file input problem

[pentiumPunk]

string fileName;

cout<<"Enter filename of datafile you wish to open \n";
cin>>fileName;

ifstream fin;

fin.open(fileNa me, ifstream::ios);
if(!fin)
{
cerr<<"blah blah blah";
exit(1);
}


i get this error message in VC++ .....Error C2664...on the
fin.open(fileNa me, ifstream::ios); line...cannot convert parameter 1 from
std::string to const char *
i cannot put a certain filename down such as fin.open("textf ile.txt"), i
need the user to enter its name, for it will change.

Thanks alot in ahead!

[John Harrison]

Re: file input problem


"pentiumPun k" <Broox@triad.rr .com> wrote in message
news:wpe5b.4437 7$5H4.2243019@t wister.southeas t.rr.com...[color=blue]
> string fileName;
>
> cout<<"Enter filename of datafile you wish to open \n";
> cin>>fileName;
>
> ifstream fin;
>
> fin.open(fileNa me, ifstream::ios);
> if(!fin)
> {
> cerr<<"blah blah blah";
> exit(1);
> }
>
>
> i get this error message in VC++ .....Error C2664...on the
> fin.open(fileNa me, ifstream::ios); line...cannot convert parameter 1 from
> std::string to const char *
> i cannot put a certain filename down such as fin.open("textf ile.txt"), i
> need the user to enter its name, for it will change.
>
> Thanks alot in ahead!
>[/color]

fin.open(fileNa me.c_str(), ifstream::ios);

As the error message says, open requires a const char*, c_str returns a
const char* from a string.

Its just one of the quirks of the standard library.

john

[Sandeep]

Re: file input problem

"John Harrison" <john_andronicu s@hotmail.com> wrote in message news:<bj3smh$ep 038$1@ID-196037.news.uni-berlin.de>...[color=blue]
> "pentiumPun k" <Broox@triad.rr .com> wrote in message
> news:wpe5b.4437 7$5H4.2243019@t wister.southeas t.rr.com...[color=green]
> > string fileName;
> >
> > cout<<"Enter filename of datafile you wish to open \n";
> > cin>>fileName;
> >
> > ifstream fin;
> >
> > fin.open(fileNa me, ifstream::ios);
> > if(!fin)
> > {
> > cerr<<"blah blah blah";
> > exit(1);
> > }
> >
> >
> > i get this error message in VC++ .....Error C2664...on the
> > fin.open(fileNa me, ifstream::ios); line...cannot convert parameter 1 from
> > std::string to const char *
> > i cannot put a certain filename down such as fin.open("textf ile.txt"), i
> > need the user to enter its name, for it will change.
> >
> > Thanks alot in ahead!
> >[/color][/color]
When we declare any string variable it is not null terminated ie not
appended with '\0' .Since in the above program u r using fstream which
requires filename to be null terminated so we can make it null
terminated by using c_str() which convert it to null terminated string
..ie use
fin.open(filena me.c_str(),ifst ream::ios) instead of fin.open(fileNa me,
ifstream::ios)

Enjoy c++ programming.... ....

[Thomas Matthews]

Re: file input problem

pentiumPunk wrote:
[color=blue]
> string fileName;
>
> cout<<"Enter filename of datafile you wish to open \n";
> cin>>fileName;
>
> ifstream fin;
>
> fin.open(fileNa me, ifstream::ios);
> if(!fin)
> {
> cerr<<"blah blah blah";
> exit(1);
> }
>
>
> i get this error message in VC++ .....Error C2664...on the
> fin.open(fileNa me, ifstream::ios); line...cannot convert parameter 1 from
> std::string to const char *
> i cannot put a certain filename down such as fin.open("textf ile.txt"), i
> need the user to enter its name, for it will change.
>
> Thanks alot in ahead![/color]

1. As others have stated:
fin.open(filena me.c_str());
or
ifstream fin(filename.c_ str());

2. What kind of file mode is "ifstream::ios" ?
Try a combination of: "ios_base:: in", "ios_base::out" ,
or "ios_base::bina ry". The default mode for ifstream
is "ios_base:: in" as a text file.

3. Don't use the exit() function. It has some nasty quirks to it.
Use the return statement instead.

4. Instead of returning a '1' from main() to the operating system,
use the predefined, portable constants EXIT_SUCCESS or
EXIT_FAILURE as defined in <cstdlib>.

5. You may want to change:
cin >> fileName;
to
getline(cin, filename, '\n');
The getline() function will read in the whole line. The other
expression _may_ not read in spaces or other characters.

--
Thomas Matthews

C++ newsgroup welcome message:

http://www.slack.net/~shiva/welcome.txt

C++ Faq: http://www.parashift.com/c++-faq-lite
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alt.comp.lang.l earn.c-c++ faq:

http://www.raos.demon.uk/acllc-c++/faq.html

Other sites:
http://www.josuttis.com -- C++ STL Library book

[Rolf Magnus]

Re: file input problem

Sandeep wrote:
[color=blue]
> "John Harrison" <john_andronicu s@hotmail.com> wrote in message
> news:<bj3smh$ep 038$1@ID-196037.news.uni-berlin.de>...[color=green]
>> "pentiumPun k" <Broox@triad.rr .com> wrote in message
>> news:wpe5b.4437 7$5H4.2243019@t wister.southeas t.rr.com...[color=darkred]
>> > string fileName;
>> >
>> > cout<<"Enter filename of datafile you wish to open \n";
>> > cin>>fileName;
>> >
>> > ifstream fin;
>> >
>> > fin.open(fileNa me, ifstream::ios);
>> > if(!fin)
>> > {
>> > cerr<<"blah blah blah";
>> > exit(1);
>> > }
>> >
>> >
>> > i get this error message in VC++ .....Error C2664...on the
>> > fin.open(fileNa me, ifstream::ios); line...cannot convert parameter
>> > 1 from std::string to const char *
>> > i cannot put a certain filename down such as
>> > fin.open("textf ile.txt"), i need the user to enter its name, for it
>> > will change.
>> >
>> > Thanks alot in ahead!
>> >[/color][/color]
> When we declare any string variable it is not null terminated ie not
> appended with '\0' .[/color]

What do you mean by "any string variable"? If you're talking about
std::string, then it's not defined (and shouldn't matter) how the data
is stored internally. The only thing that matters is that c_str()
returns a null terminated array of the string's content.
An implementation of std::string that works on null terminated
characters internally (and thus doesn't need any extra work for
c_str() would be valid, too.