2012/4/27 Jim Michaels <[email protected]>
> by the way, this was a bug report, but a support request. but really this
> "bug report" has just morphed into an ECO for a serious reason.
>
> please note that I am not doing getline. totally different operation. I
> will show you why.
>
> I would like to submit an Engineering Change Order against iostream and
> ifstream. .good() includes .eof() and is better. I tried modifying the
> code for .eof and it still doesn't work. checking for .eof() after cin'ing
> a number doesn't work.
> the problem seems to be the whitespace (space, cr, lf) AFTER the 1 in the
> text file which is in SO MANY windows systems when people simply do
> echo 1 > somefile.txt
> in windows systems.
>
> no amount of checking for .eof() or .good() will remedy that, because EOF
> hasn't come yet! there's still whitespace to chew up. By behavior I
> believe istream::operator>>() eats whitespace and THEN gets the value.
> this is the way it's acting. but it sure doesn't act nice when it's the
> last value! you always get the same one twice unless you make your own
> input system.
>
> because nothing short of a function that chews up that trailing whitespace
> is going to fix that and get you to that EOF so you can detect .eof() or
> .good() which includes .eof().
> in fact, such a method OUGHT to be part of istream and ifstream and part
> of everybody's input reading method before checking for .eof() or .good()
>
> for example, usage of the method I am proposing might look something like
> this
>
> //this assumes the existence of std::istream.eatws() and
> std::ifstream.eatws()
>
>
> #include <fstream>
> #include <iostream>
> int main(int argc, char * argv[], char * envp[]) {
> int n;
> std::ifstream filein;
> filein.open("somefile.txt", std::ifstream::in);
> if (!filein.good() && !filein.eof()) {
>
> filein.close();
> std::cerr << "ERROR: unable to open file \"somefile.txt\"" <<
> std::endl;
> return 1;
> }
> do {
> filein >> n;
> filein.eatws();
>
> std::cout << n << std::endl;
> } while (!filein.eof());
> filein.close();
> return 0;
> }
>
>
>
> manually eating whitespace after inputting a value allows you to get the
> file pointer right to the next value OR another value directly.
>
> ...instead of getting a pointer to the next whitespace before EOF.
>
>
> so I am proposing an official change in iostream and ifstream.
>
> I have not some up with my own code for such a method. but I might be
> able to.
>
> this problem is not specific to gcc. it is in borland c++ 5.5.1 as well.
>
It's not specific to any compiler/environment, it's specific to Standard
C++.
The idiomatic way to read input using C++ I/O streams (basic_istream<T>),
is by using the (C++03) implicit (C++11) explicit conversion to bool of the
stream object:
int main()
{
ifstream stream("somefile.txt");
int i;
while(stream >> i)
{
//do stuff with i
}
You must check the stream state *after the read*, how else is it going to
communicate a failure?
The double last input is because you don't change the integer because
extracting from a failed stream does not change the integer.
See for a C++ design explanation for example here:
http://stackoverflow.com/questions/1039667/why-does-stdfstream-set-the-eof-bit-the-way-it-does
>
> operator>>() doesn't figure out until it has eaten the whitespace and is
> about to try to consume a value (it thinks) that it has hit EOF. but for
> some interesting reason I guess because it doesn't know what best to do
> when it hits EOF at that point, it simply spits out the last value it had
> and throws up its hands in disgust, since it can't return EOF or give any
> indication of failure - it's a 1-trick pony.
>
> ...and it doesn't throw.
>
To make iostreams throw, *you* need to set the stream exceptions flag:
http://en.cppreference.com/w/cpp/io/basic_ios/exceptions
Please get a good C++ book, read up on what problems you are experiencing,
because all these problems will be caused by the programmer.
Ruben
>
>
>
> ------------------------------
> *From:* Ruben Van Boxem <[email protected]>
> *To:* [email protected]
> *Sent:* Wednesday, April 25, 2012 2:25 PM
> *Subject:* Re: [Mingw-w64-public] ifstream reads once too many times?
>
> Op 25 apr. 2012 23:07 schreef "Jim Michaels" <[email protected]> het
> volgende:
> >
> >
> > #include <fstream>
> > #include <iostream>
> > int main(int argc, char * argv[], char * envp[]) {
> > int n;
> > std::ifstream filein;
> > filein.open("somefile.txt", std::ifstream::in);
> > if (!filein.good()) {
> > filein.close();
> > std::cerr << "ERROR: unable to open file \"somefile.txt\"" <<
> std::endl;
> > return 1;
> > }
> > filein >> n;
> > if (filein.good()) {
> > std::cout << n << std::endl;
> > }
> > while (filein.good()) {
> > std::cout << n << std::endl;
> > filein >> n;
> > }
> > filein.close();
> > return 0;
> > }
> >
> > somefile.txt contains a 1
> > but mingw-w64 outputs
> > 1
> > 1
> > this happens with 20111127. it happens with
> >
> > I have tried rearranging this to
> > #include <fstream>
> > #include <iostream>
> > int main(int argc, char * argv[], char * envp[]) {
> > int n;
> > std::ifstream filein;
> > filein.open("somefile.txt", std::ifstream::in);
> > if (!filein.good()) {
> > filein.close();
> > std::cerr << "ERROR: unable to open file \"somefile.txt\"" <<
> std::endl;
> > return 1;
> > }
> > if (filein.good()) {
> > filein >> n;
> > std::cout << n << std::endl;
> > }
> > while (filein.good()) {
> > filein >> n;
> > std::cout << n << std::endl;
> > }
> > filein.close();
> > return 0;
> > }
> >
> > but it makes no difference. same result. this also happens with mingw
> also.
> http://lmgtfy.com/?q=c%2B%2B+getline+one+line+too+many
> This is not the first time I'm telling you this: this is not a general
> programming question mailing list.
> This has to be the most easy thing to Google...
> Ruben
> >
> >
> >
> > -------------
> > Jim Michaels
> > [email protected]
> > [email protected]
> > http://JimsComputerRepairandWebDesign.com
> > http://JesusnJim.com (my personal site, has software)
> > ---
> > Computer memory measurements, SSD measurements, microsoft disk size
> measurements (note: they will say GB or MB or KB or TB when it is not!):
> > [KiB] [MiB] [GiB] [TiB]
> > [2^10B=1,024B=1KiB]
> > [2^20B=1,048,576B=1MiB]
> > [2^30B=1,073,741,824B=1GiB]
> > [2^40B=1,099,511,627,776B=1TiB]
> > hard disk industry disk size measurements:
> > [KB] [MB] [GB] [TB]
> > [10^3B=1,000B=1KB]
> > [10^6B=1,000,000B=1MB]
> > [10^9B=1,000,000,000B=1GB]
> > [10^12B=1,000,000,000,000B=1TB]
> >
> >
> >
> >
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> >
>
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