libstdc++/1834: istream manipulators and sentry

[snyder@fnal.gov]

>Number:         1834
>Category:       libstdc++
>Synopsis:       istream manipulators and sentry
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Feb 01 13:36:01 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     scott snyder
>Release:        2.97 20010129 (experimental)
>Organization:
>Environment:
System: Linux karma 2.2.16-22 #1 Tue Aug 22 16:49:06 EDT 2000 i686 unknown
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ../egcs/configure --prefix=/usr/local/egcs --enable-threads=posix
>Description:

Consider the following program:

---------------------------------------------------------
#include <iostream>

int main ()
{
  std::cin >> std::hex;
  return 0;
}
---------------------------------------------------------

I expect this program to exit without doing anything.
However, when i run it with libstdc++, it blocks waiting for input
before it exits.  This is because the operator>> implementations
for manipulators in istream.tcc construct sentry objects.
However, this is incorrect. This conclusion is clarified
by library issue #60.  A couple excerpts from it:

   ... I
   assume that 27.6.1.2.1 is intended to apply to the arithmetic
   extractors (27.6.1.2.2), but I assume that it is not intended to apply
   to extractors like
    basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));   
   
   and
    basic_istream& operator>>(basic_streammbuf*);
   ...
   Comments from Judy Ward: It seems like the problem is that the 
   basic_istream and basic_ostream operator <<()'s that are used for the
   manipulators and streambuf* are in the wrong section and should have  
   their own separate section or be modified to make it clear that the
   "Common requirements" listed in section 27.6.1.2.1 (for basic_istream)
   and section 27.6.2.5.1 (for basic_ostream) do not apply to them.
   ...
   Proposed Resolution:
   ...
   In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an 
   effects clause. "Effects: None. This extractor does not behave as a
   formatted input function (as described in 27.6.1.2.1).
   ...
   etc.



>How-To-Repeat:
  See above.

>Fix:

2001-01-29  scott snyder  <sss@fnal.gov>

	* include/bits/istream.tcc (operator>> (manip types)): Don't do
	anything other than calling __pf.

Index: include/bits/istream.tcc
===================================================================
RCS file: /cvs/gcc/egcs/libstdc++-v3/include/bits/istream.tcc,v
retrieving revision 1.4
diff -u -p -c -r1.4 istream.tcc
*** istream.tcc	2001/01/16 07:55:25	1.4
--- istream.tcc	2001/02/01 20:41:14
*************** namespace std {
*** 81,100 ****
      basic_istream<_CharT, _Traits>::
      operator>>(__istream_type& (*__pf)(__istream_type&))
      {
!       sentry __cerb(*this, false);
!       if (__cerb) 
! 	{
! 	  try {
! 	    __pf(*this);
! 	  }
! 	  catch(exception& __fail){
! 	    // 27.6.1.2.1 Common requirements.
! 	    // Turn this on without causing an ios::failure to be thrown.
! 	    this->setstate(ios_base::badbit);
! 	    if ((this->exceptions() & ios_base::badbit) != 0)
! 	      throw;
! 	  }
! 	}
        return *this;
      }
  
--- 81,87 ----
      basic_istream<_CharT, _Traits>::
      operator>>(__istream_type& (*__pf)(__istream_type&))
      {
!       __pf(*this);
        return *this;
      }
  
*************** namespace std {
*** 103,122 ****
      basic_istream<_CharT, _Traits>::
      operator>>(__ios_type& (*__pf)(__ios_type&))
      {
!       sentry __cerb(*this, false);
!       if (__cerb) 
! 	{
! 	  try {
! 	    __pf(*this);
! 	  }
! 	  catch(exception& __fail){
! 	    // 27.6.1.2.1 Common requirements.
! 	    // Turn this on without causing an ios::failure to be thrown.
! 	    this->setstate(ios_base::badbit);
! 	    if ((this->exceptions() & ios_base::badbit) != 0)
! 	      throw;
! 	  }
! 	}
        return *this;
      }
    
--- 90,96 ----
      basic_istream<_CharT, _Traits>::
      operator>>(__ios_type& (*__pf)(__ios_type&))
      {
!       __pf(*this);
        return *this;
      }
    
*************** namespace std {
*** 125,144 ****
      basic_istream<_CharT, _Traits>::
      operator>>(ios_base& (*__pf)(ios_base&))
      {
!       sentry __cerb(*this, false);
!       if (__cerb) 
! 	{
! 	  try {
! 	    __pf(*this);
! 	  }
! 	  catch(exception& __fail){
! 	    // 27.6.1.2.1 Common requirements.
! 	    // Turn this on without causing an ios::failure to be thrown.
! 	    this->setstate(ios_base::badbit);
! 	    if ((this->exceptions() & ios_base::badbit) != 0)
! 	      throw;
! 	  }
! 	}
        return *this;
      }
    
--- 99,105 ----
      basic_istream<_CharT, _Traits>::
      operator>>(ios_base& (*__pf)(ios_base&))
      {
!       __pf(*this);
        return *this;
      }
    

>Release-Note:
>Audit-Trail:
>Unformatted: