NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/17455

Internet Message Format | 1996-08-05 | 5.4 KB

Path: bcarh8ab.bnr.ca!jcobban
From: [e-mail] (Jim Cobban)
Newsgroups: comp.lang.c++
Subject: Deriving class from IOSTREAMs
Date: 15 Apr 1996 19:59:06 GMT
Organization: Bell-Northern Research Canada
Message-ID: <4ku9qa$[e-mail]>
Reply-To: [e-mail] (Jim Cobban)
NNTP-Posting-Host: bcarsf1f.bnr.ca

I am trying to define a class which has the following characteristics:

  1) It supports the insertion (<<) operator of the ostream class.
  2) Output is organized into units of work.  No part of a unit of work
     can be written to the output device until the unit of work is complete.
     Then all of the text in the unit of work must be placed on the same line
     of output.  In other words if there is enough room on the current line
     to hold all of the text then it will be printed on the current line,
     otherwise a new line is started and the text is written on the new line.
  3) When a new line is started, including the very first line in the output,
     an application specified prefix is written before the first unit of
     work.

The first requirement dictates that my class be derived directly or
indirectly from ostream.  The second requirement seems to me to dictate that
my class be derived, in particular, from ostrstream, which provides a buffer
in which output can be held until a unit of work is complete.  I would like
the end of a unit of work to be signalled by a manipulator, but I have had a
great deal of difficulty in getting that to work.

A stripped down sample program which illustrates some of the problems I am
having follows:

    10  #include <strstream.h>
    11
    12  class xstream : public ostrstream
    13  {
    14    char          buffer[80];
    15
    16    public:
    17
    18    xstream();
    19    friend ostream& operator <<(ostream& Ostream, ostream & (*
func)(xstream &))\
    20          { return (*func)((xstream &)Ostream); }
    21  };              //* end of declaration of class xstream
    22
    23  xstream::xstream()
    24  {
    25    rdbuf()->setbuf(buffer, sizeof(buffer));
    26  }
    27
    28  ostream & endw(xstream & Stream)
    29  {
    30    Stream << ends;
    31    cout << "pcount=" << Stream.pcount() << " \"" << Stream.str() <<
"\"" << endl;
    32    Stream.seekp(0, ios::beg);
    33    return Stream;
    34  }
    35
    36  int main()
    37  {
    38    xstream       os;
    39    double        pi=3.14159;
    40
    41    cout << "pi=" << pi << " and so on" << endl;
    42    os << "pi=" << pi << " and so on" << endw;
    43    os << "testing" << endw;
    44    os << "short" << endw;
    45    os << "testing " << 5 << " more";
    46    cout << "outside os.pcount()=" << os.pcount() << endl;
    47    os << endw;
    48  }

The first problem is that the behavior of my manipulator for signalling the
end of a unit of work is dependent upon access to methods of the xstream
class (In the example these are all methods of the ostrstream class, but that
is because I haven't included the code for printing the prefix in this
example).  But normal manipulators, like endl, take an ostream as their
parameter.  So I have the ugly, dangerous, cast in the definition of the
operator << in line 19, which exists only to convince C++ to call endw.  I
say dangerous because there is nothing to prevent the endw manipulator from
being called for any output stream.  As a specific example the statement
"cout << endw;" compiles cleanly, but would crash because the stream is not
actually an ostrstream.  But the only alternatives I can see are:

  1) invoke endw as a member function of xstream, rather than as a
     manipulator, which is syntactically clumsy.
  2) redefine all of the inserters under xstream so that they return
     references to xstream rather than to ostream, something I can't do
     because the original operators were not defined as virtual, for obvious
     performance reasons.

Quite aside from that, although the above example compiles cleanly, it does
not produce the expected output, for reasons which escape me.  The actual
output is:

pi=3.14159 and so on
pcount=-268438404 "pi=3"
pcount=-268438407 ""
pcount=-268438407 ""
outside os.pcount()=0
pcount=-268438407 ""

The first line is the result of line 41 above.  The lines starting with
pcount are produced by the endw manipulator.  There are a couple of glaring
problems:

  1) the value of pcount is garbage.
  2) the inherited inserters don't seem to work, starting with the
     inserter for double which stopped after putting out one digit.  Nothing
     comes out for any subsequent inserter.
  3) if I omit using the double inserter then in other examples I got
     more reasonable output, except that the ends manipulator seemed
     incapable of shortening the string.  For example if I deleted lines 41
     and 42 from the test program then the output of lines 43 and 44 is

pcount=-268438401 "testing"
pcount=-268438403 "shortng"

If I cannot quickly resolve this bizarre behavior then I am simply going to
conclude that the C++ IOSTREAMS system is completely fucked up, and go back
to using printf.
--
Jim Cobban   |  [e-mail]                   |  Phone: [phone]
Nortel (MCS) |                                      |  FAX:   [phone]
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