

         declare
            R : constant String := S & Buffer (1 .. Last);
         begin
            if Last < Buffer'Last then
               return R;

            else
               pragma Assert (Last = Buffer'Last);

               --  If the String has the same length as the buffer, and there
               --  is no end of line, check whether we are at the end of file,
               --  in which case we have the full String in the buffer.

               if End_Of_File (File) then
                  return R;

               else
                  return Get_Rest (R);
               end if;
            end if;
         end;
      end Get_Rest;

   --  Start of processing for Get_Line

   begin
      return Get_Rest ("");
   end Get_Line;

- Bob