[STDCXX-243] std::basic_istream::gcount() returns wrong value after a successful peek()

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 4.1.2, 4.1.3, 4.1.4, 4.2.0
• Fix Version/s: 4.2.2
• Component/s: 27. Input/Output

Environment

all

• Severity: Incorrect Behavior
• Assignee: Unassigned
• Reporter: Martin Sebor
• Created: 29/Jun/06 03:55
• Updated: 02/Dec/08 03:05

Description

Moved from the Rogue Wave bug tracking database:

Class/File: istream.cc
Fix Priority: Should Fix
Long Description:
*** Sep 23 1999 9:52AM *** sebor ***

Partially resolved - gcount now more consistent with other implementations (still needs to be revisited when WG21 addresses the issue - can't find it on the issues list, though).

*** Aug 24 1999 9:51AM *** sebor ***

RW implementation of gcount() currently returns 1 after a successful peek(), 0 after an error. That's wrong according to 27.6.1.3, p2, since peek() doesn't extract any characters. The correct behavior is either to have gcount() always return 0 after a peek() or not to have peek() affect gcount() at all. The standard doesn't seem to specify which.

To: C++ libraries mailing list
Message c++std-lib-6965

Should basic_istream::peek() affect the value returned by
basic_istream::gcount()? If it does affect gcount in any
way, then clearly it ought to always set gcount to zero.
The description in 27.6.1.3 [lib.istream.unformatted],
paragraph 27, doesn't say whether peek affects gcount.

Another way to phrase this issue: is peek() an unformatted input
function? Textually it's located in [lib.istream.unformatted],
but that doesn't mean very much since that section also
contains several member functions that clearly do not behave as
unformatted input functions as described in 27.6.1.3/1. Textual
location is not a sufficient guide.

Survey:
Classic AT&T: peek doesn't affect gcount
SGI (MIPSpro 7.3): peek doesn't affect gcount
Microsoft 6.0: peek sets gcount to zero.
Dietmar: peek sets gcount to zero.
others?

--Matt

Here's a test program.

#include <stdio.h>
#include <assert.h>
#include <string.h>

#ifndef CLASSIC_IOSTREAMS
#include <iostream>
#include <fstream>
using std::cout;
using std::endl;
using std::ifstream;
using std::ofstream;
#else
#include <iostream.h>
#include <fstream.h>
#endif

int main()
{
  // Create a temporary file.
  char name_buf[L_tmpnam];
  const char* name = tmpnam(name_buf);
  assert(name != 0 && strlen(name) > 0);

  {
    ofstream out(name);
    out << "1234 abcd" << endl;
  }


  {
    ifstream in(name);
    const int N = 128;
    char buf[N];

    in.read(buf, 6);
    assert(in.gcount() == 6);
    assert(strncmp(buf, "1234 a", 6) == 0);

    // The real point of this test: what about peek and gcount?
    assert(in.peek() == 'b');

    cout << "gcount = " << in.gcount() << endl;
  }

  // Destroy the temporary file.
  remove(name);
}