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Report From: Delphi-BCB/RTL/C++/STLPort [ Add a report in this area ]

Report #: 3470

Status: Closed

basic_istream::operator>> returns an undefined value from an empty stream

Project: C++Builder

Build #: 6.0.10.165

Version: 6.0

Submitted By: Jussi Sirpoma

Report Type: Minor failure / Design problem

Date Reported: 2/2/2003 7:47:48 PM

Severity: Infrequently encountered problem

Last Updated: 2/8/2005 1:15:41 PM

Platform: All versions

Internal Tracking #:

Resolution: Won't Do (Resolution Comments)

Resolved in Build: : None

Duplicate of: None

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Description

The basic_istream::operator>>(int&) and basic_istream::operator>>(short&) return an undefined value if the stream is or goes to error state.

Other >> operators do not modify the value given as parameter.

From stl\_istream.h:

  _Self& operator>> (int& __val) {
    long __lval;
    unsigned int __uval;
    _M_get_num(*this, __lval);
    __val = __lval;
    __uval = __lval;
    // check if we lose digits
    //    if ((__val != __lval) && ((unsigned int)__val != __lval))
    if ((__val != __lval) && ((long)__uval != __lval))
      this->setstate(ios_base::failbit);
    return *this;
  }

To make this operator behave the same way as the other >> operators do, the __lval could be initialized with __val.

This problem exist only when using STLPort.

Steps to Reproduce:

Run this program and check the value in tmp after reading it from the stream.

Expected: tmp has 0 in it
Actual Result: tmp has a random value

tmp2 has the expected 0.

//---------------------------------------------------------------------------

#include <vcl.h>
#pragma hdrstop
#include <sstream>
#include <string>

//---------------------------------------------------------------------------

#pragma argsused
int main(int argc, char* argv[])
{
     std::string str;
     std::stringstream is;

     int tmp = 0;
     is >> tmp;

     unsigned int tmp2 = 0;
     is >> tmp2;

     return 0;
}
//---------------------------------------------------------------------------

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